different types of cell organelles essay

Biology Article

Cell Organelles

More than 8.7 million species are living on the planet. Every single species is composed of a cell and it includes both single-celled and multicellular organisms.

The cells provide shape, structure and carry out different types of functions to keep the entire system active. The cell contains different functional structures which are collectively called organelles, and they are involved in various cellular functions.

Also Read: Difference between organ and organelle

What are Cell Organelles?

List of cell organelles and their functions, plasma membrane, endoplasmic reticulum, mitochondria, golgi apparatus, microbodies, cytoskeleton, cilia and flagella, centrosome and centrioles, a brief summary on cell organelles.

different types of cell organelles essay

A cell is the basic structural and functional unit of a living organism. According to cell theory postulates, a cell is the basic building block of life, which makes anything alive and is self-sufficient to carry out all the fundamental functions of an organism.

Explore more about Cells .

The cellular components are called cell organelles. These cell organelles include both membrane and non-membrane bound organelles, present within the cells and are distinct in their structures and functions. They coordinate and function efficiently for the normal functioning of the cell. A few of them function by providing shape and support, whereas some are involved in the locomotion and reproduction of a cell. There are various organelles present within the cell and are classified into three categories based on the presence or absence of membrane.

Organelles without membrane: The Cell wall, Ribosomes, and Cytoskeleton are non-membrane-bound cell organelles. They are present both in the prokaryotic cell and the eukaryotic cell.

Single membrane-bound organelles: Vacuole, Lysosome, Golgi Apparatus, Endoplasmic Reticulum are single membrane-bound organelles present only in a eukaryotic cell.

Double membrane-bound organelles: Nucleus, mitochondria and chloroplast are double membrane-bound organelles present only in a eukaryotic cell.

Let us learn more in detail about the different cell organelles in brief.

The plasma membrane is also termed as a Cell Membrane or Cytoplasmic Membrane. It is a selectively permeable membrane of the cells, which is composed of a lipid bilayer and proteins.

The plasma membrane is present both in plant and animal cells. It functions as the selectively permeable membrane, by permitting the entry of selective materials in and out of the cell according to the requirement. In an animal cell, the cell membrane functions by providing shape and protects the inner contents of the cell. Based on the structure of the plasma membrane, it is regarded as the fluid mosaic model. According to the fluid mosaic model, the plasma membranes are subcellular structures, made of a lipid bilayer in which the protein molecules are embedded.

Also refer to the Difference Between Cell Membrane and Plasma Membrane

The cytoplasm is present both in plant and animal cells. They are jelly-like substances, found between the cell membrane and nucleus. They are mainly composed of water, organic and inorganic compounds. The cytoplasm is one of the essential components of the cell, where all the cell organelles are embedded. These cell organelles contain enzymes, mainly responsible for controlling all metabolic activity taking place within the cell and are the site for most of the chemical reactions within a cell.

The nucleus is a double-membraned organelle found in all eukaryotic cells. It is the largest organelle, which functions as the control centre of the cellular activities and is the storehouse of the cell’s DNA. By structure, the nucleus is dark, round, surrounded by a nuclear membrane. It is a porous membrane (like cell membrane) and forms a wall between cytoplasm and nucleus. Within the nucleus, there are tiny spherical bodies called nucleolus. It also carries an essential structure called chromosomes.

Chromosomes are thin and thread-like structures which carry another important structure called a gene. Genes are a hereditary unit in organisms i.e., it helps in the inheritance of traits from one generation (parents) to another (offspring). Hence, the nucleus controls the characters and functions of cells in our body. The primary function of the nucleus is to monitor cellular activities including metabolism and growth by making use of DNA’s genetic information. Nucleoli in the nucleus are responsible for the synthesis of protein and RNA.

Frequently Asked Questions on Cell Organelles :

Which cell organelle is called the Powerhouse of the cell?

Mitochondria is the cell organelle and is called the Powerhouse of the cell as they carry out the cellular respiration and generate the energy molecules called ATP or Adenosine Triphosphate.

Where do we find Chloroplasts and Chromoplast pigments in plants?

Chloroplasts and Chromoplasts are the plastids present in all plant cells. Chloroplasts contain the green colour pigments, present in the leaves, green-coloured stems, etc. Chromoplasts contain thee colourful pigments present in all colourful parts of the plant like flowers and fruits, etc.

Why Lysosomes are known as suicide bags?

Lysosomes are called the suicidal bags because they are capable of breaking down or digesting all the wastes, dead and damaged cells.

What is Nucleoid?

Nucleoid is a non-membrane, irregular-shaped cell organelle present in all prokaryotic cells. They are the carriers of the genetic material of a cell.

T he largest membrane-bound organelle in a eukaryotic cell is?

Organelles are special and organized structures seen in living cells. Some of the membrane-bound organelles are vacuoles, nucleus, chloroplasts, lysosomes etc. The nucleus is the largest organelle in the cell.

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Microbe Notes

Cell Organelles: Definition, Structure, Functions, Diagram

Cell organelles are specialized entities present inside a particular type of cell that performs a specific function.

There are various cell organelles, out of which, some are common in most types of cells like cell membranes, nucleus, and cytoplasm. However, some organelles are specific to one particular type of cell-like plastids and cell walls in plant cells.

Cell Organelles- Structure and Functions with diagram

Table of Contents

Interesting Science Videos

List of 24 Cell Organelles

Cell membrane (plasma membrane/ plasmalemma), cilia and flagella, chloroplast, cytoskeleton, endoplasmic reticulum (er), golgi apparatus/ golgi complex/ golgi body, intermediate filaments, microfilaments, microtubules, mitochondria, peroxisomes, plasmodesmata, storage granules.

A plasma membrane is composed of lipids and proteins where the composition might fluctuate based on fluidity, external environment, and the different stages of development of the cell.

Structure of Cell Membrane

Structurally, it consists of a phospholipid bilayer along with two types of proteins viz. embedded proteins and peripheral proteins that function in providing shape and allowing the movement of particles in and out of the cell.
The most abundant lipid which is present in the cell membrane is a phospholipid that contains a polar head group attached to two hydrophobic fatty acid tails.
The embedded proteins act as channels for the transfer of particles across the cell with some proteins acting as receptors for the binding of various components.
The peripheral proteins function as to provide fluidity as well as mechanical support to the structure of the cell.

Functions of Cell Membrane

The cell membrane provides mechanical support that facilities the shape of the cell while enclosing the cell and its components from the external environment.
It regulates what can be allowed to enter and exit the cell through channels, acting as a semi-permeable membrane, which facilities the exchange of essential compounds required for the survival of the cell.
It generates and distributes signals in and outside of the cell for the proper functioning of the cell and all the organelles.
It allows the interaction between cells required during tissue formation and cell fusion.

An additional non-living layer present outside the cell membrane in some cells that provides structure, protection, and filtering mechanism to the cell is the cell wall.

Structure of Cell Wall

In a plant cell, the cell wall is made up of cellulose, hemicellulose, and proteins while in a fungal cell, it is composed of chitin.
A cell wall is multilayered with a middle lamina, a primary cell wall, and a secondary cell wall.
The middle lamina contains polysaccharides that provide adhesion and allow binding of the cells to one another.
After the middle lamina is the primary cell wall which is composed of cellulose. The last layer, which is not always present, is the secondary cell wall made of cellulose and hemicellulose.

Functions of Cell Wall

The critical function of the cell wall is protecting and maintaining the shape of the cell. It also helps the cell withstand the turgor pressure of the cell.
It initiates cell division by providing signals to the cell and allows the passage of some molecules into the cell while blocking others.

Centrioles are tubular structures mostly found in eukaryotic cells which are composed mainly of the protein tubulin.

Structure of Centriole

A centriole consists of a cylindrical structure made with nine triplets microtubules that surround the periphery of the centriole while the center has a Y-shaped linker and a barrel-like structure that stabilizes the centriole.
Another structure called cartwheel is present in a centriole which is made up of a central hub with nine spokes/filaments radiating from it. Each of these filaments/spokes is connected to the microtubules through a pinhead.

Functions of Centriole

During cell division, centrioles have a crucial role in forming spindle fibers which assist the movement of chromatids towards their respective sides.
They are involved in the formation of cilia and flagella.

Cilia and Flagella are tiny hair-like projections from the cell made of microtubules and covered by the plasma membrane.

Structure of Cilia and Flagella

Cilia are hair-like projections that have a 9+2 arrangement of microtubules with a radial pattern of 9 outer microtubule doublet that surrounds two singlet microtubules. This arrangement is attached to the bottom with a basal body.
Flagella is a filamentous organelle, the structure of which, is different in prokaryotes and eukaryotes.
In prokaryotes, it is made up of the protein called flagellin wrapped around in a helical manner creating a hollow structure at the center throughout the length.
In eukaryotes, however, the protein is absent and the structure is replaced with microtubules.

Functions of Cilia and Flagella

The most critical role of cilia and flagella is movement. These are responsible for the movement of the organisms as well as for the movement of various particles present around the organisms.
Some cilia present in some particular organs may have the function of sense. The cilium in the blood vessels, which helps in controlling the flow of blood is an example.

A chloroplast is a type of plastic that is involved in photosynthesis in plants and algae. Chloroplast contains an essential pigment called chlorophyll necessary to trap sunlight for the production of glucose.

Structure of Chloroplast

It is a double-membraned structure with its own DNA which is inherited from the previous chloroplast.
These are usually lens-shaped with shape and number varying according to cells. They have an outer membrane, an inner membrane, and a thylakoid membrane that enclosed the gel-like matric called the stroma.
The outer and inner membrane is porous and allows transport of materials while the stroma contains DNA, chloroplast ribosomes, proteins, and starch granules.

Functions of Chloroplast

The chloroplast is the primary center for light-dependent and light-independent reactions during photosynthesis.
Different proteins present in chlorophyll are involved in the regulation of photorespiration.

Cytoplasm refers to everything present inside the cell except the nucleus.

Structure of Cytoplasm

The cytoplasm consists of a cytosol; a gel-like substance that contains other matter; cell organelles; smaller cell-like bodies bound by separate membranes; and cytoplasmic inclusions; insoluble molecules that store energy and are not surrounded by any layer.
The cytoplasm is colorless and has about 80% water along with various nutrients required for the cell.
It is known to have the properties of both viscous matter as well as elastic matter. Under its elasticity, cytoplasm helps in the movement of materials inside the cell by a process termed cytoplasmic streaming.

Functions of Cytoplasm

Most of the vital cellular and enzymatic reactions like cellular respiration and translation of mRNA into proteins occur in the cytoplasm.
It acts as a buffer and protects genetic materials as well as other organelles from damage due to collision or change in the pH of the cytosol.
The process called cytoplasmic streaming helps in the distribution of various nutrients and facilitates the movement of cell organelles within the cell.

A number of fibrous structures are present in the cytosol that helps give shape to the cell while supporting cellular transport.

Structure of Cytoskeleton

Around three different classes of fibers make up the cytoskeleton which is: microtubules, microfilaments, and intermediate filaments.
These are separated based on a protein present in them.

Functions of Cytoskeleton

The critical function of the cytoskeleton is to provide shape and mechanical support to the cell against deformation.
It allows the expansion and contraction of the cell which assists in the movement of the cell.
It is also involved in the intracellular and extracellular transport of materials.

Endoplasmic Reticulum (ER) is present as an interconnection of tubules that are connected to the nuclear membrane in eukaryotic cells.

There are two types of ER based on the presence or absence of ribosomes on them:

Rough ER (RER) with ribosomes attached on the cytosolic face of Endoplasmic Reticulum and thus is involved in protein synthesis
Smooth ER (SER) lacks ribosomes and has a function during lipid synthesis.

Structure of Endoplasmic Reticulum (ER)

Endoplasmic Reticulum exists in three forms viz. cisternae, vesicles, and tubules.
Cisternae are sac-like flattened, unbranched structures that remain stacked one on top of another.
Vesicles are spherical structures that carry proteins throughout the cell.
Tubules are tubular branched structures forming a connection between cisternae and vesicles.

Functions of Endoplasmic Reticulum (ER)

ER contains many of the enzymes required for several metabolic processes, and the surface of the ER is essential for other operations like diffusion, osmosis, and active transport.
One of the crucial functions of ER is the synthesis of lipids like cholesterol and steroids.
Rough ER allows for the modification of polypeptides emerging out of the ribosomes to prepare secondary and tertiary structures of the protein.
ER also synthesizes various membrane proteins and has a crucial role in preparing the nuclear envelope after cell division.

Endosomes are membrane-bound compartments within a cell originating from the Golgi network

Structure of Endosomes

There are different types of endosomes based on morphology and the time it takes for the endocytosed materials to reach them.
The early endosomes are made with the tubular-vesicular network while the late endosomes lack tubules but contain many close-packed intraluminal vesicles. The recycling endosomes are found with microtubules and are mainly composed of tubular structures.

Functions of Endosomes

Endosomes allow the sorting and delivery of internalized materials from the cell surface and transport of materials to the Golgi or the lysosomes.

The Golgi Apparatus is the cell organelle mostly present in eukaryotic cells which is responsible for the packaging of macromolecules into vesicles so that they can be sent out to their site of action.

Structure of Golgi Apparatus

The structure of the Golgi Complex is pleomorphic; however, it typically exists in three forms, i.e. cisternae, vesicles, and tubules.
The cisternae, which is the smallest unit of the Golgi Complex, has a flattened sac-like structure that is arranged in bundles in a parallel fashion.
Tubules are present as tubular and branched structures that radiate from the cisternae and are fenestrated at the periphery.
Vesicles are spherical bodies that are divided into three groups as transitional vesicles, secretory vesicles, and clathrin-coated vesicles.

Golgi apparatus (Golgi bodies or Golgi complex) Diagram

Functions of Golgi Apparatus

Golgi Complex has an essential purpose of directing proteins and lipids to their destination and thus, act as the “traffic police” of the cell.
They are involved in the exocytosis of various products and proteins like zymogen, mucus, lactoprotein, and parts of the thyroid hormone.
Golgi Complex is involved in the synthesis of other cell organelles like a cell membrane, lysozymes, among others.
They are also involved in the sulfation of various molecules.

The third class of filament that makes up the cytoskeleton is the intermediate filaments. They are designated as intermediate filaments because of the intermediate diameter of the filaments as compared to microfilaments and myosin proteins.

Structure of Intermediate filaments

Intermediate filaments contain a family of related proteins.
The individual filaments are coiled around each other in a helical structure called coiled-coil structure.

Functions of Intermediate filaments

Intermediate filaments contribute to the structural integrity of a cell while playing a crucial role in holding tissues of various organs like the skin.

Lysozymes are membrane-bound organelles that occur in the cytoplasm of animal cells. These organelles contain an array of hydrolytic enzymes required for the degradation of various macromolecules.

There are two types of lysozymes:

Primary lysozyme containing hydrolytic enzymes like lipases, amylases, proteases, and nucleases.
Secondary lysozyme formed by the fusion of primary lysozymes containing engulfed molecules or organelles.

Structure of Lysozyme

The shape of lysozymes is irregular or pleomorphic; however, mostly, they are found in spherical or granular structures.
Lysozymes are surrounded by a lysosomal membrane that contains the enzymes within the lysosome and protects the cytosol with the rest of the cell from the harmful action of the enzymes.

Functions of Lysozyme

These organelles are responsible for intracellular digestion where the larger macromolecules are degraded into smaller molecules with the help of enzymes present in them.
Lysozymes also perform the critical function of the autolysis of unwanted organelles within the cytoplasm.
Besides these, the lysosome is involved in various cellular processes, including secretion, plasma membrane repair, cell signaling, and energy metabolism.

Microfilaments are a part of the cytoskeleton of a cell made up of actin protein in the form of parallel polymers. These are the smallest filaments of the cytoskeleton with high rigidity and flexibility, providing strength and movement to the cell.

Structure of Microfilaments

The filaments are present either in cross-linked forming networks or as bundles. The chains of protein remain twisted around each other in a helical arrangement.
One of the polar ends of the filament is positively charged and barbed, whereas the other end is negatively charged and pointed.

Functions of Microfilaments

It generates the strength for the structure and movement of the cell in association with myosin protein.
They help in cell division and are involved in the products of various cell surface projections.

Microtubules are also a part of the cytoskeleton differing from microfilaments in the presence of tubulin protein

Structure of Microtubules

They are long hollow, beaded tubular structures of a diameter of about 24nm.
The wall of the microtubules consists of globular subunits present at a helical array of a and b tubulin.
Similar to microfilaments, the ends of microtubules also have a defined polarity with one end being positively charged while the other being negatively charged.

Functions of Microtubules

As a part of the cytoskeleton, they provide shape and movement to the cell.
Microtubules facilitate the movement of other cell organelles within the cell through binding proteins.

Microvilli are tiny finger-like structures that project on or out of the cells. These exist either on their own or in conjunction with villi.

Structure of Microvilli

Microvilli are bundles of protuberances loosely arranged on the surface of the cell with little or no cellular organelles.
These are surrounded by a plasma membrane enclosing cytoplasm and microfilaments.
These are bundles of actin filaments bound by fimbrin, villin, and epsin.

Functions of Microvilli

Microvilli increase the surface area of the cell, thus, enhancing the absorption and secretion functions.
The membrane of microvilli is packed with enzymes that allow the break down of larger molecules into smaller allowing more effective absorption.
Microvilli act as an anchoring agent in white blood cells and in sperms during fertilization.

Mitochondria are double membrane-bound cell organelles responsible for the supply and storage of energy for the cell. The oxidation of various substrates in the cell to release energy in the form of ATP (Adenosine Triphosphate) is the primary purpose of mitochondria.

Structure of Mitochondria

A mitochondrion contains two membranes with the outer layer being smooth while the inner layer is marked with folding and finger-like structures called cristae.
The inner mitochondrial membrane contains various enzymes, coenzymes, and components of multiple cycles along with pores for the transport of substrates, ATP, and phosphate molecules.
Within the membranes is a matrix that contains various enzymes of metabolic processes like Kreb’s cycle.
In addition to these enzymes, mitochondria are also home to single or double-stranded DNA called mtDNA that is capable of producing 10% of the proteins present in the mitochondria.

Functions of Mitochondria

The primary function of mitochondria is the synthesis of energy in the form of ATP required for the proper functioning of all the cell organelles.
Mitochondria also help in balancing the amount of Ca+ ions within the cell and assists the process of apoptosis.
Different segments of hormones and components of blood are built within mitochondria.
Mitochondria in the liver have the ability to detoxify ammonia.

The nucleus is a double membrane-bound structure responsible for controlling all cellular activities as well as a center for genetic materials, and it’s transferring. It is one of the large cell organelles occupying 10% of the total space in the cell. It is often termed the “brain of the cell” as it provides commands for the proper functioning of other cell organelles. A nucleus is clearly defined in the case of a eukaryotic cell; however, it is absent in prokaryotic organisms with the genetic material distributed in the cytoplasm.

Structure of Nucleus

Structurally, the nucleus consists of a nuclear envelope, chromatin, and nucleolus.
The nuclear envelope is similar to the cell membrane in structure and composition. It has pores that allow the movement of proteins and RNA in and outside the nucleus. It enables the interaction with other cell organelles while keeping nucleoplasm and chromatin within the envelope.
The chromatin in the nucleus contains RNA or DNA along with nuclear proteins, as genetic material that is responsible for carrying the genetic information from one generation to another. It is present in a sense and compact structure which might be visible as a chromosome under powerful magnification.
The nucleolus is like a nucleus within the nucleus. It is a membrane-less organelle that is responsible for the synthesis of rRNA and the assembly of ribosomes required for protein synthesis.

Functions of Nucleus

The nucleus is responsible for storage as well as the transfer of genetic materials in the form of DNA or RNA.
It aids in the process of transcription by the synthesis of mRNA molecules.
The nucleus controls the activity of all other organelles while facilitating processes like cell growth, cell division, and the synthesis of proteins .

Peroxisomes are oxidative membrane-bound organelles found in the cytoplasm of all eukaryotes. The name is accredited due to their hydrogen peroxide generating and removing activities.

Structure of Peroxisomes

Peroxisome consists of a single membrane and granular matrix scattered in the cytoplasm.
They exist either in the form of interconnected tubules or as individual peroxisomes.
The compartments within every peroxisome allow the creation of optimized conditions for different metabolic activities.
They consist of several types of enzymes with major groups being urate oxidase, D-amino acid oxidase, and catalase.

Functions of Peroxisomes

Peroxisomes are involved in the production and elimination of hydrogen peroxide during biochemical processes.
Oxidation of fatty acids takes place within peroxisomes.
Additionally, peroxisomes are also involved in the synthesis of lipid-like cholesterol and plasmalogens.

Plasmodesmata are tiny passages or channels that allow the transfer of material and communication between different cells.

Structure of Plasmodesmata

There are 103 – 105 plasmodesmata connecting two adjacent cells with 50-60 nm in diameter.
The plasma membrane is continuous with the plasma membrane of the cell and has the same phospholipid bilayer.
The cytoplasmic sleeve is continuous with the cytosol that allows the exchange of materials between two cells.
Desmotubule which is a part of the endoplasmic reticulum that provides a network between two cells and allows the transport of some molecules.

Functions of Plasmodesmata

Plasmodesmata are the primary site for the communication of two cells. It allows the transfer of molecules like proteins, RNA, and viral genomes.

Plastids are double membrane-bound structures present in plants and other eukaryotes involved in the synthesis and storage of food.

Structure of Plastids

Plastids are usually oval or spherical with an outer and an inner membrane between which lies the intermembrane space.
The inner membrane enclosed a matrix called stroma that contains small structures called grana.
Each granum consists of several sac-like thylakoids piled one on the other and connected by stroma lamellae.
Plastids contain DNA and RNA that allows it to synthesize necessary proteins for different processes.

Functions of Plastids

Chloroplasts are the center for many metabolic activities, including photosynthesis as it contains enzymes and other components required for it.
They are also involved in the storage of food, primarily starch.

Ribosomes are ribonucleoproteins containing equal parts RNA and proteins along with an array of other essential components required for protein synthesis. In prokaryotes, they exist freely while in eukaryotes, they are found either free or attached to the endoplasmic reticulum.

Structure of Ribosomes

The ribonucleoprotein consists of two subunits.
In the case of prokaryotic cells, the ribosomes are of the 70S with the larger subunit of 50S and the smaller one of 30S.
Eukaryotic cells have 80S ribosomes with 60S larger subunit and 40S smaller subunit.
Ribosomes are short-lived as after the protein synthesis, the subunits split up and can be either reused or remain broken up.

Functions of Ribosomes

Ribosomes are the site of biological protein synthesis in all living organisms.
They arrange the amino acids in the order indicated by tRNA and assist in protein synthesis.

Storage granules are membrane-bound organelles, also called zymogen granules storing cells’ energy reserve and other metabolites.

Structure of Storage granules

These granules are surrounded by a lipid bilayer and are composed mostly of phosphorus and oxygen.
The components inside these storage granules depend on their location in the body with some even containing degradative enzymes yet to participate in digestive activities.

Figure: Diagram of Storage Granules. Image Source: Slide Player

Functions of storage granules.

Many prokaryotes and eukaryotes store nutrients and reserves in the form of storage granules in the cytoplasm.
Sulfur granules are characteristic of prokaryotes that utilize hydrogen sulfide as a source of energy.

Vacuoles are membrane-bound structures varying in size in cells of different organisms.

Structure of Vacuoles

The vacuole is surrounded by a membrane called tonoplast, which encloses fluid containing inorganic materials like water and organic materials like nutrients and even enzymes.
These are formed by the fusion of various vesicles, so vacuoles are very similar to vesicles in structure.

Functions Vacuoles

Vacuoles act as a storage for nutrients as well as waste materials to protect the cell from toxicity.
They have an essential function of homeostasis as it allows the balance of pH of the cell by influx and outflow of H+ ions to the cytoplasm.
Vacuoles contain enzymes that play an important role in different metabolic processes.

Vesicles are structures present inside the cell that are either formed naturally during processes like exocytosis, endocytosis, or transport of materials throughout the cell, or they might form artificially, which are called liposomes. There are different types of vesicles like vacuoles, secretory, and transport vesicles based on their function

Structure of Vesicles

A vesicle is a structure containing liquid or cytosol which is enclosed by a lipid bilayer.
The outer layer enclosing the liquid is called a lamellar phase which is similar to the plasma membrane. One end of the lipid bilayer it hydrophobic whereas the other end is hydrophilic.

Vesicles- Structure, Types, and Functions

Figure: A liposome (left) and dendrimersome. The blue parts of their molecules are hydrophilic, the green parts are hydrophobic. Credit: Image courtesy of University of Pennsylvania

Functions of vesicles.

Vesicles facilitate the storage and transport of materials in and outside the cell. It even allows the exchange of molecules between two cells.
Because vesicles are enclosed inside a lipid bilayer, vesicles also function in metabolism and enzyme storage.
They allow temporary storage of food and also control the buoyancy of the cell.
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Images created using biorender.com

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18 thoughts on “Cell Organelles: Definition, Structure, Functions, Diagram”

Microvilli act as an anchoring agent in white blood cells and in sperms during fertilization. …. i didnt understand this line… can u explain properly?

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Organelles – Definition, List, and Functions

An organelle is a compartment within a eukaryotic cell that has a specific function. The name “organelle” comes from the idea that these structures are to cells what an organ is to the body. Typically, organelles are enclosed within their own lipid bilayers. They are essential for various cellular functions, including energy production, protein synthesis, and cell division.

Importance of Organelles

Just like an organism depends on organs for survival, a cell depends on organelles. Organelles play critical roles in:

Maintaining Cellular Health : They perform specific tasks that are vital for the survival of cells.
Energy Conversion : Organelles like mitochondria and chloroplasts convert energy into forms usable by the cell.
Protein Synthesis : Organelles such as ribosomes are crucial in making proteins.

History and Etymology

The term “organelle” was coined in the late 19th century. The discovery of organelles dates back to the 1830s with the identification of the nucleus. The development of electron microscopy in the 20th century allowed for the detailed study of organelles.

List of Organelles

Eukaryotic (animal and plant) cells share several organelles in common:

Cell Membrane : Separates the cell interior from the external environment.
Nucleus : Stores genetic material and coordinates cellular activities.
Nucleolus: Found in most eukaryotic cells, it functions in pre-ribosome production.
Mitochondria : Powerhouses of the cell, producing ATP through cellular respiration.
Endoplasmic Reticulum (ER) : Synthesizes proteins (rough ER) and lipids (smooth ER).
Golgi Apparatus : Modifies, sorts, and packages proteins for secretion.
Lysosomes : Digests excess or worn-out organelles, food particles, and engulfed viruses or bacteria.
Ribosomes : Synthesize proteins.
Vacuoles: These single-membrane compartments find use in storage and transportation of materials.
Vesicles: Single-membrane compartments that perform material transport.
Flagellum: Performs sensory or locomotion tasks in some eukaryotes.
Peroxisome: Breaks down hydrogen peroxide.
Proteasome: Degrades unnecessary proteins.

Animal Cell Organelles

Animal cells have some organelles that are not found in plant cells:

Centrosomes and Centrioles : Play a role in cell division.
Lysosomes : More prominent in animal cells for digesting materials.
Melanosome: Stores pigment in some animal cells.

Plant Cell Organelles

Meanwhile, plant cells have certain organelles not found in animal cells :

Cell Wall: Plants, fungi, and some protists have a rigid cellulose-based cell wall the keeps the cell rigid and protects it from osmotic pressure.
Chloroplasts : Conduct photosynthesis to convert solar energy into chemical energy.
Central Vacuole : Stores water, maintains turgor pressure.
Glyoxysome: Converts fat into sugars.

Additionally, there are many other organelles found in specific eukaryotic cells that carry out specialized tasks.

Membrane-Bound vs. Non-Membrane-Bound Organelles

One method of classifying organelles is according to whether they are membrane-bound or non-membrane-bound. So, strictly speaking, not all organelles are packaged within membranes.

Membrane-Bound Organelles

Examples : Mitochondria, Nucleus, ER, Golgi Apparatus.
Characteristics : Enclosed by membranes, have distinct internal environments.

Non-Membrane-Bound Organelles

Examples : Ribosomes, Centrosomes.
Characteristics : Lack a surrounding membrane, more open to the cytoplasm.

Organelles in Eukaryotic vs. Prokaryotic Cells

Eukaryotic Cells : Possess membrane-bound organelles.
Prokaryotic Cells : Generally lack membrane-bound organelles, but show some compartmentalization.

Prokaryotic cells are typically simpler and smaller than eukaryotic cells. While prokaryotic cells do not have membrane-bound organelles, they have some internal organization and compartmentalization that is analogous to organelles. Here’s a comprehensive list of these structures, their functions, and examples of prokaryotic organisms that contain them:

Function : Region in the cell where the genetic material (DNA) is located. Unlike a nucleus, it is not enclosed by a membrane.
Example Organisms : Escherichia coli , Bacillus subtilis .
Function : Sites of protein synthesis. Prokaryotic ribosomes are smaller (70S) compared to eukaryotic ribosomes (80S).
Example Organisms : All prokaryotes including Streptococcus pneumoniae , Mycobacterium tuberculosis .
Function : Small, circular DNA molecules separate from chromosomal DNA. They often carry genes beneficial for survival under specific conditions, like antibiotic resistance.
Example Organisms : Agrobacterium tumefaciens (carries plasmids used in genetic engineering of plants).
Function : Provides structural support and protection. Made of peptidoglycan in bacteria.
Example Organisms : Staphylococcus aureus (Gram-positive bacteria), E. coli (Gram-negative bacteria).
Function : Controls the movement of substances in and out of the cell.
Example Organisms : All prokaryotes.
Function : Gel-like substance inside the cell membrane containing enzymes, nutrients, and other molecules needed for cell survival.
Function : Hair-like structures that help in attachment to surfaces and in conjugation (transfer of genetic material between bacteria).
Example Organisms : Neisseria gonorrhoeae (uses pili for attachment to host cells).
Function : Long, whip-like structures used for movement.
Example Organisms : Salmonella typhi (uses flagella to move).
Function : Resistant, dormant structures formed under stress conditions, ensuring survival.
Example Organisms : Bacillus anthracis (forms endospores).
Function : A thick polysaccharide layer for protection against environmental hazards and in some cases, helps in evading the host immune system.
Example Organisms : Streptococcus pneumoniae (has a capsule that contributes to its pathogenicity).
Function : Storage of nutrients, enzymes, or metabolic end products.
Example Organisms : Many cyanobacteria store glycogen, lipids, or other compounds in inclusion bodies.
Function : Microcompartments that contain enzymes for carbon fixation in photosynthetic bacteria.
Example Organisms : Cyanobacteria like Synechococcus .
Function : Organelles in some bacteria containing iron oxide, aiding in navigation by orienting the bacteria in line with Earth’s magnetic field.
Example Organisms : Magnetospirillum magnetotacticum .
Function : Hollow structures that provide buoyancy in aquatic environments.
Example Organisms : Halobacterium salinarum .
Function : Membrane systems in photosynthetic bacteria where photosynthesis occurs.
Example Organisms : Cyanobacteria.

Origin of Organelles

The prevailing theory for the origin of organelles is endosymbiosis. This suggests that organelles like mitochondria and chloroplasts were once independent prokaryotic organisms that were taken inside another cell and evolved into the organelles we see today.

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Kerfeld, C.A.; Sawaya, M.R.; et al. (2005). “Protein structures forming the shell of primitive organelles”. Science . 309 (5736): 936–8. doi: 10.1126/science.1113397
Mullins, C. (2004). “Theory of Organelle Biogenesis: A Historical Perspective”. The Biogenesis of Cellular Organelles . Springer Science+Business Media, National Institutes of Health. ISBN 978-0-306-47990-8.
Murat, Dorothee; Byrne, Meghan; Komeili, Arash (2010). “Cell Biology of Prokaryotic Organelles”. Cold Spring Harbor Perspectives in Biology . 2 (10): a000422. doi: 10.1101/cshperspect.a000422

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Cell Organelles

An organelle is a membrane-bound structure found within a eukaryotic cell . It is similar to an organ in an animal body. There are many cell organelles in a cell, each having a distinct structure and specific functions. Some are without membrane, while others are either single or double-membrane-bound. They collectively help in the functioning of the cell.

Membrane-bound organelles are essential as they allow proper segregation for their functioning in eukaryotic cells. All organelles in a cell are found embedded in a fluid-like material called cytoplasm . It is made of water, inorganic, and organic substances. In contrast to eukaryotes, prokaryotic cells like bacteria lack membrane-bound organelles.

The organelles found in a generalized eukaryotic cell ( plant and animals) are listed below:

1. Cell Membrane

It is also called the plasma membrane, which forms the outermost boundary of an animal cell , while in plants, they are enclosed by a cell wall .

Cell membranes are made of phospholipid bilayers and are selectively permeable, allowing passage of certain substances while blocking others from entering the cell.

It is a membrane-bound organelle filled with cell sap. Plant cells have a large central vacuole , while in an animal cell, many small vacuoles are scattered throughout the cell. Vacuoles store reserve water and food materials and are also responsible for maintaining the osmotic pressure of the cell.

It is a double-membrane bound cell organelle that carries the cell’s genetic information (DNA or RNA). The nucleus is also where transcription takes place in a cell. It has several parts:

Nuclear membrane or nuclear envelope is made of a phospholipid bilayer, similar to a cell membrane, and is selectively permeable.
Nucleoplasm , known as karyoplasm, is a gelatinous substance that protects the cell’s genetic material.
A nucleolus is a dense structure within the nucleus composed of RNA, proteins, granules, and fibers involved in protein synthesis .
Chromatin is a complex of genetic material (DNA or RNA) and proteins that participate in cell division .

The nucleus is thus called the control center of the cell.

4. Endoplasmic Reticulum (ER)

They are a network of membrane-bound sac-like structures scattered throughout the cytoplasm. ER packages and transports proteins to the Golgi apparatus . They are of two types:

Rough Endoplasmic Reticulum (RER) is studded with ribosomes and is thus rough in appearance. RER participates in protein synthesis.
Smooth Endoplasmic Reticulum (SER) is without ribosomes with a smooth appearance that synthesizes carbohydrates, lipids, and steroids.

5. Ribosomes

They make proteins and are thus called the protein factories of a cell. It is composed of a smaller and a larger subunit, each made of ribosomal RNA (rRNA) and proteins; thus, they are also called ribonucleoproteins. They are bound to the endoplasmic reticulum or scattered throughout the cytosol.

6. Golgi Apparatus

It is a series of staked membrane-bound structures responsible for modifying, packaging, and transporting proteins and lipids to their destinations within and outside the cell.

7. Mitochondria

They are called the ‘powerhouse of the cell’ as they produce the high-energy molecule – ATP that provides the energy responsible for all cellular activities. Mitochondria have their DNA and are a cell’s cellular respiration site.

They are more active than all other organelles in causing apoptosis or programmed cell death.

8. Peroxisomes

They are single membrane-bound vesicular structures that contain digestive enzymes that help cells break down and remove toxic materials through oxidation. They are also involved in biochemical pathways such as β-oxidation of fatty acids, releasing energy as ATP, synthesizing lipids, and plasmalogens.

9. Cytoskeleton

They are a complex network of interlinking protein filaments that maintain a cell’s shape and internal organization. It also provides a cell with mechanical support. There are three types of cytoskeleton :

Microtubules are hollow protein tubular rods and heterodimers of α- and β-tubulin. They are located in cilia , flagella, and structures associated with cell movement.
Intermediate filaments are smaller than microtubules but larger than microfilaments .
Microfilaments are the thinnest cytoskeleton, made of actin filaments. They are thus strong and flexible and are involved in cell movement.

Apart from the above organelles found in both plant and animal cells, some are exclusively found in either of the two types.

Organelles Found Only in Plant Cells

1. cell wall.

It is the outermost boundary of the plant cell , providing protection and structural support. The cell wall is non-living.

2. Plastids

They are double membrane-bound organelles responsible for manufacturing or storing food in plants and algae. Chloroplast , chromoplast , and leucoplast are the types of plastids commonly found in them.

Chloroplast harbors chlorophyll that helps plants to prepare their food through photosynthesis . In contrast, chromoplast is found in flowers and fruits that help pollinate, while leucoplast stores food.

Organelles Found Only in Animal Cells

1. lysosomes.

They are the site of intracellular digestion and, thus, the cell’s recycling center. Lysosomes are filled with hydrolytic enzymes that remove unwanted substances, such as the worn-out – dead cells and organelles that are no longer helpful, destroy the invading pathogens, and degrade macromolecules.

Lysosomes are rarely found in plant cells and are found to have no role in them.

2. Centrosomes

They are specialized cell structures in animal cells participating in cell division. Centrosomes help separate the replicated chromosomes into the daughter cells equally.

The collaborative functions of all these organelles within cells allow plants and animals to carry out essential functions, survive, and adapt to their environments.

Cell Organelles Chart

Here is a chart with the list of organelles found in a eukaryotic plant or animal cell for reference:

Ans . The nucleus and mitochondria contain their DNA and thus are called semi-autonomous organelles.

Ans . Mitochondria and chloroplast are involved in energy conversions in a cell.

Ans . No, viruses are acellular structures and thus have no cell organelles.

Organelle – Genome.gov
Cellular Organelles and Structure – Khanacademy.org
Organelles – Education.nationalgeographic.org
What Is an Organelle? – Thoughtco.com
What Are Organelles? – News-medical.net
Cell Structure- Training.seer.cancer.gov

Article was last reviewed on Thursday, October 12, 2023

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Eukaryotic Cells

What Defines an Organelle?

In addition to the nucleus, eukaryotic cells may contain several other types of organelles , which may include mitochondria , chloroplasts, the endoplasmic reticulum, the Golgi apparatus, and lysosomes. Each of these organelles performs a specific function critical to the cell's survival. Moreover, nearly all eukaryotic organelles are separated from the rest of the cellular space by a membrane, in much the same way that interior walls separate the rooms in a house. The membranes that surround eukaryotic organelles are based on lipid bilayers that are similar (but not identical) to the cell's outer membrane. Together, the total area of a cell's internal membranes far exceeds that of its plasma membrane.

Like the plasma membrane, organelle membranes function to keep the inside "in" and the outside "out." This partitioning permits different kinds of biochemical reactions to take place in different organelles. Although each organelle performs a specific function in the cell, all of the cell's organelles work together in an integrated fashion to meet the overall needs of the cell. For example, biochemical reactions in a cell's mitochondria transfer energy from fatty acids and pyruvate molecules into an energy-rich molecule called adenosine triphosphate ( ATP ). Subsequently, the rest of the cell's organelles use this ATP as the source of the energy they need to operate.

Because most organelles are surrounded by membranes, they are easy to visualize — with magnification. For instance, researchers can use high resolution electron microscopy to take a snapshot through a thin cross-section or slice of a cell. In this way, they can see the structural detail and key characteristics of different organelles — such as the long, thin compartments of the endoplasmic reticulum or the compacted chromatin within the nucleus. An electron micrograph therefore provides an excellent blueprint of a cell's inner structures. Other less powerful microscopy techniques coupled with organelle-specific stains have helped researchers see organelle structure more clearly, as well as the distribution of various organelles within cells. However, unlike the rooms in a house, a cell's organelles are not static. Rather, these structures are in constant motion, sometimes moving to a particular place within the cell, sometimes merging with other organelles, and sometimes growing larger or smaller. These dynamic changes in cellular structures can be observed with video microscopic techniques, which provide lower-resolution movies of whole organelles as these structures move within cells.

Why Is the Nucleus So Important?

Of all eukaryotic organelles, the nucleus is perhaps the most critical. In fact, the mere presence of a nucleus is considered one of the defining features of a eukaryotic cell. This structure is so important because it is the site at which the cell's DNA is housed and the process of interpreting it begins.

Recall that DNA contains the information required to build cellular proteins. In eukaryotic cells, the membrane that surrounds the nucleus — commonly called the nuclear envelope — partitions this DNA from the cell's protein synthesis machinery, which is located in the cytoplasm. Tiny pores in the nuclear envelope, called nuclear pores, then selectively permit certain macromolecules to enter and leave the nucleus — including the RNA molecules that carry information from a cellular DNA to protein manufacturing centers in the cytoplasm. This separation of the DNA from the protein synthesis machinery provides eukaryotic cells with more intricate regulatory control over the production of proteins and their RNA intermediates.

In contrast, the DNA of prokaryotic cells is distributed loosely around the cytoplasm, along with the protein synthesis machinery. This closeness allows prokaryotic cells to rapidly respond to environmental change by quickly altering the types and amount of proteins they manufacture. Note that eukaryotic cells likely evolved from a symbiotic relationship between two prokaryotic cells, whereby one set of prokaryotic DNA eventually became separated by a nuclear envelope and formed a nucleus. Over time, portions of the DNA from the other prokaryote remaining in the cytoplasmic part of the cell may or may not have been incoporated into the new eukaryotic nucleus (Figure 3).

Why Are Mitochondria and Chloroplasts Special?

Besides the nucleus, two other organelles — the mitochondrion and the chloroplast — play an especially important role in eukaryotic cells. These specialized structures are enclosed by double membranes, and they are believed to have originated back when all living things on Earth were single-celled organisms. At that time, some larger eukaryotic cells with flexible membranes "ate" by engulfing molecules and smaller cells — and scientists believe that mitochondria and chloroplasts arose as a result of this process. In particular, researchers think that some of these "eater" eukaryotes engulfed smaller prokaryotes, and a symbiotic relationship subsequently developed. Once kidnapped, the "eaten" prokaryotes continued to generate energy and carry out other necessary cellular functions, and the host eukaryotes came to rely on the contribution of the "eaten" cells. Over many generations, the descendants of the eukaryotes developed mechanisms to further support this system, and concurrently, the descendants of the engulfed prokaryotes lost the ability to survive on their own, evolving into present-day mitochondria and chloroplasts. This proposed origin of mitochondria and chloroplasts is known as the endosymbiotic hypothesis .

In addition to double membranes, mitochondria and chloroplasts also retain small genomes with some resemblance to those found in modern prokaryotes. This finding provides yet additional evidence that these organelles probably originated as self-sufficient single-celled organisms.

Today, mitochondria are found in fungi, plants, and animals, and they use oxygen to produce energy in the form of ATP molecules, which cells then employ to drive many processes. Scientists believe that mitochondria evolved from aerobic , or oxygen-consuming, prokaryotes. In comparison, chloroplasts are found in plant cells and some algae, and they convert solar energy into energy-storing sugars such as glucose. Chloroplasts also produce oxygen, which makes them necessary for all life as we know it. Scientists think chloroplasts evolved from photosynthetic prokaryotes similar to modern-day cyanobacteria (Figure 4). Today, we classify prokaryotes and eukaryotes based on differences in their cellular contents (Figure 5).

How Do Eukaryotic Cells Handle Energy?

Mitochondria — often called the powerhouses of the cell — enable eukaryotes to make more efficient use of food sources than their prokaryotic counterparts. That's because these organelles greatly expand the amount of membrane used for energy-generating electron transport chains. In addition, mitochondria use a process called oxidative metabolism to convert food into energy, and oxidative metabolism yields more energy per food molecule than non-oxygen-using, or anaerobic , methods. Energywise, cells with mitochondria can therefore afford to be bigger than cells without mitochondria.

Within eukaryotic cells, mitochondria function somewhat like batteries, because they convert energy from one form to another: food nutrients to ATP. Accordingly, cells with high metabolic needs can meet their higher energy demands by increasing the number of mitochondria they contain. For example, muscle cells in people who exercise regularly possess more mitochondria than muscle cells in sedentary people.

Prokaryotes, on the other hand, don't have mitochondria for energy production, so they must rely on their immediate environment to obtain usable energy. Prokaryotes generally use electron transport chains in their plasma membranes to provide much of their energy. The actual energy donors and acceptors for these electron transport chains are quite variable, reflecting the diverse range of habitats where prokaryotes live. (In aerobic prokaryotes, electrons are transferred to oxygen, much as in the mitochondria.) The challenges associated with energy generation limit the size of prokaryotes. As these cells grow larger in volume, their energy needs increase proportionally. However, as they increase in size, their surface area — and thus their ability to both take in nutrients and transport electrons — does not increase to the same degree as their volume. As a result, prokaryotic cells tend to be small so that they can effectively manage the balancing act between energy supply and demand (Figure 6).

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The Cellular Level of Organization

The Cytoplasm and Cellular Organelles

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Learning Objectives

By the end of this section, you will be able to:

Describe the structure and function of the cellular organelles associated with the endomembrane system, including the endoplasmic reticulum, Golgi apparatus, and lysosomes
Describe the structure and function of mitochondria and peroxisomes
Explain the three components of the cytoskeleton, including their composition and functions

Now that you have learned that the cell membrane surrounds all cells, you can dive inside of a prototypical human cell to learn about its internal components and their functions. All living cells in multicellular organisms contain an internal cytoplasmic compartment, and a nucleus within the cytoplasm. Cytosol , the jelly-like substance within the cell, provides the fluid medium necessary for biochemical reactions. Eukaryotic cells, including all animal cells, also contain various cellular organelles. An organelle (“little organ”) is one of several different types of membrane-enclosed bodies in the cell, each performing a unique function. Just as the various bodily organs work together in harmony to perform all of a human’s functions, the many different cellular organelles work together to keep the cell healthy and performing all of its important functions. The organelles and cytosol, taken together, compose the cell’s cytoplasm . The nucleus is a cell’s central organelle, which contains the cell’s DNA ( [link] ).

This diagram shows an animal cell with all the intracellular organelles labeled.

Organelles of the Endomembrane System

A set of three major organelles together form a system within the cell called the endomembrane system. These organelles work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular products. The organelles of the endomembrane system include the endoplasmic reticulum, Golgi apparatus, and vesicles.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a system of channels that is continuous with the nuclear membrane (or “envelope”) covering the nucleus and composed of the same lipid bilayer material. The ER can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice. The ER provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface area that supports its many functions ( [link] ).

Endoplasmic reticulum can exist in two forms: rough ER and smooth ER. These two types of ER perform some very different functions and can be found in very different amounts depending on the type of cell. Rough ER (RER) is so-called because its membrane is dotted with embedded granules—organelles called ribosomes, giving the RER a bumpy appearance. A ribosome is an organelle that serves as the site of protein synthesis. It is composed of two ribosomal RNA subunits that wrap around mRNA to start the process of translation, followed by protein synthesis. Smooth ER (SER) lacks these ribosomes.

One of the main functions of the smooth ER is in the synthesis of lipids. The smooth ER synthesizes phospholipids, the main component of biological membranes, as well as steroid hormones. For this reason, cells that produce large quantities of such hormones, such as those of the female ovaries and male testes, contain large amounts of smooth ER. In addition to lipid synthesis, the smooth ER also sequesters (i.e., stores) and regulates the concentration of cellular Ca ++ , a function extremely important in cells of the nervous system where Ca ++ is the trigger for neurotransmitter release. The smooth ER additionally metabolizes some carbohydrates and performs a detoxification role, breaking down certain toxins.

In contrast with the smooth ER, the primary job of the rough ER is the synthesis and modification of proteins destined for the cell membrane or for export from the cell. For this protein synthesis, many ribosomes attach to the ER (giving it the studded appearance of rough ER). Typically, a protein is synthesized within the ribosome and released inside the channel of the rough ER, where sugars can be added to it (by a process called glycosylation) before it is transported within a vesicle to the next stage in the packaging and shipping process: the Golgi apparatus.

The Golgi Apparatus

The Golgi apparatus is responsible for sorting, modifying, and shipping off the products that come from the rough ER, much like a post-office. The Golgi apparatus looks like stacked flattened discs, almost like stacks of oddly shaped pancakes. Like the ER, these discs are membranous. The Golgi apparatus has two distinct sides, each with a different role. One side of the apparatus receives products in vesicles. These products are sorted through the apparatus, and then they are released from the opposite side after being repackaged into new vesicles. If the product is to be exported from the cell, the vesicle migrates to the cell surface and fuses to the cell membrane, and the cargo is secreted ( [link] ).

Some of the protein products packaged by the Golgi include digestive enzymes that are meant to remain inside the cell for use in breaking down certain materials. The enzyme-containing vesicles released by the Golgi may form new lysosomes, or fuse with existing, lysosomes. A lysosome is an organelle that contains enzymes that break down and digest unneeded cellular components, such as a damaged organelle. (A lysosome is similar to a wrecking crew that takes down old and unsound buildings in a neighborhood.) Autophagy (“self-eating”) is the process of a cell digesting its own structures. Lysosomes are also important for breaking down foreign material. For example, when certain immune defense cells (white blood cells) phagocytize bacteria, the bacterial cell is transported into a lysosome and digested by the enzymes inside. As one might imagine, such phagocytic defense cells contain large numbers of lysosomes.

Under certain circumstances, lysosomes perform a more grand and dire function. In the case of damaged or unhealthy cells, lysosomes can be triggered to open up and release their digestive enzymes into the cytoplasm of the cell, killing the cell. This “self-destruct” mechanism is called autolysis , and makes the process of cell death controlled (a mechanism called “apoptosis”).

Watch this video to learn about the endomembrane system, which includes the rough and smooth ER and the Golgi body as well as lysosomes and vesicles. What is the primary role of the endomembrane system?

Organelles for Energy Production and Detoxification

In addition to the jobs performed by the endomembrane system, the cell has many other important functions. Just as you must consume nutrients to provide yourself with energy, so must each of your cells take in nutrients, some of which convert to chemical energy that can be used to power biochemical reactions. Another important function of the cell is detoxification. Humans take in all sorts of toxins from the environment and also produce harmful chemicals as byproducts of cellular processes. Cells called hepatocytes in the liver detoxify many of these toxins.

Mitochondria

A mitochondrion (plural = mitochondria) is a membranous, bean-shaped organelle that is the “energy transformer” of the cell. Mitochondria consist of an outer lipid bilayer membrane as well as an additional inner lipid bilayer membrane ( [link] ). The inner membrane is highly folded into winding structures with a great deal of surface area, called cristae. It is along this inner membrane that a series of proteins, enzymes, and other molecules perform the biochemical reactions of cellular respiration. These reactions convert energy stored in nutrient molecules (such as glucose) into adenosine triphosphate (ATP), which provides usable cellular energy to the cell. Cells use ATP constantly, and so the mitochondria are constantly at work. Oxygen molecules are required during cellular respiration, which is why you must constantly breathe it in. One of the organ systems in the body that uses huge amounts of ATP is the muscular system because ATP is required to sustain muscle contraction. As a result, muscle cells are packed full of mitochondria. Nerve cells also need large quantities of ATP to run their sodium-potassium pumps. Therefore, an individual neuron will be loaded with over a thousand mitochondria. On the other hand, a bone cell, which is not nearly as metabolically-active, might only have a couple hundred mitochondria.

This figure shows the structure of a mitochondrion. The inner and outer membrane, the cristae and the intermembrane space are labeled. The right panel shows a micrograph with the structure of a mitochondrion in detail.

Peroxisomes

Like lysosomes, a peroxisome is a membrane-bound cellular organelle that contains mostly enzymes ( [link] ). Peroxisomes perform a couple of different functions, including lipid metabolism and chemical detoxification. In contrast to the digestive enzymes found in lysosomes, the enzymes within peroxisomes serve to transfer hydrogen atoms from various molecules to oxygen, producing hydrogen peroxide (H 2 O 2 ). In this way, peroxisomes neutralize poisons such as alcohol. In order to appreciate the importance of peroxisomes, it is necessary to understand the concept of reactive oxygen species.

This diagram shows a peroxisome, which is a vesicular structure with a lipid bilayer on the outside and a crystalline core on the inside.

Peroxisomes, on the other hand, oversee reactions that neutralize free radicals. Peroxisomes produce large amounts of the toxic H 2 O 2 in the process, but peroxisomes contain enzymes that convert H 2 O 2 into water and oxygen. These byproducts are safely released into the cytoplasm. Like miniature sewage treatment plants, peroxisomes neutralize harmful toxins so that they do not wreak havoc in the cells. The liver is the organ primarily responsible for detoxifying the blood before it travels throughout the body, and liver cells contain an exceptionally high number of peroxisomes.

Defense mechanisms such as detoxification within the peroxisome and certain cellular antioxidants serve to neutralize many of these molecules. Some vitamins and other substances, found primarily in fruits and vegetables, have antioxidant properties. Antioxidants work by being oxidized themselves, halting the destructive reaction cascades initiated by the free radicals. Sometimes though, ROS accumulate beyond the capacity of such defenses.

Oxidative stress is the term used to describe damage to cellular components caused by ROS. Due to their characteristic unpaired electrons, ROS can set off chain reactions where they remove electrons from other molecules, which then become oxidized and reactive, and do the same to other molecules, causing a chain reaction. ROS can cause permanent damage to cellular lipids, proteins, carbohydrates, and nucleic acids. Damaged DNA can lead to genetic mutations and even cancer. A mutation is a change in the nucleotide sequence in a gene within a cell’s DNA, potentially altering the protein coded by that gene. Other diseases believed to be triggered or exacerbated by ROS include Alzheimer’s disease, cardiovascular diseases, diabetes, Parkinson’s disease, arthritis, Huntington’s disease, and schizophrenia, among many others. It is noteworthy that these diseases are largely age-related. Many scientists believe that oxidative stress is a major contributor to the aging process.

Cell: The Free Radical Theory

The free radical theory on aging was originally proposed in the 1950s, and still remains under debate. Generally speaking, the free radical theory of aging suggests that accumulated cellular damage from oxidative stress contributes to the physiological and anatomical effects of aging. There are two significantly different versions of this theory: one states that the aging process itself is a result of oxidative damage, and the other states that oxidative damage causes age-related disease and disorders. The latter version of the theory is more widely accepted than the former. However, many lines of evidence suggest that oxidative damage does contribute to the aging process. Research has shown that reducing oxidative damage can result in a longer lifespan in certain organisms such as yeast, worms, and fruit flies. Conversely, increasing oxidative damage can shorten the lifespan of mice and worms. Interestingly, a manipulation called calorie-restriction (moderately restricting the caloric intake) has been shown to increase life span in some laboratory animals. It is believed that this increase is at least in part due to a reduction of oxidative stress. However, a long-term study of primates with calorie-restriction showed no increase in their lifespan. A great deal of additional research will be required to better understand the link between reactive oxygen species and aging.

The Cytoskeleton

Much like the bony skeleton structurally supports the human body, the cytoskeleton helps the cells to maintain their structural integrity. The cytoskeleton is a group of fibrous proteins that provide structural support for cells, but this is only one of the functions of the cytoskeleton. Cytoskeletal components are also critical for cell motility, cell reproduction, and transportation of substances within the cell.

The cytoskeleton forms a complex thread-like network throughout the cell consisting of three different kinds of protein-based filaments: microfilaments, intermediate filaments, and microtubules ( [link] ). The thickest of the three is the microtubule , a structural filament composed of subunits of a protein called tubulin. Microtubules maintain cell shape and structure, help resist compression of the cell, and play a role in positioning the organelles within the cell. Microtubules also make up two types of cellular appendages important for motion: cilia and flagella. Cilia are found on many cells of the body, including the epithelial cells that line the airways of the respiratory system. Cilia move rhythmically; they beat constantly, moving waste materials such as dust, mucus, and bacteria upward through the airways, away from the lungs and toward the mouth. Beating cilia on cells in the female fallopian tubes move egg cells from the ovary towards the uterus. A flagellum (plural = flagella) is an appendage larger than a cilium and specialized for cell locomotion. The only flagellated cell in humans is the sperm cell that must propel itself towards female egg cells.

This figure shows the different cytoskeletal components in an animal cell. The left panel shows the microtubules with the structure of the column formed by tubulin dimers. The middle panel shows the actin filaments and the helical structure formed by the filaments. The right panel shows the fibrous structure of the intermediate filaments with the different keratins coiled together.

A very important function of microtubules is to set the paths (somewhat like railroad tracks) along which the genetic material can be pulled (a process requiring ATP) during cell division, so that each new daughter cell receives the appropriate set of chromosomes. Two short, identical microtubule structures called centrioles are found near the nucleus of cells. A centriole can serve as the cellular origin point for microtubules extending outward as cilia or flagella or can assist with the separation of DNA during cell division. Microtubules grow out from the centrioles by adding more tubulin subunits, like adding additional links to a chain.

In contrast with microtubules, the microfilament is a thinner type of cytoskeletal filament (see [link] b ). Actin, a protein that forms chains, is the primary component of these microfilaments. Actin fibers, twisted chains of actin filaments, constitute a large component of muscle tissue and, along with the protein myosin, are responsible for muscle contraction. Like microtubules, actin filaments are long chains of single subunits (called actin subunits). In muscle cells, these long actin strands, called thin filaments, are “pulled” by thick filaments of the myosin protein to contract the cell.

Actin also has an important role during cell division. When a cell is about to split in half during cell division, actin filaments work with myosin to create a cleavage furrow that eventually splits the cell down the middle, forming two new cells from the original cell.

The final cytoskeletal filament is the intermediate filament. As its name would suggest, an intermediate filament is a filament intermediate in thickness between the microtubules and microfilaments (see [link] c ). Intermediate filaments are made up of long fibrous subunits of a protein called keratin that are wound together like the threads that compose a rope. Intermediate filaments, in concert with the microtubules, are important for maintaining cell shape and structure. Unlike the microtubules, which resist compression, intermediate filaments resist tension—the forces that pull apart cells. There are many cases in which cells are prone to tension, such as when epithelial cells of the skin are compressed, tugging them in different directions. Intermediate filaments help anchor organelles together within a cell and also link cells to other cells by forming special cell-to-cell junctions.

Chapter Review

The internal environmental of a living cell is made up of a fluid, jelly-like substance called cytosol, which consists mainly of water, but also contains various dissolved nutrients and other molecules. The cell contains an array of cellular organelles, each one performing a unique function and helping to maintain the health and activity of the cell. The cytosol and organelles together compose the cell’s cytoplasm. Most organelles are surrounded by a lipid membrane similar to the cell membrane of the cell. The endoplasmic reticulum (ER), Golgi apparatus, and lysosomes share a functional connectivity and are collectively referred to as the endomembrane system. There are two types of ER: smooth and rough. While the smooth ER performs many functions, including lipid synthesis and ion storage, the rough ER is mainly responsible for protein synthesis using its associated ribosomes. The rough ER sends newly made proteins to the Golgi apparatus where they are modified and packaged for delivery to various locations within or outside of the cell. Some of these protein products are enzymes destined to break down unwanted material and are packaged as lysosomes for use inside the cell.

Cells also contain mitochondria and peroxisomes, which are the organelles responsible for producing the cell’s energy supply and detoxifying certain chemicals, respectively. Biochemical reactions within mitochondria transform energy-carrying molecules into the usable form of cellular energy known as ATP. Peroxisomes contain enzymes that transform harmful substances such as free radicals into oxygen and water. Cells also contain a miniaturized “skeleton” of protein filaments that extend throughout its interior. Three different kinds of filaments compose this cytoskeleton (in order of increasing thickness): microfilaments, intermediate filaments, and microtubules. Each cytoskeletal component performs unique functions as well as provides a supportive framework for the cell.

Interactive Link Questions

Processing, packaging, and moving materials manufactured by the cell.

Review Questions

Choose the term that best completes the following analogy: Cytoplasm is to cytosol as a swimming pool containing chlorine and flotation toys is to ________.

the walls of the pool
the chlorine
the flotation toys

The rough ER has its name due to what associated structures?

Golgi apparatus

Which of the following is a function of the rough ER?

production of proteins
detoxification of certain substances
synthesis of steroid hormones
regulation of intracellular calcium concentration

Which of the following is a feature common to all three components of the cytoskeleton?

They all serve to scaffold the organelles within the cell.
They are all characterized by roughly the same diameter.
They are all polymers of protein subunits.
They all help the cell resist compression and tension.

Which of the following organelles produces large quantities of ATP when both glucose and oxygen are available to the cell?

mitochondria
peroxisomes

Critical Thinking Questions

Explain why the structure of the ER, mitochondria, and Golgi apparatus assist their respective functions.

The structure of the Golgi apparatus is suited to its function because it is a series of flattened membranous discs; substances are modified and packaged in sequential steps as they travel from one disc to the next. The structure of Golgi apparatus also involves a receiving face and a sending face, which organize cellular products as they enter and leave the Golgi apparatus. The ER and the mitochondria both have structural specializations that increase their surface area. In the mitochondria, the inner membrane is extensively folded, which increases surface area for ATP production. Likewise, the ER is elaborately wound throughout the cell, increasing its surface area for functions like lipid synthesis, Ca ++ storage, and protein synthesis.

Compare and contrast lysosomes with peroxisomes: name at least two similarities and one difference.

Peroxisomes and lysosomes are both cellular organelles bound by lipid bilayer membranes, and they both contain many enzymes. However, peroxisomes contain enzymes that detoxify substances by transferring hydrogen atoms and producing H 2 O 2 , whereas the enzymes in lysosomes function to break down and digest various unwanted materials.

Kolata, G. Severe diet doesn’t prolong life, at least in monkeys. New York Times [Internet]. 2012 Aug. 29 [cited 2013 Jan 21]; Available from:

http://www.nytimes.com/2012/08/30/science/low-calorie-diet-doesnt-prolong-life-study-of-monkeys-finds.html?_r=2&ref=caloricrestriction&

Chapter 6. Cell Structure and Function

Ayda Basgul Martin

Unit Outline

Part 1. Characteristics of Life

Organization

Exchange of material, responsiveness, development, growth, and reproduction.

Part 2. Structural Organization of the Body

The Levels of Organization

The cellular level of organization.

Part 3. Cell Structure, Cellular Organelles, and Functions

General Cell Structure: Plasma Membrane, Cytoplasm and Nucleus

Organelles of the Endomembrane System

Organelles for Energy Pocessing

Part 4 . Cellular Processes Involved in Growth

Cell Division, Growth, and Differentiation

Cell specialization, learning objectives.

At the end of this unit, you should be able to:

I. Specify the characteristics associated with life and explain why the cell is the basic unit of life.

II. Describe the levels of structural organization in the body.

III. Describe the structure and the functions of the major components of a cell.

IV. Define metabolism and distinguish between anabolism and catabolism.

V. Describe the cellular processes involved in the growth of the human body from a fertilized egg to an adult.

VI. Describe the importance of cell differentiation to an organism.

VII. Describe the general characteristics of each of the following cell types and relate their characteristics to their functions: nerve cell, muscle cell, red blood cell (erythrocyte), and white blood cell (leukocyte).

Part 1: Characteristics of Life

The different organ systems each have different functions and therefore unique roles to perform in the body. These many functions can be summarized in terms of a few that we might consider definitive of human life: organization, metabolism , exchange of materials, responsiveness, movement, development, growth, and reproduction.

A human body consists of trillions of cells organized in a way that maintains distinct internal compartments. These compartments keep body cells separated from external environmental threats and keep the cells moist and nourished. They also separate internal body fluids from the countless microorganisms that grow on body surfaces, including the lining of certain tracts, or passageways. The intestinal tract, for example, is home to even more bacteria cells than the total of all human cells in the body, yet these bacteria are outside the body and cannot circulate freely inside the body.

Cells, for example, have a membrane (also referred to as the plasma membrane) that keeps the intracellular environment—the fluids and organelles —separate from the environment outside the cell (the extracellular environment). Blood vessels keep blood inside a closed system, and nerves and muscles are wrapped in tissue sheaths that separate them from surrounding structures. In the chest and abdomen, a variety of internal membranes keep major organs such as the lungs, heart, and kidneys protected and separate from others.

The body’s largest organ system is the integumentary system , which includes the skin and its associated structures, such as hair and nails. The surface tissue of the skin is a barrier that protects internal structures and fluids from potentially harmful microorganisms, toxins, and the external environment.

The first law of thermodynamics holds that energy can neither be created nor destroyed—it can only change form. Your basic function as an organism is to consume (ingest) energy and molecules in the foods you eat, convert some of it into fuel for movement, sustain your body functions, and build and maintain your body structures. There are two types of reactions that accomplish this: anabolism and catabolism .

Anabolism is the process whereby smaller, simpler molecules are combined into larger, more complex substances. For example, amino acids can be combined together to make proteins. Your body can assemble, by utilizing energy, the complex chemicals it needs by combining small molecules derived from the foods you eat.
Catabolism is the process by which larger, more complex substances are broken down into smaller, simpler molecules. For example, sugars are broken down into carbon dioxide and water. Catabolism releases energy. The complex molecules found in foods are broken down so the body can use their parts to assemble the structures and substances needed for life.

Taken together, these two processes are called metabolism. Metabolism is the sum of all anabolic and catabolic reactions that take place in the body. Both anabolism and catabolism occur simultaneously and continuously to keep you alive.

Every cell in your body makes use of a chemical compound, adenosine triphosphate (ATP) , to store and release energy. The cell stores energy in the molecule of ATP and then moves the ATP molecules to the location where energy is needed to fuel cellular activities. Then the ATP is broken down, and a controlled amount of energy is released, which is used by the cell to perform a particular job.

Organisms do not exist solely within their own boundaries but interact with the external environment that surrounds them. One of the ways in which they do this is by exchanging materials with their external environment: taking in materials from their external environment and expelling waste products out into their external environment. These materials and waste products may be anything from very small, relatively simple molecules (e.g., glucose, carbon dioxide) that must cross an individual cell’s plasma membrane to whole cells or foods that were ingested but not fully digested and/or absorbed and so must be excreted from the organism.

Responsiveness is the ability of an organism to adjust to changes in its internal and external environments. An example of responsiveness to external stimuli could include moving toward sources of food and water and away from perceived dangers. Changes in an organism’s internal environment, such as increased body temperature, can cause the responses of sweating and the dilation of blood vessels in the skin in order to decrease body temperature.

Human movement includes not only actions at the joints of the body but also the motion of individual organs and even individual cells. As you read these words, red and white blood cells are moving throughout your body, muscle cells are contracting and relaxing to maintain your posture and to focus your vision, and glands are secreting chemicals to regulate body functions. Your body is coordinating the action of entire muscle groups to enable you to move air into and out of your lungs, to push blood throughout your body, and to propel the food you have eaten through your digestive tract. Consciously, of course, you contract your skeletal muscles to move the bones of your skeleton to get from one place to another and to carry out all of the activities of your daily life.

Development is all of the changes the body goes through in life. The development includes the process of cell differentiation , in which unspecialized cells become specialized in structure and function to perform certain tasks in the body. The development also includes the processes of growth and repair, both of which involve cell differentiation.
Growth is the increase in body size. Humans, like all multicellular organisms, grow by increasing the number of existing cells, increasing the amount of non-cellular material around cells (such as mineral deposits in bone), and within very narrow limits, increasing the size of existing cells.
Reproduction is the formation of a new organism from parent organisms. In humans, reproduction is carried out by the male and female reproductive systems. Because death will come to all complex organisms, without reproduction, the line of organisms would end.

Test Your Knowledge- Part 1: Characteristics of Life

List, explain, and provide examples of each of the characteristics of life.
In reference to your answer to question #1, above, explain in one sentence why the cell is the basic unit of life.

Part 2: Structural Organization of the Human Body

Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms , molecules , organelles , cells, tissues, organs, organ systems, and organisms (Figure 1).

To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, atoms, and molecules. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles, such as the proton, electron, and neutron. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.

A cell is the smallest independently functioning unit of a living organism. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells. Even bacteria, which are extremely small single-celled, independently-living organisms, have a cellular structure.

A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based fluid, together with a variety of tiny functioning units called organelles. In humans, as in all organisms, cells perform all functions of life. A tissue is a group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function. An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet the physiological needs of the body. Assigning organs to organ systems can be imprecise, since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system.

The organism level is the highest level of organization. An organism is a living being that has a cellular structure and that can independently perform all physiologic functions necessary for life. In multicellular organisms, including humans, all cells, tissues, organs, and organ systems of the body work together to maintain the life and health of the organism.

You developed from a single fertilized egg cell into a complex organism containing trillions of cells that you see when you look in a mirror. Early during this developmental process, cells differentiate and become specialized in their structure and function. These different cell types form specialized tissues that work in concert to perform all of the functions necessary for the living organism. Cellular and developmental biologists study how the continued division of a single cell leads to such complexity.

Consider the difference between a cell in the skin and a nerve cell. A skin cell may be shaped like a flat plate (squamous) and live only for a short time before it is shed and replaced. Packed tightly into rows and sheets, the squamous skin cells provide a protective barrier for the cells and tissues that lie beneath. A nerve cell, on the other hand, may be shaped something like a star, sending out long processes up to a meter in length, and may live for the entire lifetime of the organism. With their long winding processes, nerve cells can communicate with one another and with other types of body cells and send rapid signals that inform the organism about its environment and allow it to interact with that environment. These differences illustrate one very important theme that is consistent at all organizational levels of biology: the form of a structure is optimally suited to perform particular functions assigned to that structure. Keep this theme in mind as you tour the inside of a cell and are introduced to the various types of cells in the body.

The concept of a cell started with microscopic observations of dead cork tissue by scientist Robert Hooke in 1665. Without realizing their function or importance, Hook coined the term “cell” based on the resemblance of the small subdivisions in the cork to the rooms that monks inhabited, called cells. About ten years later, Antonie van Leeuwenhoek became the first person to observe living and moving cells under a microscope. In the century that followed, the theory that cells represented the basic unit of life would develop. These tiny fluid-filled sacs house components responsible for the thousands of biochemical reactions necessary for an organism to grow and survive. In this chapter, you will learn about the major components and functions of a generalized cell and discover some of the different types of cells in the human body.

Test Your Knowledge- Part 2: Structural Organization of the Body

Describe the levels of structural organization in the body.

Chemical level
Cellular level
Tissue level
Organ level
Organ system level
Organismal level
Write a clear description of the relationships between the chemical, cellular, tissue, organ, organ system, and organismal levels of organization in the body.

Part 3: Cell Structure, Cellular Organelles, and Functions

General cell structure: plasma membrane, cytoplasm, and nucleus.

The cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its external environment. This membrane provides a protective barrier around the cell and regulates which materials can pass in or out. It is primarily composed of phospholipids arranged in two layers but also contains cholesterol and a mosaic of different proteins. You will learn more about the structure and function of the plasma membrane in Unit 5. All living cells in multicellular organisms contain an internal cytoplasmic compartment, composed of cytosol and organelles. Cytosol , the jelly-like substance within the cell, provides the fluid medium necessary for biochemical reactions and is mostly composed of water. Eukaryotic cells, including all animal cells, also contain various cellular organelles. An organelle (“little organ”) is one of several different types of membrane-enclosed bodies in the cell, each performing a unique function.

Just as the various bodily organs work together in harmony to perform all of a human’s functions, the many different cellular organelles work together to keep the cell healthy and perform all of its important functions. The organelles and cytosol, taken together, compose the cell’s cytoplasm. The nucleus is a cell’s central organelle, which contains the cell’s DNA (Figure 2).

Most membranous organelles found in a human cell together form a system within the cell called the endomembrane system . These organelles work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular products. The components of the endomembrane system include the nuclear envelope, endoplasmic reticulum , Golgi apparatus , vesicles, and plasma membrane.

Endoplasmic Reticulum : The endoplasmic reticulum (ER) is a system of channels that is continuous with the nuclear membrane (or “envelope”) covering the nucleus (see Part 7) and composed of the same lipid bilayer material. The ER can be thought of as a series of winding thoroughfares similar to the waterway canals in Venice. The ER provides passages throughout much of the cell that function in transporting, synthesizing, and storing materials. The winding structure of the ER results in a large membranous surface area that supports its many functions (Figure 3).

Endoplasmic reticulum can exist in two forms: rough ER and smooth ER. These two types of ER perform some very different functions and can be found in different amounts depending on the type of cell. Rough ER (RER) is so called because its membrane is dotted with embedded granules—organelles called ribosomes, giving the RER a bumpy appearance. A ribosome is an organelle that serves as the site of protein synthesis, and it is composed of two subunits. Ribosomes can either be bound (attached to ER) or free (floating in the cytosol). Smooth ER (SER) lacks ribosomes.

One of the main functions of the smooth ER is in the synthesis of lipids . The smooth ER synthesizes phospholipids , the main component of biological membranes, as well as steroid hormones .

For this reason, cells that produce large quantities of such hormones, such as those of the female ovaries and male testes, contain large amounts of smooth ER. In addition to lipid synthesis, the smooth ER also sequesters (i.e., stores) and regulates the concentration of cellular calcium (Ca 2+ ), which is extremely important in cells of the nervous system where Ca 2+ is the trigger for neurotransmitter release. Additionally, the smooth ER, especially in the liver, performs a detoxification role, breaking down certain toxins.

In contrast with the smooth ER, the primary job of the rough ER is the synthesis and modification of proteins destined for the cell membrane or for export from the cell. For this protein synthesis, many ribosomes attach to the ER (giving it the studded appearance of rough ER). Typically, a protein is synthesized within the ribosome and released inside the channel of the rough ER, where sugars can be added to it (by a process called glycosylation) before it is transported within a vesicle (a small fluid-filled sac) to the next stage in the packaging and shipping process: the Golgi apparatus .

The Golgi Apparatus: The Golgi apparatus is responsible for sorting, modifying, and shipping off the products that come from the rough ER, much like a post office. The Golgi apparatus looks like stacked flattened discs, almost like stacks of oddly shaped pancakes. Like the ER, these discs are membranous. The Golgi apparatus has two distinct sides, each with a different role. One side (the cis face) of the apparatus receives products in vesicles . These products are sorted through the apparatus, and then they are released from the opposite side (the trans face) after being repackaged into new vesicles. If the product is to be exported from the cell, the vesicle migrates to the cell surface and fuses to the cell membrane, and the cargo is secreted (Figure 4).

Lysosomes : Some of the protein products from the Golgi include digestive enzymes that are meant to remain inside the cell for use in breaking down certain materials. These enzymes are packaged into vesicles called lysosomes. A lysosome is an organelle that contains enzymes that break down and digest unneeded cellular components, such as a damaged organelle in a process called autophagy (“self-eating”).

Lysosomes are also important for breaking down foreign material. For example, when certain immune defense cells, like white blood cells, phagocytize (engulf) bacteria, the bacterial cell is transported to a lysosome and digested by the enzymes inside. Under certain circumstances, lysosomes perform a more grand and dire function. In the case of damaged or unhealthy cells, lysosomes can be triggered to open up and release their digestive enzymes into the cytoplasm of the cell, killing the cell. This “self-destruct” mechanism is called autolysis and makes the process of cell death controlled (a mechanism called “ apoptosis ”).

Organelles for Energy Processing

Mitochondrion : A mitochondrion (plural = mitochondria) is a membranous, bean-shaped organelle that is the “energy transformer” of the cell. Mitochondria consist of an outer lipid bilayer membrane as well as an additional inner lipid bilayer membrane (Figure 5). The inner membrane is highly folded into winding structures with a great deal of surface area, called cristae. It is along this inner membrane that a series of proteins , enzymes , and other molecules perform the biochemical reactions of cellular respiration .

These reactions harvest the energy stored in nutrient molecules (such as glucose) to power the synthesis of ATP , which provides usable energy to the cell. Cells use ATP constantly, so the mitochondria are constantly at work. Oxygen molecules are required during cellular respiration, which is why you must constantly breathe them in. One of the organ systems in the body that uses huge amounts of ATP is the muscular system because ATP is required to sustain muscle contraction. As a result, muscle cells are packed full of mitochondria.

Nerve cells also need large quantities of ATP to run their sodium-potassium pumps, which are used to generate an action potential. Therefore, an individual neuron will be loaded with over a thousand mitochondria. On the other hand, a bone cell, which is not nearly as metabolically active, might only have a couple of hundred mitochondria.

The Nucleus : The nucleus is the largest and most prominent of a cell’s organelles (Figure 6). The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions for manufacturing proteins. Interestingly, some cells in the body, such as muscle cells, contain more than one nucleus (Figure 7), which is known as multinucleated. Other cells, such as mammalian red blood cells (RBCs), do not contain nuclei at all. RBCs eject their nuclei as they mature, making space for the large numbers of hemoglobin molecules that carry oxygen throughout the body.

Inside the nucleus lies the blueprint that dictates everything a cell will do and all of the products it will make. This information is stored within DNA. The nucleus sends “commands” to the cell via molecular messengers that translate the information from DNA. Each cell in your body (with the exception of the cells that produce eggs and sperm) contains the complete set of your DNA. When a cell divides, the DNA must be duplicated so that each new cell receives a full complement of DNA.

Organization of the Nucleus and Its DNA : Like most other cellular organelles, the nucleus is surrounded by a membrane called the nuclear envelope . This membranous covering consists of two adjacent lipid bilayers with a thin fluid space in between them. Spanning these two bilayers are nuclear pores. A nuclear pore is a tiny passageway for the passage of proteins, RNA , and solutes between the nucleus and the cytoplasm .

Inside the nuclear envelope is a gel-like nucleoplasm with solutes that include the building blocks of nucleic acids. There also can be a dark-staining mass often visible under a simple light microscope, called a nucleolus (plural = nucleoli). The nucleolus is a region of the nucleus that is responsible for manufacturing the RNAs necessary for the construction of ribosomes . Once synthesized, newly made ribosomal subunits exit the cell’s nucleus through the nuclear pores.

The genetic instructions that are used to build and maintain an organism are arranged in an orderly manner in strands of DNA. Within the nucleus are threads of chromatin composed of DNA and associated proteins (Figure 8). Along the chromatin threads, the DNA is wrapped around a set of histone proteins. When a cell is in the process of division, the chromatin condenses into chromosomes so that the DNA can be safely transported to the “daughter cells.” The chromosome is composed of DNA and proteins; it is the condensed form of chromatin. It is estimated that humans have almost 22,000 genes distributed on 46 chromosomes.

Test Your Knowledge- Part 3: Cell Structure, Cellular Organelles, and Functions

Describe the structure and the functions of the major components of a cell.

The cell membrane (plasma membrane)
Endoplasmic reticulum
Golgi apparatus (Golgi complex)
Mitochondria
Nuclear envelope
Chromosomes
Plasma membrane
Smooth endoplasmic reticulum
Rough endoplasmic reticulum
Bound ribosomes
Free ribosomes
Golgi apparatus (or Golgi complex)
Describe the structure (name all the components and describe their relationships to each other) and list the general functions of the “endomembrane system.”

Part 4: Cellular Processes Involved in Growth.

Cell Division: cells in the body must replace themselves over the lifetime of a person. For example, the cells lining the gastrointestinal tract must be frequently replaced when constantly “worn off” by the movement of food through the gut. But what triggers a cell to divide, and how does it prepare for and complete cell division? The cell cycle is the sequence of events in the life of the cell from the moment it is created at the end of a previous cycle of cell division until it then divides itself, generating two new cells.

While there are a few cells in the body that do not undergo cell division (such as gametes , red blood cells, most neurons , and some muscle cells), most somatic cells divide regularly. A somatic cell is a general term for a body cell, and all human cells, except for the cells that produce eggs and sperm (which are referred to as germ cells ), are somatic cells.

Cell Growth: Once cells divide, they grow and increase in size. For example, nerve cells first appear as relatively small cells, but then they elongate to become extremely long cells. Similarly, muscle cells grow to become extremely long cells as muscles are formed.

Cell Differentiation : How does a complex organism such as a human develop from a single cell—a fertilized egg—into the vast array of cell types such as nerve cells, muscle cells, and epithelial cells that characterize the adult? Throughout development and adulthood, the process of cellular differentiation leads cells to assume their final morphology and physiology. Differentiation is the process by which unspecialized cells become specialized to carry out distinct functions.

A stem cell is an unspecialized cell that can divide without limit as needed and can, under specific conditions, differentiate into specialized cells. Stem cells are unique in that they can also continually divide and regenerate new stem cells instead of further specializing. There are different stem cells present at different stages of a human’s life. They include the embryonic stem cells of the embryo , fetal stem cells of the fetus , and adult stem cells in the adult. One type of adult stem cell is the epithelial stem cell, which gives rise to the keratinocytes in the multiple layers of epithelial cells in the epidermis of skin.

When a cell differentiates, it becomes specialized; yet if all cells in the body, beginning with the fertilized egg, contain the same DNA, how do the different cell types come to be so different? The answer is analogous to a movie script. The different actors in a movie all read from the same script; however, they are each only reading their own part of the script. Similarly, all cells contain the same full complement of DNA, but each type of cell only “reads” the portions of DNA that are relevant to its own function. In biology, this is referred to as the unique genetic expression of each cell.

As cells specialize, they may undertake major changes in size, shape, metabolic activity, and overall function. The morphology (structure) of a mature cell is closely related to the function it is specialized to serve (Figure 9). Muscle fibers, for example, are far removed in structure and function from the zygote that they ultimately arose from: they are long, slender structures that are well-suited to contracting to produce macroscopic movements over relatively long distances. Some neurons (nerve cells) are exceptionally long and slender in shape, again to act over relatively long distances, although in this case, their function is to transmit information rather than move body structures directly. Erythrocytes (red blood cells) are used to transport oxygen in the blood; their tiny size and lack of a nucleus make them well-suited to squeezing through the smallest of capillaries, and their lack of mitochondria mean they do not themselves use up the oxygen they are supposed to be delivering to other cells. Leukocytes (white blood cells) on the other hand are noticeably larger than erythrocytes and do have mitochondria . The large size of macrophages, for example, means they are capable of physically engulfing relatively large particles or whole cells, such as bacteria by phagocytosis , and their mitochondria allow them access to the chemical energy required to move through body tissues toward invading pathogens.

Test Your Knowledge- Part 4: Cellular Processes Involved in Growth.

I. Define metabolism and distinguish between anabolism and catabolism.

Define the term “metabolism.”
Write a single sentence that clearly differentiates between “anabolism” and “catabolism.”

II. Describe the cellular processes involved in the growth of the human body from a fertilized egg to an adult.

Distinguish between cell division, cell growth, and cell differentiation.
Provide two examples of cell types in the human body that do not undergo cell division.
Define the term “stem cell.”

For the image below, drag and drop the correct structures to the animal cell.

Image Descriptions

Figure 6.1. Levels of Structural Organization of the Human Body. The organization of the body often is discussed in terms of six distinct levels of increasing complexity, from the smallest chemical building blocks to a unique human organism. The tip of the pyramid represents the atoms and examples of atoms, the oxygen and hydrogen atoms are given, in the bracket, below shows water molecule to refer to atoms bonding to form molecules with dimensional structures. Next, complexity is shared at the cellular level with the example of a smooth muscle cell represented as an example of a variety of molecules combining to form the fluid and organelles of a body cell. The fourth complexity contains the illustration of smooth muscle tissue to refer to the tissue level of organization, which is defined as a community of similar cells from body tissue. One below in the pyramid organ level is presented to refer to two or more different tissues combined to form an organ, and a urinary bladder illustration is given as an example. The fifth level contains the urinary tract system to refer to the organ system level, which is defined as two or more organs working closely together to perform the functions of a body system. The last and most inclusive level is the organismal level, which is formed by many organ systems working harmoniously together to perform the functions of an independent organism. [Return to image.]

Figure 6.2. Typical Human Cell. While this image is not indicative of any one human cell, it is a typical example of an animal cell containing the primary organelles and internal structures. In the illustration, the plasma membrane is labeled as a border, in the cytoplasm; mitochondria, microtubule, centrosome, microfilament, microtubule, lysosome, smooth endoplasmic reticulum (has no ribosome attachment to their membrane), secretory vesicle, peroxisome, vacuole, cytoplasm, Golgi vesicle, Golgi apparatus, rough endoplasmic reticulum (ribosomes attached on their membrane), cytoplasmic ribosomes, which freely floats within the cytosol, intermediate filaments, and nucleus are drawn and labeled. Within the nucleus, chromatin and nucleolus are also drawn and labeled. [Return to image.]

Figure 6.4. Golgi Apparatus. (a) The illustration presents RER: transport vesicle carries substances from RER to the Golgi apparatus, and secretory vesicle carries substances from the Golgi apparatus toward the plasma membrane. The Golgi apparatus manipulates products from the rough ER. Proteins and other products of the ER are sent to the Golgi apparatus, which organizes, modifies, packages, and tags them. Some of these products are transported to other areas of the cell, and some are exported from the cell through exocytosis. Enzymatic proteins are packaged as new vesicles called lysosomes. The illustration shows a transport vesicle between the RER and Golgi apparatus and a secretory vesicle between the Golgi apparatus and the plasma membrane. Transport vesicles move materials within the cell, while secretory vesicles store and release materials into the cell or to the extracellular environment. (b) An electron micrograph of the Golgi apparatus is shown with labeled trans face and cis face. The cis face lies near the transitional region of the rough endoplasmic reticulum, while the trans face lies near the cell membrane. These two networks are responsible for the essential task of sorting proteins and lipids that are received (at the cis face) or released (at the trans face) by the organelle. [Return to image.]

Figure 6.5. Mitochondrion. The mitochondria are the energy-conversion factories of the cell. (a) Illustration of a mitochondrion shows labels for cristae and intermembrane space. The outer and inner membranes and intermembrane space are labeled within the illustration and the electron micrograph. A mitochondrion is composed of two separate lipid bilayer membranes. Along the inner membrane are various molecules that work together to produce ATP, the cell’s major energy currency. The outer mitochondrial membrane fully surrounds the inner membrane, with a small intermembrane space in between. The outer membrane has many protein-based pores that are big enough to allow the passage of ions and molecules as large as a small protein. Cristae are folds in the inner mitochondrial membrane. Mitochondria are organelles in eukaryotic cells. The major function of cristae is to increase the surface area of the mitochondrial membrane. This allows membrane processes to produce more energy at a faster rate. (b) An electron micrograph of mitochondria. EM × 236,000. (Micrograph provided by the Regents of University of Michigan Medical School © 2012). [Return to image].

Figure 6.6. The Nucleus. The nucleus is the control center of the cell. The nucleus of living cells contains the genetic material that determines the entire structure and function of that cell. Illustration of a nucleus contains nucleolus, condensed chromatin, nuclear envelope, and nuclear pores. The nuclear pore is a protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and the cytoplasm. In eukaryotic cells, the nucleus is separated from the cytoplasm and surrounded by a nuclear envelope. This envelope safeguards the DNA contained in the nucleus. In spite of this barrier, there is still communication between the nucleus and the cytoplasm. This communication is regulated by the nuclear pores. Next to the nucleus, RER is illustrated, and cisternae of the RER is labeled. The RER is morphologically distinguishable by its series of convoluted, flattened-like membrane sheets (called cisternae) that arise near the nucleus and extend across the cytoplasm. [Return to image.]

Figure 6.9. Stem Cells. Stem cells have the remarkable potential to renew themselves. They can develop into many different cell types in the body during early life and growth. The capacity of stem cells to differentiate into specialized cells make them potentially valuable in therapeutic applications designed to replace damaged cells of different body tissues. There are several main categories: the “pluripotent” stem cells (embryonic stem cells and induced pluripotent stem cells) and nonembryonic or somatic stem cells (commonly called “adult” stem cells). The illustration is showing a totipotent embryonic stem cell can divide and differentiate into pluripotent embryonic stem cells such as the endoderm line, which differentiate later into multipotent stem cells to form the cells of the lung and pancreas. The pluripotent mesoderm line differentiates into multipotent stem cells to form the heart muscle and RBC. The pluripotent ectoderm line differentiates into multipotent stem cells to form the cells of the skin and neurons. [Return to image.]

Sum of all catabolic and anabolic reactions that take place in the body.

Any of several different types of membrane-enclosed specialized structures in the cell that perform specific functions for the cell.

Skin and its accessory structures.

Reactions that build smaller molecules into larger molecules.

Chemical reaction that breaks down more complex organic molecules.

Building block of proteins; characterized by an amino and carboxyl functional groups and a variable side-chain.

Nucleotide containing ribose and an adenine base that is essential in energy transfer.

Becoming wider, larger, or more open.

Process by which unspecialized cells become more specialized in structure and function.

Consisting of more than one cell (as opposed to organisms such as bacteria, which are unicellular).

The smallest unit of an element that retains the unique properties of that element.

Two or more atoms covalently bonded together.

Group of many similar cells (though sometimes composed of a few related types) that work together to perform a specific function.

An anatomically distinct structure of the body composed of two or more tissue types.

Internal material between the cell membrane and nucleus of a cell, mainly consisting of a water-based fluid called cytosol, within which are all the other organelles and cellular solute and suspended materials.

Clear, semi-fluid medium of the cytoplasm, made up mostly of water.

One of two major divisions of living things (or their cells) that have membrane-bound nuclei and other organelles and can form large complex organisms (including all animals, plants, fungi). By contrast, bacteria are prokaryotic.

Cell’s central organelle; contains the cell’s DNA.

Set of cellular organelles that often work together to produce, package, and export certain products.

Cellular organelle that consists of interconnected membrane-bound tubules, which may or may not be associated with ribosomes (rough type or smooth type, respectively).

Cellular organelle formed by a series of flattened, membrane-bound sacs that functions in protein modification, tagging, packaging, and transport.

Cellular organelle that functions in protein synthesis.

Class of organic compounds that are composed of many amino acids linked together by peptide bonds.

Class of nonpolar organic compounds built from hydrocarbons and distinguished by the fact that they are not soluble in water.

An amphipathic lipid molecule containing a phosphate head (polar) and two fatty acid tails (non-polar). The major molecule comprising plasma membranes.

(Also, sterol) lipid compound composed of four hydrocarbon rings bonded to a variety of other atoms and molecules; not to be confused with anabolic steroids, a synthetic supplement

Secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body.

Chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell.

Membrane-bound structure that contains materials within or outside of the cell.

Membrane-bound cellular organelle originating from the Golgi apparatus and containing digestive enzymes.

Cell process (a form of endocytosis) in which a cell engulfs and ingests another large particle or cell.

Programmed cell death.

One of the cellular organelles bound by a double lipid bilayer that function primarily in the production of cellular energy (ATP).

Molecule (usually a protein) that catalyzes chemical reactions.

Production of ATP from glucose oxidation via glycolysis, the Krebs cycle, and oxidative phosphorylation.

Oxygen-carrying protein in erythrocytes (red blood cells).

Deoxyribose-containing nucleic acid that stores genetic information.

Membrane that surrounds the nucleus; consisting of a double lipid-bilayer.

One of the small, protein-lined openings found scattered throughout the nuclear envelope.

Ribose-containing nucleic acid that helps manifest the genetic code as protein.

Small region of the nucleus that functions in ribosome synthesis.

Substance consisting of DNA and associated proteins.

A long DNA molecule, combined with proteins that contains a number of genes. The normal chromosome complement is 23 pairs of homologous chromosomes, one each from mother and father.

Life cycle of a single cell, from its birth until its division into two new daughter cells.

Haploid reproductive cell (egg or sperm in humans) that contributes genetic material to form an offspring.

Excitable neural cell that transfer nerve impulses.

A body cell, excluding germ cells. Normally diploid, each cell containing a complete set of genes.

Cell that gives rise to a gamete.

Type of tissue that serves primarily as a covering or lining of body parts, protecting the body; it also functions in absorption, transport, and secretion.

Cell that is oligo-, multi-, or pleuripotent that has the ability to produce additional stem cells rather than becoming further specialized.

Developing human during weeks 3–8.

Developing human during the time from the end of the embryonic period (week 9) to birth.

Cell that produces keratin and is the most predominant type of cell found in the epidermis.

outermost tissue layer of the skin

Red blood cell.

White blood cell.

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Essay on Cell

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In this essay we will discuss about the cell. After reading this essay you will learn about: 1. Definition of Cell 2. Discovery of Cell 3. Cell Theory 4. Modern Cell Theory 5. Limit of Cell Size or Volume 6. Types 7. Compartmentalization for Cellular Life 8. Cell— An Open System 9. Shapes 10. Functions.

Essay Contents:

Essay on the Functions of Cell Parts

Essay # 1. Definition of Cell:

Cell is a basic unit of life as no living organism can have life without being cellular because cell is a unit of both its structure and function. All life begins as a single cell. A number of organisms are made of single cells. They are called unicellular or acellular, e.g.Amoeba, Chlamydomonas, Acetabularia, bacteria, yeast.

Here a single cell is:

(i) Capable of independent existence and

(ii) Able to perform all the essential functions of life.

Anything less than a complete cell cans neither lead an independent existence nor perform all the functions of life. A multicellular organism is made of many cells. A higher animal or plant contains billions of cells. For example, a newly born human infant has 2 x 1012 cells.

The number increases to 100 trillion (100 x 1012 or 1014) cells in the body of 60 kg human being. About 25% (25 x 1012) of them are found in the blood. A drop of blood contains several million cells. The large sized organisms do not have large sized cells. Instead they possess higher number of cells. In multicellular organisms, cells are building blocks of the body or basic units of body structure.

Of course, they become specialized for performing different functions. Human body has some 200 types of cells, e.g., erythrocytes, leucocyte types, epithelial cell types, muscle cells, nerve cells, fat cells, cartilage cells, bone cells, connective tissue cells, gland cells, germinal cells, pigment cells, etc.

Cells are grouped into tissues, tissues into organs and organs into organ systems. Occurrence of different types of tissues, organs and organ system results in division of labour or performance of different functions of the body by specialised structures.

Cells are not only the building blocks of the organisms, they are also the functional units of life. Life passes from one generation to the next in the form of cells.

The activities of an organism are actually the sum total of activities of its cells. Each cell of the body possesses the same genetic information though mature cells may become specialized to perform specific functions. A new cell always develops by division of a pre-existing cell.

Cells are totipotent, i.e., a single cell has the ability to form the whole organism. Internally each cell is build up of several organelles. The organelles perform different functions just like the ones carried on by different organ systems of the body.

All life activities of the organism are present in miniature form in each and every cell of its body. Thus, cell is a basic unit of life and structural and functional unit of an organism. It is the smallest unit capable of independent existence and performing the essential functions of life.

Essay # 2. Discovery of Cell:

Work on the study of cell has continued for more than the last three and a half centuries. It required microscopes or instruments with good resolving power and magnification. Techniques like preservation, sectioning, staining and mounting were needed to distinguish various cellular components. Improvement in tools and techniques has continued all this period to enhance our knowledge about the cell.

The first microscope was built by Zacharias Janssen in 1590. It was first modified by Galileo (1610) and then by Robert Hooke (Fig. 8.1). Robert Hooke (1635-1703) was a mathematician and physicist. He developed a new microscope with which he studied the internal structure of a number of plants. His work is famous for the study of cork cells.

In 1665, Robert Hooke wrote a book “Micrographia: or Some Physiological Descriptions of Minutae made by magnifying glasses with observations and enquiries there upon. He took a piece of cork of spanish oak and prepared thin slice by means of sharp pen knife. A deep planoconcave lens was used for throwing light on cork piece. The latter was observed under the microscope.

The piece of cork was found to have a honey comb structure with a number of boxes like compartments, each having a pore and separated from others by diaphragms (Fig. 8.2). Robert Hooke named the compartments as cellulae (singular- cellula) now known as cells (Latin cella – hollow spaces or compartments).

He did not know the significance of these structures and regarded them as passages for conducting fluids. Actually the ‘cells’ of Hooke were cell walls enclosing spaces left by dead protoplasts.

Robert Hooke found that the cells or boxes were not very deep. A cubic inch contained 1259,712,000 cells, a square inch 1, 66,400 and one inch strip 1080 cells. The term “cell” is actually a misnomer as a living cell is neither hollow nor always covered by a wall.

Cells were also observed prior to Hooke, by Malpighi (1661), who called them saccules and utricles. Leeuwenhoek (1673) was first to observe, describe and sketch a free living cell. He observed bacteria, protozoa, spermatozoa, red blood cells, etc. In the beginning of nineteenth century it became clear that the bodies of organisms are made of one or more cells.

Robert Brown (1831) discovered the presence of nucleus in the cells of orchid root. Living semifluid substance of cells was discovered by Dujardin (1835) and named sarcode. Schleiden (1838) found all plant cells to have similar structure— cell wall, a clear jelly-like substance and a nucleus.

Schwann (1838) discovered that animal cells lacked cell wall. Purkinje (1839) and von Mohl (1838, 1846) renamed sarcode or the jelly like substance of the cells as protoplasm (Gk. protos- first, plasma- form).

Cell membrane was discovered by Schwann (1838) but was provided with a name by Nageli and Cramer (1855). Soon various organelles were discovered inside the cells. Electron microscope has elaborated our knowledge about cells.

ADVERTISEMENTS: (adsbygoogle = window.adsbygoogle || []).push({}); Essay # 3. Cell Theory:

The theory was jointly put forward by Schleiden and Schwann (1839) in their paper “Microscope Investigations on the similarity of structure and growth in animals and plants.” Cell theory states that the bodies of all organisms are made up of cells and their products so that cells are units of both structure and function of living organisms.

Formulation of Cell Theory:

Development of cell theory illustrates how scientific methodology operates. It involves observation, hypothesis, formulation of theory and its modification.

Observations were started by Malthias Schleiden (1838), a German botanist who examined a large number of plant tissues. He found that all plant tissues were made of one or the other kind of cells. Therefore, he concluded that cells constitute the ultimate units of all plant tissues.

Theodore Schwann (1838), a German Zoologist, studied different types of animal tissues including development of embryos. He found that animal cells lack a cell wall.

Instead they are covered by a membrane. Otherwise cells of both plants and animals are similar. Schwann defined a cell as membrane en-locked, nucleus containing structure. He also proposed a cell hypothesis — bodies of animals and plants are made of cells and their products.

Schneider and Schwann compared their findings, discussed Schwann’s hypothesis and formulated the cell theory in their joint paper in 1839. The theory proposed that cells are the units of both structure and function of organisms.

Rudolf Virchow (1855) observed that hew cells develop by division of the pre-existing cells— Omnis cellula e cellula (theory of cell lineage or common ancestry). The finding gave cell theory its final shape. Louis Pasteur (1862) further proved that life originated from life. Soon Haeckel (1866) established that nucleus stores and transmits hereditary traits. Cell theory was modified accordingly.

Fundamental Features of Cell Theory:

Five fundamental observations of the cell theory are:

i. All living organisms are composed of cells and their products.

ii. Each cell is made of a small mass of protoplasm containing a nucleus in its inside and a plasma membrane with or without a cell wall on its outside.

iii. All cells are basically alike in their chemistry and physiology.

iv. Activities of an organism are the sum total of activities and interactions of its constituent cells.

Essay # 4. Modern Cell Theory :

It is also known as cell doctrine or cell principle.

Modem cell theory states that:

i. The bodies of all living beings are made up of cells and their products.

ii. Cells are units of structure in the body of living organisms. Every cell is made up of a mass of protoplasm having a nucleus, organelles and a covering membrane.

iii. Cells are units of function in living organisms, that is, the activities of an organism are the sum total of the activities of its cells.

iv. While a cell can survive independently, its organelles cannot do so.

v. The cells belonging to diverse organisms and different regions of the same organism have a fundamental similarity in their structure, chemical composition and metabolism.

vi. Life exists only in cells because all the activities of life are performed by cells.

vii. Depending upon specific requirement, the cells get modified, e.g. elongated in muscle and nerve cells, loss of nucleus in RBCs or cytoplasm in outer skin cells.

viii. Growth of an organism involves the growth and multiplication of its cells.

ix. Genetic information is stored and expressed inside cells.

x. Life passes from one generation to the next in the form of a living cell.

xi. New cells arise from pre-existing cells through division. All new cells contain the same amount and degree of genetic information as contained in the parent cell.

xii. All the present day cells/organisms have a common ancestry because they are derived from the first cell that evolved on the planet through continuous line of cell generations.

xiii. Basically the cells are totipotent (i.e., a single cell can give rise to the whole organism) unless and until they have become extremely specialized.

xiv. No organism, organ or tissue can have activity that is absent in its cells.

Objections:

(i) Viruses are acellular and do not have a cellular machinery. Even then they are considered to be organisms.

(ii) In some organisms, the body is not differentiated into cells though it may have numerous nuclei (coenocytes, e.g., Rhizopus).

(iii) Protozoans and many thallophytes have a uninucleate differentiated body (e.g., Acetabularia) which cannot be divided into cells. They are acellular.

(iv) Bacteria and cyanobacteria do not have nucleus and membrane bound organelles.

(v) RBCs and sieve tube cells continue to live without nucleus.

(vi) Protoplasm is replaced by non-living materials in the surface cells of skin and cork.

(vii) Schleiden and Schwann did not know the mechanism of cell formation. Schwann believed cells to develop spontaneously like a crystal. Schleiden thought new cells to develop from cytoblast or nucleus.

Significance of Cell Theory:

(i) There is a structural similarity in cells belonging to diverse groups of organisms,

(ii) All the cells perform similar metabolic activities,

(iii) Life exists only in the form of cells,

(iv) Life passes from one generation to the next as cells,

(v) All living beings are descendants of a primitive cell that developed on earth as the first eukaryote and prior to that as the first prokaryote.

Essay # 5. Limit of Cell Size or Volume:

The factors which set the limit of cell size or volume are:

(i) Nucleocytoplasmic or kern-plasma ratio (ratio of nucleus to cytoplasm) which determines the range of control of metabolic activities by nucleus.

(ii) Ability of oxygen and other materials to reach every part of the cell.

(iii) Ability of waste products to pass to the outside.

(iv) Rate of metabolic activity.

(v) Ratio of surface area to the volume of the cell.

Metabolically active cells are usually smaller due to higher nucleocytoplasmic ratio and higher surface volume ratio. The former will allow the nucleus to have better control of metabolic activities while the latter will allow quicker exchange of materials between the cell and its outside environment.

Surface volume ratio decreases with the increase in cell size or volume as surface increases by the square of the size while volume increases by the cube of the size.

Take three cubic cells which have the surface area of 6 mm2 (6 x 1 x 1), 24 mm2 (6 x 2×2) and 54 mm2 (6×3 x 3) and a volume of 1 ram I3 (1 x 1 x 1), 8 mm3 (2x2x2) and 27 mm3 (3x3x3) respectively (Fig. 8.4). The surface to volume ratio in the three would be 6: 1, 3: 1 and 2: 1.

Therefore, larger cells have lesser surface volume ratio. They tend to become less efficient. All passive cells like eggs are, therefore, larger in size. All active cells are smaller. If larger cells are to remain active, they are either cylindrical in shape or possess several extensions of the cell membrane.

Microvilli are one of such developments. They are found in all those cells which are active in absorption. Membrane infolding’s also occur in transfer cells found in plants in the region of absorption or secretion of nutrients.

Essay # 6. Types of Cells:

A multicellular organism is composed of numerous cells. The cells are of three main types undifferentiated (stem cells), differentiated (post-mitotic cells) and dedifferentiated.

(a) Undifferentiated or Stem Cells:

They are un-specialised cells which usually possess the power of division, e.g., stem apical meristem, root apical meristem, vascular cambium, cork cambium, stratum germinativum of skin, germinal epithelium, bone marrow, etc. Zygote is also an undifferentiated cell.

(b) Differentiated or Post-mitotic Cells:

The cells are specialized to perform specific functions. Differentiation occurs in shape, size, structure and function through an orderly switching on and off of some particular genes of the cells by means of chemicals named as inducers and repressors. It leads to better organisation, division of labour and higher efficiency. Duplication of work is avoided.

They are differentiated cells which revert to undifferentiated state to take over the function of division. The process by which they lose their specialization is called dedifferentiation. It involves reactivation of certain genes that prevent differentiation, allow limited growth and induce division.

Cork cambium of plants is always produced through dedifferentiation. Dedifferentiation helps in healing of wounds, regeneration in animals, or vegetative propagation in plants. Cell culture experiments are based on this dedifferentiation of cells.

Essay # 7. Compartmentalization for Cellular Life:

Every cell behaves as a compartment because it is completely covered over by a membrane known as plasma membrane or plasma lemma. It may also possess some internal compartments in the form of membrane lined organelles like mitochondria, plastids, lysosomes, Golgi bodies, nucleus, etc. Non membranous organelles occur in both prokaryotic and eukaryotic cells, e.g. ribosomes.

i. Separation from Extracellular Medium:

Plasma membrane of the cell segregates its protoplasm from the extracellular medium. As a result, the protoplasm does not mix with the latter. It allows the cell to maintain its chemical pool, orderliness of structure and reactions in contrast to disorderly distribution and random interaction of molecules in the extra-cellular medium.

ii. Selective Permeability:

A cell is not a closed compartment. Its plasma membrane is selectively permeable, i.e., it allows selective exchange of materials between the cell interior and extracellular medium. The cell is thus able to maintain its internal composition quite different from that of the extracellular medium.

iii. Accumulation:

Most cells accumulate inorganic nutrients against their concentration gradient. Sea weeds have iodine in concentration 2 million times the one present in sea water.

iv. Interconnections:

Compartmentalization helps the cells to maintain their individuality. However, cells of a multicellular organism do not remain isolated. Cells of plant tissues are often connected with one another through cytoplasmic bridges called plasmodesmata. Junctions occur amongst animal cells.

v. Recognition:

Cells are able to recognize one another due to presence of specific chemicals on their surface. Thus separated cells of different species of sponges would segregate species-wise if they are allowed to come together. Similar cells of a higher animal would segregate tissue-wise.

vi. Comparatively very large:

A striated muscle cell can be 1-40 mm long and 30-80 pm in thickness. Longest cells of human body are the nerve cells which may reach a length of 90cm.

vii. Intracellular Compartmentalization:

Membrane lined cell organelles act as intracellular compartments. They allow the cells to separate diverse types of chemical reactions.

Essay # 8. Cell— An Open System:

An open system is the one which is separated from its surroundings by a boundary that allows transfer of materials and energy across it. Cell is an open system because it receives a number of materials including energy containing nutrients from outside. It liberates energy as heat and sends out excretions.

There is a wide variation in the size shaped and activities of cells. The smallest cells are those of Mycoplasma. They have a size of 0.1-0.5 pm. Bacteria measure 3-5 pm in length. Viruses are still smaller. They do not have a cellular structure.

The smallest virus has a volume of 7.0 x 10 -7 µm 3 . The smallest mycoplasma has a volume of 1.0 x 10 -3 µm 3 while the smallest bacterium possesses a volume of 2.0 x 10 -2 µm 3 . Unicellular eukaryotes have a size of 1-1000 µm.

Sporozoite of Plasmodium is only 2 µm long. Cells of multicellular eukaryotes have a size range of 5-100 µm. Among multicellular organisms, human erythrocytes (RBC) are about 7 µm in diameter. Some lymphocytes are still smaller (6 µm). Cells of kidney, liver, skin and intestine are 20-30 pm in diameter.

Muscle and nerve cells are comparatively very large. A striated muscle cell can be 1-40 mm long and 30-80 µm in thickness. Longest cells of human body are the nerve cells which may reach a length of 90cm.

Amongst plants, large cells occur in many algae. Intermodal cells of Char a are 1—10 cm in length. Acetabularia (Fig. 8.4B), a unicellular alga, is up to 10 cm in length. It is differentiated into rhizoid, stalk and cap. Plant fibres are still longer— 4 cm in Cotton, 55 cm in Ramie, 30-90 cm in Jute and over a metre in Hemp.

In general, eggs are large sized cells because they store food for partial or complete development of the embryo. Human egg is slightly over 0.1 mm or 100 µm in diameter. It has a volume 1.4 x 10 6 µm 3 or 0.1 million times that of the human sperm (1.7 x 10 1 µm 3 tables 8.1). Avian eggs are the largest. Hen egg is 60 x 45 mm with a volume of 5.0 x 10 µm 13 while the egg of Ostrich is 170 x 150 mm with a volume of 1.1 x 10 15 – µm 3.

Essay # 9. Shapes of Cell:

The cells vary in their shapes. They may be disc like, polygonal columnar, cuboid, amoeboid, thread like or irregular. The shape of cell is related to its position (flat in surface cells, polygonal in cortex) and function (e.g., RBCs are biconcave to pass through capillaries and carry 02 ; WBCs are irregular to do phagocytosis, nerve cells are long to conduct impulses, sperms have tail for motility etc. ; Fig. 8.5).

On the basis of organisation of DNA, the cells are of two types prokaryotic and eukaryotic. The organisms having prokaryotic cells are called prokaryotes. They are now- a-days placed in a super kingdom called Prokaryote. Other organisms (having eukaryotic cells) are included in super kingdom Eukaryote. Prokaryotic cells occur in bacteria, blue green algae, chlamydiae, Archaebacteria and Mycoplasma or PPLO.

Essay # 10. Functions of Cell Parts:

i. Cell Wall:

Shape, rigidity and protection to cell.

ii. Plasma Membrane:

Regulation of substances leaving or entering a cell.

iii. Cytoplasm:

(a) Endoplasmic Reticulum — Cytoskeleton, channelization, synthesis of fats, steroids, proteins, formation of vacuoles and vesicles,

(b) Ribosomes— Protein synthesis,

(d) Chloroplasts— Photosynthesis,

(e) Amyloplasts— Storage of starch,

(f) Golgi Apparatus— Storage, secretion, excretion, wall synthesis, some chemical transformations, membrane transformation, lysosome formation,

(g) Centrioles— Formation of astral poles, flagella.

(h) Lysosomes— Separation and storage of hydrolytic (digestive) enzymes, digestion, autophagy.

(i) Sphaerosomes— Metabolism, storage and synthesis of fats,

(j) Glyoxysomes— Glyoxylate cycle, conversion of fat to carbohydrates,

(k) Peroxisomes— Photorespiration, peroxide metabolism.

(l) Microtubules— Cytoskeleton, formation of spindle and flagella.

(m) Microfilaments— Holding of membrane proteins, controlling cleavage and cyclosis.

(n) Vacuole— Osmotic pressure, storage.

iv. Nucleus:

Carrier of hereditary information, control of cell metabolism, cell differentiation, synthesis of DNA and RNA, formation of ribosomes, control of reproduction.

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Introduction to cells

All living things are made from one or more cells. A cell is the simplest unit of life and they are responsible for keeping an organism alive and functioning. This introduction to cells is the starting point for the area of biology that studies the various types of cells and how they work.

There is a massive variety of different types of cells but they all have some common characteristics. Almost every different type of cell contains genetic material , a membrane and cytoplasm. Cells also have many other features such as organelles and ribosomes that perform specific functions.

Many different organisms on the tree of life contain only one cell and are known as single-celled or unicellular organisms. Their single cell performs all the necessary functions to keep the organism alive. All species of bacteria and archaea are single-celled organisms. On the other hand, large organisms like humans are made from many trillions of cells that work together to keep the organism alive.

The most basic categorisation of Earth’s organisms is determined by different types of cells. All cells can be divided into one of two classifications: prokaryotic cells and eukaryotic cells. Prokaryotic cells are found in bacteria and archaea. Eukaryotic cells are found in organisms from the domain Eukaryota which includes animals, plants, fungi and protists.

This introduction to cells will take you through the basic structure of cells, the difference between prokaryotic and eukaryotic cells and you will learn about organelles.

STRUCTURE OF A CELL

The genetic material of cells is found as molecules called DNA. The DNA of a cell holds all the information that a cell needs to keep itself alive. A DNA molecule contains a code that can be translated by a cell and tells it how to perform different tasks. A gene is a specific segment of a DNA molecule and each gene tells a cell how to perform one specific task.

The gel-like substance that the genetic material is found in is called the cytoplasm. The cytoplasm fills a cell and gives it it’s shape. The cytoplasm also allows for different materials to move around the cell. All cells have other structures in their cytoplasm that help the cell stay alive.

The cytoplasm of all cells is surrounded by a membrane called the plasma membrane. The plasma membrane separates the cell from the outside world and keeps the contents of the cell together. The plasma membrane provides a barrier that substances have to pass through before they can enter or exit a cell.

EUKARYOTIC CELLS VS. PROKARYOTIC CELLS

The main difference between prokaryotic cells and eukaryotic cells is the presence of a nucleus and organelles. Prokaryotic cells do not have either a nucleus or organelles. The word prokaryotic can be translated to mean ‘before nucleus’.

Eukaryotic cells have both a nucleus and a range of different organelles. The nucleus is a structure found in eukaryotic cells that contains the cell’s DNA. Organelles are cellular ‘factories’ that perform important functions such as building different molecules of life , removing wastes and breaking down sugars.

Having organelles makes eukaryotic cells much more efficient at completing important cellular functions. Because they are more efficient, eukaryotic cells can grow much larger than prokaryotic cells.

For a cellular structure to be considered an organelle it must be surrounded by a membrane just as the nucleus is. Prokaryotic cells contain various structures that help with certain functions, such as ribosomes, but these structures are not encapsulated by membranes and are therefore not considered organelles.

Eukaryotic cells have evolved into multicellular organisms. By specializing into different types of cells, they are able to perform functions even more efficiently and are able to keep large, multicellular organisms alive.

Important organelles include the nucleus, mitochondria, chloroplasts, and the endoplasmic reticulum. Mitochondria are involved in the process of cellular respiration where sugar is broken down and converted into cellular energy.

Chloroplasts are found in the cells of plants and other photosynthetic organisms . Inside chloroplasts are where plant cells are able to use energy from the sun to create sugars from carbon dioxide and water.

The endoplasmic reticulum is a network of membranes that are attached to the membrane of the nucleus. The endoplasmic reticulum is involved with many important tasks such as producing proteins and breaking down fats and carbohydrates.

For more information on cells check out these pages on our website: Cells | Eukaryotic cells | Prokaryotic cells | Animal cells | Plant cells

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Types of cells in the human body

Author: Rachel Baxter, MSc • Reviewer: Francesca Salvador, MSc Last reviewed: September 11, 2023 Reading time: 31 minutes

There are over 200 different cell types in the human body. Each type of cells is specialised to carry out a particular function, either solely, but usually by forming a particular tissue . Different tissues then combine and form specific organs, where the organ is like a factory where every type of cell has its own job.

Since every tissue has its own function that contributes to the multifunctionality of an organ, every type of cell is equally important. The most important types of cells are listed below.

Key facts about the cell types in the human body
Stem cells	stem cells stem cells
Red blood cells
White blood cells	(neutrophils, eosinophils, basophils) (monocytes, lymphocytes)
Platelets	Fragments of megakaryocytes
Nerve cells
Muscle cells
Cartillage cells
Bone cells
Skin cells
Endothelial	Lining blood vessels
Epithelial cells	Lining body cavities
Fat cells
Sex cells

This article will discuss the histology of most important types of cells in the human organism.

Red blood cells

Neutrophils, eosinophils, lymphocytes, nerve cells, neuroglial cells, skeletal muscle cells, cardiac muscle cells, smooth muscle cells, cartilage cells, osteoclasts, osteoblasts, lining cells, endothelial cells, epithelial cells, spermatozoa.

Before a cell becomes specialised, it first starts out as a stem cell. The unique feature of stem cells is that they are pluripotent - they have the potential to become any type of cell in the body. These incredible cells are the ancestors of all cells in the body, from simple skin cells to complex neurons . Without these cells, we wouldn't be as complex or functional as human beings.

Not only this, these “magic” cells even have the power to replicate into healthy cells in order to speed up regeneration after certain pathological conditions. The process that allows stem cells to transform into any kind of cell is known as cell differentiation and is controlled by a combination of internal genetics and external factors such as chemicals and physical contact with other cells. Stem cells have the ability to divide and replicate themselves for long periods of time.

There are two types of stem cells, embryonic stem cells and adult stem cells. Embryonic stem cells are from embryos. Generally used in a research setting, embryonic stem cells are harvested from fertilised eggs. Adult (or somatic) stem cells are present throughout the human body [amongst other specialised tissue cells]. They exist in order to repair and maintain surrounding specialised tissues.

As these cells are unspecialised, stem cell anatomy is that of a simple cell . Stem cells have a cell membrane, surrounding the cytoplasm. The cytoplasm contains a nucleus, mitochondria, ribosomes, endoplasmic reticulum, golgi apparatus, lysosomes and centrioles. The nucleus contains DNA and RNA, which are expressed when differentiation occurs in the cell.

Red blood cells are known as erythrocytes , and are the most common type of blood cell. They are shaped like a biconcave disc (I.e. donut shaped). They have a diameter of around 6 to 8 µm and have an average thickness of 2 µm, being 2.5 µm thick at their thickest point and 1 µm thick at the center. Red blood cells are fairly flexible, allowing them to squeeze through thin blood capillaries.

The main role of red blood cells is to transport oxygen around the body using haemoglobin. However, they also help to control pH of the blood by forming an acid-base buffer maintaining the blood at a neutral pH of 7.35 to 7.45. They also release an enzyme called carbonic anhydrase, which causes water in the blood to carry carbon dioxide to the lungs , so that it can be expelled from the body.

Haemoglobin is a molecule in red blood cells that binds to oxygen, allowing it to be transported through the blood. Haemoglobin is comprised of a heme molecule and a globin molecule. Heme molecules are formed from succinyl-CoA and glycine. Four of these molecules together bind with iron forming a heme molecule. This combines with a globin polypeptide chain forming a haemoglobin chain (also named globulin chain). Four of these chains together create a haemoglobin molecule. There are four different types of haemoglobin chains; alpha, beta, gamma and delta. The most common combination is two alpha chains and two beta chains, which form a haemoglobin A molecule.

White blood cells

White blood cells, also known as leukocytes , are a vital component of the immune system. There are five different types, which fall under two main categories; granulocytes and agranulocytes. As suggested by their names, granulocytes contain granules in the cytoplasm as agranulocytes do not. Granulocytes include neutrophils , eosinophils and basophils. Agranulocytes include lymphocytes and monocytes.

Neutrophils are the most common type of leukocyte, making up around 65% of all white blood cells. They are 12 to 14 µm in diameter, and contain a single nucleus. They contain few cell organelles and protein synthesis does not take place within them. Neutrophils originate in the bone marrow and circulate in the bloodstream for 6 to 10 hours, before entering the surrounding tissues. Once in the tissues, they destroy damaged cells and bacteria through phagocytosis , before self-destructing.

Eosinophils are rare in the bloodstream. They are 12 to 17 µm in diameter and contain toxic proteins. Like neutrophils, they originate in the bone marrow and move into the bloodstream before entering loose connective tissue in the respiratory tract and intestines. Here they destroy antigen-antibody complexes using phagocytosis .

The cells release the specialised enzymes histaminase and arylsulfatase B which are involved in the inflammatory response. Eosinophils also play a role in destroying bacteria, viruses and parasites that invade the body.

Basophils are the rarest form of white blood cell and are involved in the body’s defense against parasites . They are 14 to 16 µm in diameter. They accumulate at infected areas, releasing histamines , serotonin and prostaglandins to increase blood flow which causes an inflammatory response.

Lymphocytes can be divided into two different types, B-cells and T-cells. Lymphocytes vary in size, with most being around 6 to 9 µm in diameter, and a tenth of them being 10 to 14 µm in diameter. The largest lymphocytes tend to be favored, and contain more cytoplasm, mitochondria and ribosomes than their smaller counterparts.

Both B-cells and T-cells are involved in the adaptive immune response , but have different roles. Both originate from haematopoietic stem cells in the bone marrow. However, T-cells mature in the thymus gland between the lungs and in front of the heart . The thymus gland atrophies into fat as children become adults yet can still stimulate the maturation of T-cells. B-cells develop into plasma cells and are involved in the synthesis of antibodies which attack foreign antigens. T-cells are involved in the destruction of bacteria, viruses and other damaging cells such as cancer cells.

The final type of white blood cells are the monocytes . These are as large as 20 µm in diameter. They have a large kidney bean shaped nucleus. Monocytes circulate in the bloodstream between one and three days before entering the tissues of the body where they become macrophages. Macrophages are large phagocytic cells that engulf and kill dead cells and bacterial cells.

Learning the types of cells is tricky business! Practice your tissue identifiction skills with our free histology slide worksheets, quizzes and labeling diagrams.

Just like the white and red blood cells, platelets also form an important component of the blood. Technically platelets are fragments of cells rather than true cells, but are vital in the control of bleeding . They are fragments of large cells called megakaryocytes which are produced in the bone marrow. They have surface proteins which allow them to bind to one another, and to bind to damaged blood vessel walls. Platelets are recruited when bleeding occurs, initiating a process known as hemostasis . They plug the source of the bleeding, coagulating and sticking together to form a blood clot, together with a fibrous protein known as fibrin.

Learn everything about the blood cells with the following study unit and quiz.

Nerve cells, commonly known as neurons , transmit information throughout the body in the form of electrical signals or nerve impulses. Structurally, neurons have four specific regions; the cell body, dendrites , the axon and axon terminals. The cell body contains a nucleus and is responsible for synthesising neural proteins. The axon is long and thin, and protrudes from the cell body like a tail and can be myelinated or unmyelinated. Axons are responsible for conducting electrical impulses in the form of action potentials , away from the cell body.

Action potentials cause a change in voltage across the plasma membrane. Axons connect to other neurons via synapses , which are formed by small branches at the end of the axon called axon terminals. Impulses are received from other cells by dendrites, which are multiple branching structures protruding from the cell body.

Neurons can have multiple, two or one dendrite(s) which makes them multipolar , bipolar or unipolar respectively. They convert chemical signals from the synapse into small electrical impulses, and transmit them towards the cell body. Electrical disturbance in the dendrites is transmitted to a structure called the axon hillock at the base of the axon, and with enough voltage, generates an action potential which moves down the axon and continues its course.

Test your knowledge on the structure of the neuron with the quiz below!

Neuroglial cells, more commonly known as glial cells or glia, are cells of the nervous system that are not involved in the conduction of nervous impulses. Glia are very common in the brain , outnumbering neurons at a ratio of 3 to 1. Glia are smaller than neurons, and do not have axons or dendrites. They have a variety of roles in the nervous system , they modulate synaptic action and rate of impulse propagation, they provide a scaffold for neural development, and aid recovery from neural injuries.

There are four types of glial cells in the central nervous system; astrocytes , oligodendrocytes, microglial cells, and ependymal cells . Astrocytes are found in the brain and spinal cord , and have a starlike appearance. They are involved in the maintenance of the chemical environment required for neuron signalling. Oligodendrocytes are responsible for forming a lipid-rich myelin sheath around axons, increasing the speed at which action potentials are conducted. Microglial cells are very small and are involved in the removal of debris from sites of injury. Ependymal cells line the ventricles and central canal of the brain to produce cerebrospinal fluid. In the peripheral nervous system , Schwann cells are responsible for the myelination of axons and satellite cells regulate the neural cell environment.

Muscle cells

There are 3 types of muscle cells, known as myocytes , in the human body. These types are skeletal, cardiac and smooth muscle. Skeletal and cardiac muscle cells are known as striated , due to the aligned arrangement of myosin and actin proteins within them. Actin and myosin allow muscle contraction by sliding past one another, as described by sliding filament theory. Actin and myosin are arranged more randomly in smooth muscle cells, creating a smooth rather than striated appearance.

Skeletal muscle cells are attached to bones and tendons and can reach up to 30 cm in length, although they are usually 2 to 3 cm long. Skeletal muscle cells are responsible for voluntary movements . They are multinucleated and comprise a sarcolemma (cell membrane), sarcoplasm (cytoplasm), myofibrils (actin and myosin), sarcosomes (mitochondria) and a sarcoplasmic reticulum , which is like the smooth endoplasmic reticulum of other cells. They also contain two proteins called troponin and tropomyosin which regulate the interaction between actin and myosin during contraction. The basic units of striated muscle cells comprising actin and myosin are known as sarcomeres .

You've almost finished learning about the types of cells in the body - but what about the parts of a cell? Learn this topic easily and fuss-free using our handy diagams and cell quizzes!

Cardiac muscle cells are also called cardiomyocytes which together make up the most important muscular tissue in the entire body, the tissue of the heart. Individually, they are about 0.02 mm wide and 0.1 mm long and linked together via gap junctions . The cells contract in unison creating the contractions of the heart. This is coordinated by nervous impulses which depolarises the cell membrane, spreading from cell to cell relatively quickly as the cells are very closely anchored via intercalated discs . Cardiomyocytes contain many sarcosomes to provide sufficient energy for contraction.

Smooth muscle cells are responsible for involuntary contractions in hollow and visceral organs like the bladder and lungs , and the walls of blood vessels. They are responsible for peristalsis , whereby food is propelled through the digestive system via wavelike contractions.

They are 10 to 600 µm long spindle-shaped cells and have a central nucleus. Smooth muscle cells are arranged in sheets allowing them to contract simultaneously. As they are smaller than cardiomyocytes and skeletal myocytes, they contain fewer cell organelles, and do not contain sarcomeres.

Cartilage cells, also known as chondrocytes , make up cartilage , a firm tissue that is vital to the body’s structure. Cartilage is found in joints between bones, in the ears and nose, in the airways as well as other locations. For example, cartilage can be found between the vertebrae in the spinal column.

Chondrocytes produce and maintain the extracellular matrix of cartilage, comprising collagen, proteoglycan and elastin fibers. They lack blood vessels meaning that cartilage is repaired slower than other tissues, and nutrients have to be absorbed by diffusion from the tissue surrounding the cartilage, known as the perichondrium . Articular cartilage (cartilage found in synovial joints) differs from other cartilages since it does not contain perichondrium.

Learn more and test your knowledge on the different types of cartilage with the following study unit and quiz!

There are four types of bone cells in the body; osteoblasts , osteoclasts, osteocytes and lining cells.

Osteoclasts are large multinucleated cells that are involved in bone resorption . This is where the bone is broken down during the process of renewal. Osteoclasts break down bone by forming sealed compartments on its surface, and releasing enzymes and acids. After they complete the process, they die by apoptosis (programmed cell death).

Osteoblasts have the opposite function, they are involved in the generation of new bone . They are cuboidal in shape and have one central nucleus. They work by synthesising protein which forms the organic matrix of the bone. They are triggered to create new bone by hormones such as vitamin D and estrogen, and have specialised receptors on their surfaces which detect them.

Osteocytes are cells that are found inside the bone. They have long branched structures protruding from them allowing cell to cell contact and access to the bone’s surface. Osteocytes can sense mechanical strain being placed on the bone, and secrete growth factors which activate bone growth in response.

The final type of bone cells are lining cells. These originate as osteoblasts before becoming flat in structure. As their name suggests, they line the surface of the bone and are responsible for the release of calcium from the bone into the bloodstream when it falls too low. Lining cells have receptors on their surfaces which are receptive to hormones and other chemicals that signify a need for bone growth and remodeling. They also work to protect the bone from chemicals in the blood which might be damaging to the bone’s structure.

Go through these resources to solidify your knowledge about bone tissue:

There are many different types of cells in the epidermis (top layer) of the skin . The epidermis contains the following cell types:

Keratinocytes : These cells make up 95% of the epidermis and are sometimes known as basal cells , as they are found in the basal layer of the epidermis. Keratinocytes generate the protein keratin , but are also important in protecting the body by blocking toxins and pathogens, and preventing loss of heat and moisture. They also stimulate inflammation and secrete inhibitory cytokines. The outermost layer of epidermis is formed by keratinized epithelial cells which are responsible for forming the protective barrier. Hair and nails are examples of fully keratinized epithelial cells.
Melanocytes: The role of melanocytes in the skin is to produce the pigment melanin , which determines skin coloration.
Langerhans cells: These are dendritic cells involved in antigen processing when the skin becomes infected, they act as antigen-processing cells. They contain large organelles known as Birbeck granules, but the exact function of these is still unknown.
Merkel cells: These act as mechanosensory cells and are involved in touch reception (the ability to feel).
Other types of sensory cells are present within the skin, however are found in the deeper layers and known as cutaneous receptors.

Why not test your knowledge of the skin with some quiz questions?

Endothelial cells are the cells that form the lining of blood vessels . They are flat in structure, and are between 1 and 2 µm thick. They have a central nucleus, and are connected to one another via intercellular junctions. Endothelial cells are highly adaptable , being able to migrate and adjust their numbers and arrangements to accommodate the body’s needs. This allows growth and repair of body tissues, as new blood vessel networks can easily form.

As well as healthy body tissues, cancer cells also rely on endothelial cells and blood vessels to survive As a result, a lot of research is focused on preventing the formation of blood vessels in cancerous tissues. Endothelial cells express different surface proteins, depending on whether they are forming veins or arteries.

Epithelial cells make up the linings of cavities in the body such as the lungs, small intestine and stomach. They are joined to one another forming sheets called epithelia , and are connected by tight junctions, adherens, desmosomes and gap junctions. Tight junctions are unique to epithelial cells and form the closest type of junction between any cell type in the body. They are supported by a basement membrane known as a basal lamina , which covers a capillary bed. The nucleus of an epithelial cell is found close to the basal lamina, towards the bottom of the cell.

Epithelial cells are innervated with nerve endings, and can become sensory cells , detecting stimuli such as scent. Epithelial cells can also specialise to become secretory cells , that release mucous, hormones and enzymes into the body. These cells contain vesicles of hormones or enzymes ready to be released. Specialised secretory epithelial cells include goblet cells and paneth cells in the intestines, which secrete mucous and antibacterial proteins respectively.

Quiz yourself to reinforce what you have learned about the epithelial cells.

Fat cells, also referred to as adipocytes and lipocytes are the cells of the body that are specialised to store energy in the form of adipose tissue , or fat. There are two types of fat cell, white fat cells and brown fat cells. White fat cells , or unilocular cells, are vacuolar cells that contain a lipid droplet and cytoplasm. They have a nucleus which is flat and at the edge of the cell, rather than the centre. White fat cells vary in size, but on average they are around 0.1 mm in diameter. The fat inside white fat cells is mainly made up of triglycerides and cholesteryl ester, and is stored in semi-liquid form.

Brown fat cells , or multilocular cells, have multiple vacuoles and are shaped like polygons. They contain more cytoplasm that white fat cells, and fat droplets are scattered throughout them. The nucleus is not flattened but round, and is found randomly positioned towards the centre of the cell. The key role of brown fat is to generate heat energy, and therefore the cells contain many mitochondria, which give them their brownish coloration.

Sexual reproduction is the result of the fusion of two different types of sex cells called gametes . Male sex cells are commonly known as sperm cells, or spermatozoa , and female gametes are known as eggs or ova . When they fuse together, fertilization occurs and a zygote is formed.

Spermatozoa and ova are structurally very different from one another. Spermatozoa are smaller, being about 50 µm long, and have a head , a midpiece region and a long tail (flagellum) for propulsion and motility. The head contains an acrosome , which is a type of covering filled with enzymes that enable penetration of the female ovum during fertilisation. The head of the cell contains a nucleus that is densely packed with DNA, with little cytoplasm present. The midpiece region of the cell contains mitochondria which provide the energy required for locomotion.

Ova are very large compared to other cell bodies, being as large as 0.2 mm in diameter. They are round in shape and are produced in the ovaries during embryological development. The cell itself comprises a nucleus, cytoplasm, the zona pellucida and the corona radiata. The zona pellucida is a membrane that surrounds the cell membrane of the cell, and the corona radiata forms protective layers which surround the zona pellucida. During the process of fertilization, the spermatozoa binds with the ovum at the zona pellucida. After, the penetration of the spermatozoa and the release of its contents into the ovum can then occur (acrosome reaction) .

Stem cells are pluripotent cells that have the potential to become any type of cell in the body through a process called differentiation. Stem cells have the ability to divide and replicate themselves for long periods of time. There are two types of stem cells, embryonic stem cells, and adult stem cells.
Red blood cells are known as erythrocytes and are the most common type of blood cell. They are shaped like a biconcave disc. The main role of red blood cells is to transport oxygen around the body using hemoglobin.
White blood cells, also known as leukocytes , are a vital component of the immune system. There are five different types, which fall under two main categories; granulocytes and agranulocytes. As suggested by their names, granulocytes contain granules in the cytoplasm as agranulocytes do not. Granulocytes include neutrophils, eosinophils, and basophils. Agranulocytes include lymphocytes and monocytes.
Platelets are fragments of cells rather than true cells but are vital in the control of bleeding. They are fragments of large cells called megakaryocytes. They have surface proteins that allow them to bind to one another, and to bind to damaged blood vessel walls.
Nerve cells, commonly known as neurons , transmit information throughout the body in the form of electrical signals or nerve impulses. Structurally, they have four specific regions; the cell body, dendrites, the axon and axon terminals. Neurons can have multiple, two or one dendrite(s) which makes them multipolar, bipolar or unipolar respectively.
Neuroglial cells, more commonly known as glial cells or glia, are cells of the nervous system that modulate synaptic action and rate of impulse propagation, provide a scaffold for neural development, and aid recovery from neural injuries. There are four types of glial cells in the central nervous system; astrocytes, oligodendrocytes, microglial cells, and ependymal cells.
There are 3 types of muscle cells, known as myocytes , in the human body. These types are skeletal, cardiac and smooth muscle. Skeletal and cardiac muscle cells are known as striated, due to the aligned arrangement of myosin and actin proteins within them. Actin and myosin allow muscle contraction by sliding past one another, as described by sliding filament theory.
Cartilage cells, also known as chondrocytes , make up cartilage , a firm tissue that is vital to the body’s structure. Chondrocytes produce and maintain the extracellular matrix of cartilage, comprising collagen, proteoglycan and elastin fibers.
There are four types of bone cells in the body; osteoblasts, osteoclasts, osteocytes, and lining cells. Osteoclasts are large multinucleated cells that are involved in bone resorption. Osteoblasts have the opposite function, they are involved in the generation of new bone. Osteocytes can sense mechanical strain being placed on the bone, and secrete growth factors that activate bone growth in response. Lining cells line the surface of the bone and are responsible for the release of calcium from the bone into the bloodstream when it falls too low.
There are many different types of cells in the epidermis (top layer) of the skin . The epidermis contains many types of cells, including keratinocytes, melanocytes, Langerhans cells, and Merkel cells.
Endothelial cells are the cells that form the lining of blood vessels and are connected to one another via intercellular junctions. Endothelial cells are highly adaptable, being able to migrate and adjust their numbers and arrangements to accommodate the body’s needs.
Epithelial cells make up the linings of cavities in the body, forming sheets called epithelia. They are connected by tight junctions, adherens, desmosomes and gap junctions.
Fat cells, also referred to as adipocytes and lipocytes, are the cells of the body that are specialized to store energy in the form of adipose tissue, or fat. There are two types of fat cells, white fat cells and brown fat cells.
Sexual reproduction is the result of the fusion of two different types of sex cells called gametes. Male sex cells are commonly known as sperm cells or spermatozoa , and female gametes are known as eggs or ova . When they fuse together, fertilization occurs and a zygote is formed.

References:

A. Mandal: What is Cartilage? News medical (accessed 2nd August 2016)
B. Alberts, A. Johnson, J. Lewis , et al. : Molecular Biology of the Cell, 4th Edition, Garland Science (2002)
D. Purves, G. J. Augustine, D. Fitzpatrick, et al.: Neuroscience, 2nd Edition, Sinauer Associates (2001)
E. A. Woodcock, S. J. Matkovich: Cardiomyocytes structure, function and associated pathologies (accessed 1st August 2016)
Excitable Tissues (Smooth Muscle) . Queen Margaret University (accessed 2nd August 2016)
H. Lodish, A. Berk, S. L. Zipursky, et al.: Molecular Cell Biology, 4th Edition, W. H. Freeman (2000)
J. McGeachie: Endothelial Cells. The University of Western Australia School of Anatomy and Human Biology (accessed 2nd August 2016)
J. P. Whickson: What is the Structure of Stem Cells? Cuteness (accessed 3rd August 2016)
J. N. George: Platelets . The Latest Information on TTP, ITP and Drug-Induced Thrombocytopenia (accessed 3rd August 2016)
Keratinocytes . Keratinocyte transfection (accessed 2nd August 2016)
M. Cichorek, M. Wachulska, A. Stasiewicz, et al.: Skin Melanocytes. biology and development. Advances in Dermatology and Allergology (2013), volume 30, issue 1, p. 30-41
M. Harms, P. Seale: Brown and beige fat: development, function and therapeutic potential. Nature Medicine (2013), volume 19, p. 1252-1263
M. J. Oursler, T. Bellido: Bone Cells. ASBMR Bone Curriculum (accessed 2nd August 2016)
M. P. Gaidhu, R. B. Ceddia: The Role of Adenosine Monophosphate Kinase in Remodeling the White Adipose Tissue Metabolism. Exercise and Sports Science Reviews (2011), volume 39, issue 2, p. 102-108
Overview of Epithelial Cells . Davidson College Biology Department (accessed 2nd August 2016)
R. Bailey: Sex Cells . About Education (accessed 2nd August 2016)
Skeletal Muscle Structure . University of California (accessed 1st August 2016)
S. Maksimovic, M. Nakatani, Y. Baba, et al.: Epidermal Merkel cells are mechanosensory cells that tune mammalian touch receptors. Nature (2014), volume 509, issue 7502, p. 617-621
What are Adult Stem Cells? National Institute of Health (accessed 3rd August 2016)
White Blood Cells . University of Leeds Histology Guide (accessed 3rd August 2016)

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Overview of Cells

Prokaryotic Cells

Eukaryotic Cells

Prokaryotic vs. Eukaryotic Cells
Monocot and Dicot Overview
Monocot and Dicot Roots
Monocot and Dicot Stems
Monocot and Dicot Leaves
Monocot Glossary
Dicot Glossary
DNA and Chromosomes Overview
Eukaryotic Chromosomes
Prokaryotic Chromosomes
Eukaryotic vs. Prokaryotic Chromosomes
DNA Structure
Red Blood Cells and Platelets
Granular Myeloid White Blood Cells
Agranular Myeloid White Blood Cells
Lymphoid White Blood Cells
Reactants and Products
Leaf Structures
Photosynthesis Reactions
Evolution Overview
Mechanisms of Evolution

What are cells and what do they do?

Eukaryotic Cell

Cells are the microscopic units that make up humans and every other living organism. Some organisms consist of only one cell, while others (like humans) have trillions of cells!

1. The vast majority of cells share several characteristics: they are bound by a plasma membrane and contain cytoplasm, DNA, and ribosomes

There are many, many types of cells, but there are a few key things most of them have in common:

All cells are bound by a plasma membrane .
The interior of all cells consists of cytoplasm filled with a jelly-like substance called cytosol. Structures inside the cell are suspended in the cytosol.
All living organisms have cells that contain genetic material ( DNA ).
Most cells contain ribosomes , which are structures that combine amino acids to create proteins.

Cells have a plasma membrane and contain ribosomes and genetic material.

2. Cells perform many functions, from synthesizing proteins to passing on genetic material

Our cells do a lot for us: they synthesize proteins, convert nutrients from our food into energy we can use, and make up the tissues and organs in our bodies. Eukaryotic cells contain smaller structures, called organelles , that help it carry out these functions.

3. Cells replicate themselves

Most cells make more cells by dividing. Most cells in the human body can divide via a processes called mitosis. Mitosis occurs when a cell divides and creates two genetically identical copies of itself. Specialized sex cells can divide by meiosis , which occurs when a sex cell creates four daughter cells that are all genetically distinct. This helps get genetic material shuffled around, so sexually reproducing organisms have offspring that are a little different from themselves.

In the human body, sex cells divide via meiosis and somatic cells divide via mitosis

4. There are two main types of cells: prokaryotic cells and eukaryotic cells.

Prokaryotic cells include bacteria and archaea. Prokaryotes—organisms composed of a prokaryotic cell—are always single-celled (unicellular). Prokaryotic cells don’t contain a nucleus. Instead, their DNA can be found in the cytoplasm in a region called the nucleoid or in circular chromosomes called plasmids.

Eukaryotic cells can be found in animals, plants, protists, and fungi. Eukaryotes—organisms composed of eukaryotic cells—are multicellular or complex unicellular organisms. Eukaryotic cells have a membrane-bound nucleus where their DNA is stored.

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External Sources

A basic overview of cells from the U.S. National Library of Medicine.

Inside the Cell : an educational booklet from the National Institute of General Medical Science.

Prokaryotes vs. Eukaryotes

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Biology archive

Course: biology archive > unit 6.

Scale of cells

Cell theory

Intro to cells.

Introduction to cells

Introduction

All living things are composed of one or more cells.
The cell is the basic unit of life.
New cells arise from pre-existing cells.

Attribution:

“ Studying cells ” by OpenStax College, Biology, CC BY 3.0 . Download the original article for free at http://cnx.org/contents/[email protected]:16/Biology .
“ Introduction ” from Unit 2: The Cell by OpenStax College, Biology, CC BY 3.0 . Download the original article for free at http://cnx.org/contents/[email protected]:15/Biology .

Works cited:

Waggoner, Ben. "Robert Hooke (1635-1703)." In University of California Museum of Paleontology . 2001. http://www.ucmp.berkeley.edu/history/hooke.html .
"Microscope history: Robert Hooke (1635-1703)." In History of the microscope . 2010. http://www.history-of-the-microscope.org/robert-hooke-microscope-history-micrographia.php .
"Anton Van Leeuwenhoek: A History of the Compound Microscope." In History of the Microscope . 2010. http://www.history-of-the-microscope.org/anton-van-leeuwenhoek-microscope-history.php .
"Theodor Schwann." Wikipedia. July 18, 2015. Accessed August 9, 2015. https://en.wikipedia.org/wiki/Theodor_Schwann .
"Rudolf Virchow." Wikipedia. August 3, 2015. Accessed August 9, 2015. https://en.wikipedia.org/wiki/Rudolf_Virchow .
"Robert Remak." Wikipedia. June 17, 2015. Accessed August 9, 2015. https://en.wikipedia.org/wiki/Robert_Remak .

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Types of Cells

Reviewed by: BD Editors

There are approximately 200 different types of cells in the human body, but all cells on Earth fit into just two categories; prokaryotes, and eukaryotes.

The Two Types of Cells on Earth

All cells on Earth can be classified as either prokaryotic cells or eukaryotic cells. Eukaryotic organisms may be multicellular or unicellular, but prokaryotes are always unicellular organisms.

Eukaryotic cells are larger and more complex than prokaryotes, and usually contain organelles that are absent from prokaryotic cells. This is because eukaryotes contain membrane-bound organelles (like the nucleus, endoplasmic reticulum, Golgi apparatus, and mitochondria ), but prokaryotes do not.

The two types of cells on Earth are prokaryotes and eukaryotes

Types of Eukaryotic Cells

The four types of eukaryotic cells are animal cells, plant cells, fungi cells, and protists.

Animal Cells

Animal cells are the basic building blocks that make up all animals, including birds, fish, reptiles, mammals, and amphibians. Like eukaryotic cells, they contain membrane-bound organelles (such as a nucleus, mitochondria, Golgi apparatus, and endoplasmic reticulum), and are surrounded by a plasma membrane.

Plant Cells

Plants are made up of plant cells. Plant cells contain many of the organelles common to all eukaryotes, but they contain additional structures that are not found in animal cells. For example, plant cells are surrounded by a tough, cellulose-based structure called the cell wall. They also contain organelles called chloroplasts, which are the site of photosynthesis and allow plant cells to produce carbohydrates from carbon dioxide, water, and light energy.

Fungi Cells

The fungi kingdom consists of yeasts, mildews, molds, and mushrooms. Fungi cells contain many of the structures and organelles found in plant and animal cells, like the nucleus, mitochondria, cell membrane, mitochondria, Golgi apparatus, and endoplasmic reticulum. However, they do not contain chloroplasts. They do have a cell wall but this is mainly composed of a polysaccharide called chitin, rather than cellulose (as is the case in plant cells).

Protist Cells

Protists are a highly diverse group of organisms, and kingdom Protista is comprised of all eukaryotes that are not animals, plants, or fungi. Protist cells contain all of the membrane-bound organelles found in animal cells, and some types also contain chloroplasts. They may also have a cell wall made from cellulose.

Types of Prokaryotic Cells

Prokaryotic cells are smaller and have a simpler structure than eukaryotic cells, as they do not contain membrane-bound organelles. Prokaryotic organisms are always unicellular and may be either bacteria or archaea. Bacterial and archaeal cells have the same basic structure, but some of their components are made from different materials.

Bacteria and archaea are prokaryotic cells

Bacterial Cells

Bacteria are unicellular, prokaryotic organisms. Their cells do not contain membrane-bound organelles, so they have no nucleus, mitochondria, endoplasmic reticulum, or Golgi apparatus. However, they do have a cell membrane, cytoplasm, ribosomes, and free-floating loops of DNA. Bacterial cells also have a cell wall made from a polymer called peptidoglycan (AKA murein ). Some bacteria have additional specialized structures, like the capsule (a sticky layer of carbohydrates that surrounds the cell), or flagella (whip-like structures that allow the bacterium to move).

Archaeal Cells

Archaea are also unicellular prokaryotes, and they contain many of the same structures that are found in bacteria cells. However, they typically have a different composition. For example, the bacterial cell wall contains peptidoglycan, but the archaeal cell wall does not. The plasma membrane in bacterial cells (and eukaryotes) is a lipid bilayer, but the plasma membrane of archaeal cells is a lipid monolayer. Finally, the cell membrane in bacteria contains fatty acids, but the cell membranes of archaea contain a hydrocarbon called phytanyl .

Types of Cells in the Human Body

Adult human beings are made up of around 37 trillion individual cells, and approximately 200 different types of cells. Some key cell types of the human body include stem cells, muscle cells, blood cells, bone cells, nerve cells, fat cells, sperm cells, and egg cells.

Muscle Cells

Muscle cells are the basic units of muscle tissue. These cells are highly specialized to facilitate muscle contraction and contain protein filaments (called myofibrils ) and lots of mitochondria.

Stem cells are cells that can develop into other types of cells. They differentiate to form all of the specialized cells in the human body and are found in both embryos and certain adult body tissues (such as the bone marrow ).

Stem cells can turn into other types of cells

Bones are living tissues with their own network of blood vessels, and they are made of several different types of cells. The key types of bone cells are osteoclasts (cells that dissolve bone tissue) , osteoblasts (cells that make new bone tissue) , and osteocytes (which are found inside the bone and communicate with other bone cells).

Blood Cells

Blood contains an assortment of cells. The three major types of blood cells are red blood cells (which carry oxygen), white blood cells (which are components of the immune system, and platelets (cell fragments that allow blood to clot).

Nerve Cells

Nerve cells (AKA neurons ) make up the nervous system. Their key function is to carry messages between the body and the brain.

Fat cells are also known as adipose cells. They are specially adapted to store energy in the form of fat tissue and are found all over the body. The fat tissue that is stored beneath the skin is called subcutaneous fat, and the fat that surrounds the internal organs is known as visceral fat.

Sperm Cells

Sperm cells are male reproductive cells. They are the smallest type of cell in the human body and have several adaptations for fertilization, such as a tail (for swimming) and lots of mitochondria (for energy production).

Female reproductive cells are called egg cells. Egg cells are much larger than sperm cells and contain a large amount of nutrient-rich cytoplasm to support the development of the new organism.

Sperm and egg cells are reproductive cells

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Advancements in utilizing natural compounds for modulating autophagy in liver cancer: molecular mechanisms and therapeutic targets.

1. Introduction

2. molecular mechanism of autophagy in hcc, 2.1. a dual role in hcc, 2.2. signaling cascades of autophagy in hcc, 3. natural compounds targeting autophagy in hepatocellular carcinoma, 4. recent advances in the use of natural compounds in liver cancer, 5. preclinical and clinical evidence of ncs directing liver cancer via autophagy, 6. current challenges and future directions for the use of autophagy-mediated natural compound in liver disease, 7. conclusions, author contributions, conflicts of interest.

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Click here to enlarge figure

Compounds	Source	Dosage	Administration Method	Mechanism of Action	Potential Side Effects	References
Curcumin	Turmeric	100–200 mg/kg	Oral	Inhibits mTOR signaling	Gastrointestinal discomfort	[ ]
Resveratrol	Grapes	10–100 mg/kg	Oral	Activates AMPK pathway	Headache, dizziness	[ ]
Berberine	Goldenseal	5–50 mg/kg	Oral	Induces autophagic cell death	Nausea, constipation	[ ]
Quercetin	Onions	25–50 mg/kg	Oral	Inhibits PI3K/Akt pathway	Headache, upset stomach	[ ]
Epigallocatechin	Green Tea	50–200 mg/kg	Oral	Modulates Beclin-1 and Bcl-2	Liver toxicity at high doses	[ ]
Genistein	Soybeans	10–50 mg/kg	Oral	Inhibits Akt/mTOR signaling	Gastrointestinal issues	[ ]
Lycopene	Tomatoes	10–50 mg/kg	Oral	Induces autophagy via AMPK	Low toxicity, rare allergic reactions	[ ]
Apigenin	Parsley	10–40 mg/kg	Oral	Inhibits PI3K/Akt pathway	Mild gastrointestinal discomfort	[ ]
Baicalein	Scutellaria	10–50 mg/kg	Oral	Modulates autophagy and apoptosis	Nausea, vomiting	[ ]
Honokiol	Magnolia	5–50 mg/kg	Oral, intravenous	Activates AMPK and inhibits mTOR	Sedation, drowsiness	[ ]
Silibinin	Milk Thistle	100–300 mg/kg	Oral	Inhibits mTOR and activates AMPK	Mild gastrointestinal issues	[ ]
Withaferin A	Ashwagandha	5–20 mg/kg	Oral	Modulates p62/SQSTM1 pathway	Nausea, skin rash	[ ]
Emodin	Rhubarb	10–40 mg/kg	Oral	Inhibits PI3K/Akt/mTOR signaling	Diarrhea, abdominal pain	[ ]
Diosgenin	Fenugreek	10–50 mg/kg	Oral	Inhibits Akt/mTOR signaling	Mild gastrointestinal discomfort	[ ]
Plumbagin	Black Walnut	2–10 mg/kg	Oral	Induces autophagy via ROS	Hemolysis, nephrotoxicity	[ ]
Ursolic Acid	Apple Peel	10–50 mg/kg	Oral	Inhibits mTOR signaling	Mild gastrointestinal discomfort	[ ]
Fisetin	Strawberries	10–50 mg/kg	Oral	Inhibits PI3K/Akt/mTOR pathway	Mild gastrointestinal issues	[ ]
Luteolin	Celery	10–50 mg/kg	Oral	Inhibits Akt/mTOR signaling	Mild gastrointestinal discomfort	[ ]
Ginsenoside Rg3	Ginseng	5–30 mg/kg	Oral, intravenous	Inhibits mTOR and induces autophagy	Mild gastrointestinal discomfort	[ ]
Capsaicin	Chili Peppers	2–10 mg/kg	Oral	Activates AMPK pathway	Gastrointestinal irritation

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Rahman, M.A.; Rakib-Uz-Zaman, S.M.; Chakraborti, S.; Bhajan, S.K.; Gupta, R.D.; Jalouli, M.; Parvez, M.A.K.; Shaikh, M.H.; Hoque Apu, E.; Harrath, A.H.; et al. Advancements in Utilizing Natural Compounds for Modulating Autophagy in Liver Cancer: Molecular Mechanisms and Therapeutic Targets. Cells 2024 , 13 , 1186. https://doi.org/10.3390/cells13141186

Rahman MA, Rakib-Uz-Zaman SM, Chakraborti S, Bhajan SK, Gupta RD, Jalouli M, Parvez MAK, Shaikh MH, Hoque Apu E, Harrath AH, et al. Advancements in Utilizing Natural Compounds for Modulating Autophagy in Liver Cancer: Molecular Mechanisms and Therapeutic Targets. Cells . 2024; 13(14):1186. https://doi.org/10.3390/cells13141186

Rahman, Md Ataur, S M Rakib-Uz-Zaman, Somdeepa Chakraborti, Sujay Kumar Bhajan, Rajat Das Gupta, Maroua Jalouli, Md. Anowar Khasru Parvez, Mushfiq H. Shaikh, Ehsanul Hoque Apu, Abdel Halim Harrath, and et al. 2024. "Advancements in Utilizing Natural Compounds for Modulating Autophagy in Liver Cancer: Molecular Mechanisms and Therapeutic Targets" Cells 13, no. 14: 1186. https://doi.org/10.3390/cells13141186

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Lack of cytotoxic and genotoxic effects of mPEG-silane coated iron(III) oxide nanoparticles doped with magnesium despite cellular uptake in cancerous and noncancerous lung cells

Sikorska, Malgorzata
Ruzycka-Ayoush, Monika
Rios-Mondragon, Ivan
Longhin, Eleonora Marta
Meczynska-Wielgosz, Sylwia
Wojewodzka, Maria
Kowalczyk, Agata
Kasprzak, Artur
Nowakowska, Julita
Sobczak, Kamil
Muszynska, Magdalena
Cimpan, Mihaela Roxana
Runden-Pran, Elise
Shaposhnikov, Sergey
Kruszewski, Marcin
Dusinska, Maria
Nowicka, Anna M.
Grudzinski, Ireneusz P.

Cytotoxic and genotoxic effects of novel mPEG-silane coated iron(III) oxide nanoparticles doped with magnesium (Mg 0.1 -γ-Fe 2 O 3 (mPEG-silane) 0.5 ) have been investigated on human adenocarcinomic alveolar basal epithelial (A549) and human normal bronchial epithelial (BEAS-2B) cells. In the studies several molecular and cellular targets addressing to cell membrane, cytoplasm organelles and nucleus components were served as toxicological endpoints. The as-synthesized nanoparticles were found to be stable in the cell culture media and were examined for different concentration and exposure times. No cytotoxicity of the tested nanoparticles was found although these nanoparticles slightly increased reactive oxygen species in both cell types studied. Mg 0.1 -γ-Fe 2 O 3 (mPEG-silane) 0.5 nanoparticles did not produce any DNA strand breaks and oxidative DNA damages in A549 and BEAS-2B cells. Different concentration of Mg 0.1 -γ-Fe 2 O 3 (mPEG-silane) 0.5 nanoparticles and different incubation time did not affect cell migration. The lung cancer cells' uptake of the nanoparticles was more effective than in normal lung cells. Altogether, the results evidence that mPEG-silane coated iron(III) oxide nanoparticles doped with magnesium do not elucidate any deleterious effects on human normal and cancerous lung cells despite cellular uptake of these nanoparticles. Therefore, it seems reasonable to conclude that these novel biocompatible nanoparticles are promising candidates for further development towards medical applications.

iron oxide nanoparticles;
mPEG-silane;
Cytotoxicity;
Genotoxicity;

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CELL ORGANELLES CLASS 9TH BIOLOGY LECTURE 3 CELL
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Cell Organelles
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An organelle is a compartment within a eukaryotic cell that has a specific function. The name "organelle" comes from the idea that these structures are to cells what an organ is to the body. Typically, organelles are enclosed within their own lipid bilayers. They are essential for various cellular functions, including energy production ...
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Cell membrane	A double membrane composed of lipids and proteins. Present both in plant and animal cells.	Provides shape, protects the inner organelles of the cell and acts as a selectively permeable membrane.
Centrosomes	Composed of centrioles and found only in the animal cells.	It plays a major role in organizing the microtubule and cell division.
Chloroplasts	Present only in plant cells and contains a green-coloured pigment known as chlorophyll.	Sites of photosynthesis.
Cytoplasm	A jelly-like substance, which consists of water, dissolved nutrients and waste products of the cell.	Responsible for the cell’s metabolic activities.
Endoplasmic Reticulum	A network of membranous tubules, present within the cytoplasm of a cell.	Forms the skeletal framework of the cell, involved in the detoxification, production of lipids and proteins.
Golgi apparatus	Membrane-bound, sac-like organelles, present within the cytoplasm of the eukaryotic cells.	It is mainly involved in secretion and intracellular transport.
Lysosomes	A tiny, circular-shaped, single membrane-bound organelles, filled with digestive enzymes.	Helps in the digestion and removes wastes and digests dead and damaged cells. Therefore, it is also called as the “suicidal bags”.
Mitochondria	An oval-shaped, membrane-bound organelle, also called as the “Powerhouse of The Cell”.	The main site of cellular respiration and also involved in storing energy in the form of ATP molecules.
Nucleus	The largest, double membrane-bound organelles, which contains all the cell’s genetic information.	Controls the activity of the cell, helps in cell division and controls the hereditary characters.
Peroxisome	A membrane-bound cellular organelle present in the cytoplasm, which contains the reducing enzyme.	Involved in the metabolism of lipids and catabolism of long-chain fatty acids.
Plastids	Double membrane-bound organelles. There are 3 types of plastids: –Colourless plastids. –Blue, red, and yellow colour plastids. – Green coloured plastids.	Helps in the process of photosynthesis and pollination, imparts colour to leaves, flowers, fruits and stores starch, proteins and fats.
Ribosomes	Non-membrane organelles, found floating freely in the cell’s cytoplasm or embedded within the endoplasmic reticulum.	Involved in the synthesis of proteins.
Vacuoles	A membrane-bound, fluid-filled organelle found within the cytoplasm.	Provide shape and rigidity to the plant cell and help in digestion, excretion, and storage of substances.

Cell Organelles

What are Cell Organelles?

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Cell Organelles: Definition, Structure, Functions, Diagram

List of 24 Cell Organelles

Structure of Cell Membrane

Functions of Cell Membrane

Structure of Cell Wall

Functions of Cell Wall

Structure of Centriole

Functions of Centriole

Structure of Cilia and Flagella

Functions of Cilia and Flagella

Structure of Chloroplast

Functions of Chloroplast

Structure of Cytoplasm

Functions of Cytoplasm

Structure of Cytoskeleton

Functions of Cytoskeleton

Structure of Endoplasmic Reticulum (ER)

Functions of Endoplasmic Reticulum (ER)

Structure of Endosomes

Functions of Endosomes

Structure of Golgi Apparatus

Functions of Golgi Apparatus

Structure of Intermediate filaments

Functions of Intermediate filaments

Structure of Lysozyme

Functions of Lysozyme

Structure of Microfilaments

Functions of Microfilaments

Structure of Microtubules

Functions of Microtubules

Structure of Microvilli

Functions of Microvilli

Structure of Mitochondria

Functions of Mitochondria

Structure of Nucleus

Functions of Nucleus

Structure of Peroxisomes

Functions of Peroxisomes

Structure of Plasmodesmata

Functions of Plasmodesmata

Structure of Plastids

Functions of Plastids

Structure of Ribosomes

Functions of Ribosomes

Structure of Storage granules

Figure: Diagram of Storage Granules. Image Source: Slide Player

Structure of Vacuoles

Functions Vacuoles

Structure of Vesicles

Figure: A liposome (left) and dendrimersome. The blue parts of their molecules are hydrophilic, the green parts are hydrophobic. Credit: Image courtesy of University of Pennsylvania

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Organelles – Definition, List, and Functions

Importance of Organelles

History and Etymology

List of Organelles

Animal Cell Organelles

Plant Cell Organelles

Membrane-Bound vs. Non-Membrane-Bound Organelles

Membrane-Bound Organelles

Non-Membrane-Bound Organelles

Organelles in Eukaryotic vs. Prokaryotic Cells

Origin of Organelles

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Cell Organelles

1. Cell Membrane

4. Endoplasmic Reticulum (ER)

5. Ribosomes

6. Golgi Apparatus

7. Mitochondria

8. Peroxisomes

9. Cytoskeleton

Organelles Found Only in Plant Cells

2. Plastids

Organelles Found Only in Animal Cells