• 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

Let us learn more in detail about the different types and functions of Cell Organelles.

Table of Contents

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.

different types of cell organelles essay

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.

Nucleus Cell Organelles

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.

Also read about the Nucleus

The Endoplasmic Reticulum is a network of membranous canals filled with fluid. They are the transport system of the cell, involved in transporting materials throughout the cell. There are two different types of Endoplasmic Reticulum:

  • Rough Endoplasmic Reticulum – They are composed of cisternae, tubules, and vesicles, which are found throughout the cell and are involved in protein manufacture.
  • Smooth Endoplasmic Reticulum – They are the storage organelle, associated with the production of lipids, steroids, and also responsible for detoxifying the cell.

Also, read about  Endoplasmic Reticulum


Mitochondria are called the powerhouses of the cell as they produce energy-rich molecules for the cell. The mitochondrial genome is inherited maternally in several organisms. It is a double membrane-bound, sausage-shaped organelle, found in almost all eukaryotic cells.

The double membranes divide its lumen into two distinct aqueous compartments. The inner compartment is called a ‘matrix’ which is folded into cristae whereas the outer membrane forms a continuous boundary with the cytoplasm. They usually vary in their size and are found either round or oval in shape. Mitochondria are the sites of aerobic respiration in the cell, produces energy in the form of ATP and helps in the transformation of the molecules.

For instance, glucose is converted into adenosine triphosphate – ATP. Mitochondria have their own circular DNA, RNA molecules, ribosomes (the 70s), and a few other molecules that help in protein synthesis.

Also read about Mitochondria

Plastids are large, membrane-bound organelles which contain pigments. Based on the type of pigments, plastids are of three types:


  • Chloroplasts – Chloroplasts are double membrane-bound organelles, which usually vary in their shape – from a disc shape to spherical, discoid, oval and ribbon. They are present in mesophyll cells of leaves, which store chloroplasts and other carotenoid pigments. These pigments are responsible for trapping light energy for photosynthesis. The inner membrane encloses a space called the stroma. Flattened disc-like chlorophyll-containing structures known as thylakoids are arranged in a stacked manner like a pile of coins. Each pile is called a granum (plural: grana) and the thylakoids of different grana are connected by flat membranous tubules known as stromal lamella. Just like the mitochondrial matrix, the stroma of chloroplast also contains a double-stranded circular DNA, 70S ribosomes, and enzymes which are required for the synthesis of carbohydrates and proteins.
  • Chromoplasts – The chromoplasts include fat-soluble, carotenoid pigments like xanthophylls, carotene, etc. which provide the plants with their characteristic color – yellow, orange, red, etc.
  • Leucoplasts – Leucoplasts are colorless plastids which store nutrients. Amyloplasts store carbohydrates (like starch in potatoes), aleuroplasts store proteins, and elaioplasts store oils and fats.

Also read about Plastids


Ribosomes are non membrane-bound and important cytoplasmic organelles found in close association with the endoplasmic reticulum. Ribosomes are found in the form of tiny particles in a large number of cells and are mainly composed of 2/3rd of RNA and 1/3rd of protein. They are named as the 70s (found in prokaryotes) or 80s (found in eukaryotes) The letter S refers to the density and the size, known as Svedberg’s Unit. Both 70S and 80S ribosomes are composed of two subunits. Ribosomes are either encompassed within the endoplasmic reticulum or are freely traced in the cell’s cytoplasm. Ribosomal RNA and Ribosomal proteins are the two components that together constitute ribosomes. The primary function of the ribosomes includes protein synthesis in all living cells that ensure the survival of the cell.

Also read about Ribosomes

Golgi Apparatus is also termed as Golgi Complex. It is a membrane-bound organelle, which is mainly composed of a series of flattened, stacked pouches called cisternae. This cell organelle is primarily responsible for transporting, modifying, and packaging proteins and lipids to targeted destinations. Golgi Apparatus is found within the cytoplasm of a cell and is present in both plant and animal cells.

Golgi Apparatus

Also read about the Golgi Apparatus


Microbodies are membrane-bound, minute, vesicular organelles, found in both plant and animal cells . They contain various enzymes and proteins and can be visualized only under the electron microscope.

Also read about  Microbodies

It is a continuous network of filamentous proteinaceous structures that run throughout the cytoplasm, from the nucleus to the plasma membrane. It is found in all living cells, notably in the eukaryotes. The cytoskeleton matrix is composed of different types of proteins that can divide rapidly or disassemble depending on the requirement of the cells. The primary functions include providing the shape and mechanical resistance to the cell against deformation, the contractile nature of the filaments helps in motility during cytokinesis.

Also read about  Cytoskeleton

Cilia and Flagella

Cilia are hair-like projections, small structures, present outside the cell wall and work like oars to either move the cell or the extracellular fluid. Flagella are slightly bigger and are responsible for the cell movements. The eukaryotic flagellum structurally differs from its prokaryotic counterpart. The core of the cilium and flagellum is called an axoneme, which contains nine pairs of gradually  arranged peripheral  microtubules and a set of central microtubules running parallel to the axis. The central tubules are interconnected by a bridge and are embedded by a central sheath. One of the peripheral microtubular pairs is also interconnected to the central sheath by a radial spoke. Hence there are a total of 9 radial spokes. The cilia and flagella emerge from centriole-like structures called basal bodies.

Also read about the Difference Between Cilia And Flagella

Centrosome and Centrioles

The centrosome organelle is made up of two mutually perpendicular structures known as centrioles. Each centriole is composed of 9 equally spaced peripheral fibrils of tubulin protein, and the fibril is a set of interlinked triplets. The core part of the centriole is known as a hub and is proteinaceous. The hub connects the peripheral fibrils via radial spoke, which is made up of proteins. The centrioles from the basal bodies of the cilia and flagella give rise to spindle fibres during cell division.

Also read about  Centrosomes

Vacuoles are mostly defined as storage bubbles of irregular shapes which are found in cells. They are fluid-filled organelles enclosed by a membrane. The vacuole stores the food or a variety of nutrients that a cell might need to survive. In addition to this, it also stores waste products. The waste products are eventually thrown out by vacuoles. Thus, the rest of the cell is protected from contamination. The animal and plant cells   have different size and number of vacuoles. Compared to the animals, plant cells have larger vacuoles.

Also read about  Vacuoles

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.

Stay tuned with BYJU’S to learn more about the different types of Cell Organelles, their functions and other related topics at   BYJU’S Biology

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The Cellular Level of Organization

The Cytoplasm and Cellular Organelles


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] ).

This figure shows structure of the endoplasmic reticulum. The diagram highlights the rough and smooth endoplasmic reticulum and the nucleus is labeled. Two micrographs show the structure of the endoplasmic reticulum in detail. The left micrograph shows the rough endoplasmic reticulum in a pancreatic cell and the right micrograph shows a smooth endoplasmic reticulum.

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] ).

This figure shows the structure of the Golgi apparatus. The diagram in the left panel shows the location and structure of the Golgi apparatus. The right panel shows a micrograph showing the folds of the Golgi in detail.

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”).

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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.


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.


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:


The Cytoplasm and Cellular Organelles Copyright © 2013 by OpenStaxCollege is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Biology LibreTexts

5.6: Cell Organelles

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  • Suzanne Wakim & Mandeep Grewal
  • Butte College

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Ribosome Review

Figure \(\PageIndex{1}\) represents an important structure in living cells. It is a component of a ribosome, the cell structure where proteins are synthesized. Large ribosomal subunit (50S) of Haloarcula marismortui , facing the 30S subunit. The ribosomal proteins are shown in blue, the rRNA in ochre (a shade of brown and yellow), the active site in red. All living cells contain ribosomes, whether they are prokaryotic or eukaryotic cells. However, only eukaryotic cells also contain a nucleus and several other types of organelles.

50S subunit of the ribosome ribbon model

An organelle is a structure within the cytoplasm of a eukaryotic cell that is enclosed within a membrane and performs a specific job. Organelles are involved in many vital cell functions. Organelles in animal cells include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, vesicles, and vacuoles. Ribosomes are not enclosed within a membrane but are still commonly referred to as organelles in eukaryotic cells.

The Nucleus

The nucleus is the largest organelle in a eukaryotic cell and is considered to be the cell’s control center. It contains most of the cell’s DNA, which makes up chromosomes and is encoded with the genetic instructions for making proteins. The function of the nucleus is to regulate gene expression, including controlling which proteins the cell makes. In addition to DNA, the nucleus contains a thick liquid called nucleoplasm that is similar in composition to the cytosol found in the cytoplasm outside the nucleus (Figure \(\PageIndex{2}\)). Most eukaryotic cells contain just a single nucleus, but some types of cells, such as red blood cells, contain no nucleus. A few other types of cells, such as muscle cells, contain multiple nuclei.

Cell Nucleus

As you can see from the model in Figure \(\PageIndex{2}\), the membrane enclosing the nucleus is called the nuclear envelope . This is actually a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm. Tiny holes, called nuclear pores , allow large molecules to pass through the nuclear envelope with the help of special proteins. Large proteins and RNA molecules must be able to pass through the nuclear envelope so proteins can be synthesized in the cytoplasm and the genetic material can be maintained inside the nucleus. The nucleolus shown in the model below is mainly involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they are involved in the synthesis of proteins.


The mitochondrion (plural, mitochondria) is an organelle that makes energy available to the cell (Figure \(\PageIndex{3}\)). This is why mitochondria are sometimes referred to as the power plants of the cell. They use energy from organic compounds such as glucose to make molecules of ATP (adenosine triphosphate) , an energy-carrying molecule that is used almost universally inside cells for energy.

Scientists think that mitochondria were once free-living organisms because they contain their own DNA. They theorize that ancient prokaryotes infected (or were engulfed by) larger prokaryotic cells, and the two organisms evolved a symbiotic relationship that benefited both of them. The larger cells provided the smaller prokaryotes with a place to live. In return, the larger cells got extra energy from the smaller prokaryotes. Eventually, the smaller prokaryotes became permanent guests of the larger cells, as organelles inside them. This theory is called the endosymbiotic theory , and it is widely accepted by biologists today

Mitochondrial Compartments

The double membrane nature of the mitochondria results in five distinct compartments, each with an important role in cellular respiration. These compartments are:

  • the outer mitochondrial membrane,
  • the intermembrane space (the space between the outer and inner membranes),
  • the inner mitochondrial membrane,
  • the cristae (formed by infoldings of the inner membrane), and
  • the matrix (space within the inner membrane).

Endoplasmic Reticulum

The endoplasmic reticulum (ER) (plural, reticuli) is a network of phospholipid membranes that form hollow tubes, flattened sheets, and round sacs. These flattened, hollow folds and sacs are called cisternae . The ER has two major functions:

  • Transport: Molecules, such as proteins, can move from place to place inside the ER, much like on an intracellular highway.
  • Synthesis: Ribosomes that are attached to the ER, similar to unattached ribosomes, make proteins. Lipids are also produced in the ER.

There are two types of endoplasmic reticulum, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER):

  • Rough endoplasmic reticulum is studded with ribosomes, which gives it a “rough” appearance. These ribosomes make proteins that are then transported from the ER in small sacs called transport vesicles. The transport vesicles pinch off the ends of the ER. The rough endoplasmic reticulum works with the Golgi apparatus to move new proteins to their proper destinations in the cell. The membrane of the RER is continuous with the outer layer of the nuclear envelope.
  • Smooth endoplasmic reticulum does not have any ribosomes attached to it, and so it has a smooth appearance. SER has many different functions, some of which include lipid synthesis, calcium ion storage, and drug detoxification. The smooth endoplasmic reticulum is found in both animal and plant cells and it serves different functions in each. The SER is made up of tubules and vesicles that branch out to form a network. In some cells, there are dilated areas like the sacs of RER. Smooth endoplasmic reticulum and RER form an interconnected network.

One drawings and two micrographs of smooth and rough Endoplasmic Reticulum

Golgi Apparatus

The Golgi apparatus (Figure \(\PageIndex{5}\)) is a large organelle that processes proteins and prepares them for use both inside and outside the cell. It was identified in 1898 by the Italian physician Camillo Golgi. The Golgi apparatus modifies, sorts, and packages different substances for secretion out of the cell, or for use within the cell. The Golgi apparatus is found close to the nucleus of the cell where it modifies proteins that have been delivered in transport vesicles from the Rough Endoplasmic Reticulum. It is also involved in the transport of lipids around the cell. Pieces of the Golgi membrane pinch off to form vesicles that transport molecules around the cell. The Golgi apparatus can be thought of as similar to a post office; it packages and labels "items" and then sends them to different parts of the cell. The Golgi apparatus tends to be larger and more numerous in cells that synthesize and secrete large quantities of materials; for example, the plasma B cells and the antibody-secreting cells of the immune system have prominent Golgi complexes.

The Golgi apparatus manipulates products from the Rough Endoplasmic Reticulum (ER) and also produces new organelles called lysosomes. 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 lysosomes.

Golgi Apparatus involved in endomembrane system export import.

The stack of cisternae has four functional regions: the cis-Golgi network , medial-Golgi, endo-Golgi, and trans-Golgi network. Vesicles from the ER fuse with the network and subsequently progress through the stack from the cis- to the trans-Golgi network , where they are packaged and sent to their destination. Each cisterna includes special Golgi enzymes which modify or help to modify proteins that travel through it. Proteins may be modified by the addition of a carbohydrate group (glycosylation) or phosphate group (phosphorylation). These modifications may form a signal sequence on the protein, which determines the final destination of the protein. For example, the addition of mannose-6-phosphate signals the protein for lysosomes.

Vesicles and Vacuoles

Both vesicles and vacuoles are sac-like organelles that store and transport materials in the cell. Vesicles are much smaller than vacuoles and have a variety of functions. The vesicles that pinch off from the membranes of the ER and Golgi apparatus store and transport protein and lipid molecules. You can see an example of this type of transport vesicle in the figure above. Some vesicles are used as chambers for biochemical reactions. Other vesicles include:

  • Lysosomes, which use enzymes to break down foreign matter and dead cells.
  • Peroxisomes, which use oxygen to break down poisons.
  • Transport vesicles, transport contents between organelle as well as between cell exterior and interior.

Centrioles are organelles involved in cell division. The function of centrioles is to help organize the chromosomes before cell division occurs so that each daughter cell has the correct number of chromosomes after the cell divides. Centrioles are found only in animal cells and are located near the nucleus. Each centriole is made mainly of a protein named tubulin . The centriole is cylindrical in shape and consists of many microtubules, as shown in the model pictured below.


Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome is formed of two subunits, like the one pictured at the top of this section. Both subunits consist of proteins and RNA. RNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in RNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm as well as on the RER.

  • Define organelle.
  • Describe the structure and function of the nucleus.
  • Explain how the nucleus, ribosomes, rough endoplasmic reticulum, and Golgi apparatus work together to make and transport proteins.
  • Why are mitochondria referred to as the power plants of the cell?
  • What roles are played by vesicles and vacuoles?
  • Why do all cells need ribosomes, even prokaryotic cells that lack a nucleus and other cell organelles?
  • Explain endosymbiotic theory as it relates to mitochondria. What is one piece of evidence that supports this theory?
  • A. Organelles
  • B. Vesicles
  • C. Vacuoles
  • D. Both A and B
  • a. Contains the genetic instructions for the production of proteins
  • b. Organizes chromosomes before cell division
  • c. Provides a framework for ribosomes
  • d. Packages and labels proteins
  • e. Assembles ribosomes
  • True or False. All eukaryotic cells have a nucleus.
  • True or False. The outer surface of the nucleus of a eukaryotic cell is not completely solid.

Explore More


  • 50S-subunit of the ribosome by Yikrazuul , licensed CC BY-SA 3.0 via Wikimedia Commons
  • Cell nucleus by Blausen.com staff (2014). " Medical gallery of Blausen Medical 2014 ". WikiJournal of Medicine 1 (2). DOI : 10.15347/wjm/2014.010 . ISSN 2002-4436 . licensed CC BY 3.0 via Wikimedia Commons
  • Animal mitochondrion by LadyofHats , released into the public domain via Wikimedia Commons
  • Endoplasmic reticulum by OpenStax, licensed CC BY 4.0 via Wikimedia Commons
  • Golgi Apparatus by Openstax , licensed CC BY 4.0 via Wikimedia Commons
  • Centrioles by Blausen.com staff (2014). " Medical gallery of Blausen Medical 2014 ". WikiJournal of Medicine 1 (2). DOI : 10.15347/wjm/2014.010 . ISSN 2002-4436 . licensed CC BY 3.0 via Wikimedia Commons
  • Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0


Cell organelles.

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Every cell in your body contains organelles (structures that have specific functions). Just like organs in the body, each organelle contributes in its own way to helping the cell function well as a whole. The nucleus, mitochondria and chloroplasts are all organelles.

Chloroplasts (left) Storage granules (centre) Microvilli (right)

Specialised organelles

Some organelles are found only in some cell types. They have roles that are important to the specific function of that cell type. Chloroplasts (left) are the site of photosynthesis in plant cells, storage granules (centre) provide a storage site for proteins in secreting cells, and microvilli (right) aid absorption of nutrients during digestion by increasing the surface area of cells in the intestinal wall.

Despite their central importance to cell function (and therefore to all life), organelles have only been studied closely following the invention of the transmission electron microscope , which allowed them to be seen in detail for the first time.

Core organelles

Core organelles are found in virtually all eukaryotic cells. They carry out essential functions that are necessary for the survival of cells – harvesting energy, making new proteins, getting rid of waste and so on. Core organelles include the nucleus, mitochondria, endoplasmic reticulum and several others. The primary cilium (which has recently been shown to help cells sense their surroundings) may also be a core organelle because it seems to be present on most cells.

Primary cilium under a transmission electron microscope (TEM)

Primary cilium (width)

The primary cilium is a small organelle that acts like an antenna, co-ordinating information about the cell’s surroundings. At just 200 nm wide, the primary cilium is only just big enough to be viewed through an optical microscope, but its structure can be studied in detail by using a transmission electron microscope (TEM).

Associate Professor Tony Poole uses TEM microscopes to unlock the mystery of how the primary cilium works . Tony’s story is an excellent example of the changing nature of scientific knowledge and how new information can change the way we think about things.

Micrograph image: Tony Poole

Different types of cells have different amounts of some organelles. For instance, cells that use a lot of energy tend to contain large numbers of mitochondria (the organelle responsible for harvesting energy from food). That’s why very active muscle cells are often full of mitochondria.

Some cell types have their own specialised organelles that carry out functions that aren’t required by all cells. Here are just a few of the specialised organelles that we know about:

  • Chloroplasts are found in plant cells and other organisms that conduct photosynthesis (such as algae). They are the site where photosynthesis occurs.
  • Storage granules are found in cells that produce a lot of material for secretion (release from the cell). For instance, some pancreas cells (which make insulin for release into the bloodstream) contain large numbers of storage granules that store insulin until the cell receives a signal to release it.
  • Microvilli are tiny finger-like protrusions on the surface of a cell. Their main function is to increase the surface area of the part of the cell in which they’re found. Cells in the intestinal wall have many microvilli so they can absorb as many nutrients as possible from the gut.

Diagram of a Core cell organelles with detailed info.

Core cell organelles

Some organelles are found in virtually every eukaryotic cell. These organelles have key roles that are important to all cells, such as making energy available and synthesising proteins.

Location, location, location

Within cells, organelles tend to cluster close to where they do their job. In sperm cells, for instance, mitochondria are concentrated around the base of the tail, where they provide energy for the sperm’s rapid ‘swim’ towards the ovum during fertilisation. In intestinal wall cells, microvilli are clustered on the side of the cell that faces the intestinal space so that the cells maximise their surface area for absorbing nutrients.

Zooming in on organelles

Microscopes have been crucial for understanding organelles. In fact, without microscopes, we wouldn’t even know that organelles existed! However, most organelles are not clearly visible by light microscopy, and those that can be seen (such as the nucleus, mitochondria and Golgi) can’t be studied in detail because their size is close to the limit of resolution of the light microscope. The detailed structure of organelles only became clear after the development of the transmission electron microscope (TEM), which made it possible to look at individual organelles at high resolution.

Mitochondria under electron microscopy and electron tomography.

Mitochondria under the microscope

Microscopes have been crucial for our understanding of mitochondrial structure and function. Mitochondria are visible under the light microscope although little detail can be seen. Transmission electron microscopy (left) shows the complex internal membrane structure of mitochondria, and electron tomography (right) gives a three-dimensional view.

Having detailed information about organelle structure has been very important for understanding how they work. For instance, the TEM showed that mitochondria contained two membranes and that the inner one was highly folded inside the outer one. This helped scientists to understand how mitochondria harvest energy from food.

Related content

Use these articles to explore investigating what is inside our bodies and how they work, whilst also understanding the important role microscopes play.

  • Animal cells and their shapes
  • A closer look at the cell’s antenna
  • Mitochondria – cell powerhouses
  • The human digestive system
  • Preparing samples for the electron microscope
  • The microscopic scale

Activity ideas

In Modelling animal cells in 3D , students make 3D models of specialised animal cells, imitating what can be seen under high-resolution microscopes.

In the Inside a cell activity, students learn about the contents of a cell. They explore some of the main organelles within a cell using the analogy of a school, an online game and/or by making something edible.

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Medicine LibreTexts

4.1: Cell Structure and Function

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Learning ObjectiveS

  • Define a cell, identify the main common components of human cells, and differentiate between intracellular fluid and extracellular fluid
  • Describe the structure and functions of the plasma (cell) membrane
  • Describe the nucleus and its function
  • Identify the structure and function of cytoplasmic organelles

A cell is the smallest living thing in the human organism, and all living structures in the human body are made of cells. There are hundreds of different types of cells in the human body, which vary in shape (e.g. round, flat, long and thin, short and thick) and size (e.g. small granule cells of the cerebellum in the brain (4 micrometers), up to the huge oocytes (eggs) produced in the female reproductive organs (100 micrometers) and function. However, all cells have three main parts, the plasma membrane , the cytoplasm and the nucleus. The plasma membrane (often called the cell membrane) is a thin flexible barrier that separates the inside of the cell from the environment outside the cell and regulates what can pass in and out of the cell. Internally, the cell is divided into the cytoplasm and the nucleus. The cytoplasm ( cyto- = cell; - plasm = “something molded”) is where most functions of the cell are carried out. It looks a bit-like mixed fruit jelly, where the watery jelly is called the cytosol ; and the different fruits in it are called organelles . The cytosol also contains many molecules and ions involved in cell functions. Different organelles also perform different cell functions and many are also separated from the cytosol by membranes. The largest organelle, the nucleus is separated from the cytoplasm by a nuclear envelope (membrane). It contains the DNA (genes) that code for proteins necessary for the cell to function.

Generally speaking, the inside environment of a cell is called the intracellular fluid (ICF) , (intra- = within; referred to all fluid contained in cytosol, organelles and nucleus) while the environment outside a cell is called the extracellular fluid (ECF) (extra- = outside of; referred to all fluid outside cells). Plasma, the fluid part of blood, is the only ECF compartment that links all cells in the body.


Figure \(\PageIndex{1}\) 3-D representation of a simple human cell. The top half of the cell volume was removed. Number 1 shows the nucleus, numbers 3 to 13 show different organelles immersed in the cytosol, and number 14 on the surface of the cell shows the plasma membrane

Concepts, terms, and facts check

Study Questions Write your answer in a sentence form (do not answer using loose words)

1. What is a cell? 2. What is a plasma membrane? 3. What is a cytoplasm? 4. What is the intracellular fluid (ICF)? 5. What is the extracellular fluid (ECF)?

The plasma (cell) membrane separates the inner environment of a cell from the extracellular fluid. It is composed of a fluid phospholipid bilayer (two layers of phospholipids) as shown in figure \(\PageIndex{2}\) below, and other molecules. Not many substances can cross the phospholipid bilayer, so it serves to separate the inside of the cell from the extracellular fluid. Other molecules found in the membrane include cholesterol, proteins, glycolipids and glycoproteins , some of which are shown in figure \(\PageIndex{3}\) below. Cholesterol, a type of lipid, makes the membrane a little stronger. Different proteins found either crossing the bilayer (integral proteins) or on its surface (peripheral proteins) have many important functions. Channel and transporter (carrier) proteins regulate the movement of specific molecules and ions in and out of cells. Receptor proteins in the membrane initiate changes in cell activity by binding and responding to chemical signals, such as hormones (like a lock and key). Other proteins include those that act as structural anchors to bind neighboring cells and enzymes. Glycoproteins and glycolipids in the membrane act as identification markers or labels on the extracellular surface of the membrane. Thus, the plasma membrane has many functions and works as both a gateway and a selective barrier.


Figure \(\PageIndex{2}\) Phospholipids form the basic structure of a cell membrane. Hydrophobic tails of phospholipids are facing the core of the membrane, avoiding contact with the inner and outer watery environment. Hydrophilic heads are facing the surface of the membrane in contact with intracellular fluid and extracellular fluid.


Figure \(\PageIndex{3}\) Small area of the plasma membrane showing lipids (phospholipids and cholesterol), different proteins, glycolipids and glycoproteins.

1. What is the function of the cell membrane? 2. Which are the three types of biomolecules that form the cell membrane?

Almost all human cells contain a nucleus where DNA, the genetic material that ultimately controls all cell processes, is found. The nucleus is the largest cellular organelle, and the only one visible using a light microscope. Much like the cytoplasm of a cell is enclosed by a plasma membrane, the nucleus is surrounded by a nuclear envelope that separates the contents of the nucleus from the contents of the cytoplasm. Nuclear pores in the envelope are small holes that control which ions and molecules (for example, proteins and RNA) can move in and out the nucleus. In addition to DNA, the nucleus contains many nuclear proteins. Together DNA and these proteins are called chromatin . A region inside the nucleus called the nucleolus is related to the production of RNA molecules needed to transmit and express the information coded in DNA. See all these structures below in figure \(\PageIndex{4}\).


Figure \(\PageIndex{4}\) Nucleus of a human cell. Find DNA, nuclear envelope, nucleolus, and nuclear pores. The figure also shows how the outer layer of the nuclear envelope continues as rough endoplasmic reticulum, which will be discussed in the next learning objective.

1. What is the nuclear envelope? 2. What is a nuclear pore? 3. What is the function of the nucleus?

An organelle is any structure inside a cell that carries out a metabolic function. The cytoplasm contains many different organelles, each with a specialized function. (The nucleus discussed above is the largest cellular organelle but is not considered part of the cytoplasm). Many organelles are cellular compartments separated from the cytosol by one or more membranes very similar in structure to the cell membrane, while others such as centrioles and free ribosomes do not have a membrane. See figure \(\PageIndex{5}\) and table \(\PageIndex{1}\) below to learn the structure and functions of different organelles such as mitochondria (which are specialized to produce cellular energy in the form of ATP) and ribosomes (which synthesize the proteins necessary for the cell to function). Membranes of the rough and smooth endoplasmic reticulum form a network of interconnected tubes inside of cells that are continuous with the nuclear envelope. These organelles are also connected to the Golgi apparatus and the plasma membrane by means of vesicles. Different cells contain different amounts of different organelles depending on their function. For example, muscle cells contain many mitochondria while cells in the pancreas that make digestive enzymes contain many ribosomes and secretory vesicles.


Figure \(\PageIndex{5}\) Typical example of a cell containing the primary organelles and internal structures. Table \(\PageIndex{1}\) below describes the functions of mitochondrion, rough and smooth endoplasmic reticulum, Golgi apparatus, secretory vesicles, peroxisomes, lysosomes, microtubules and microfilaments (fibers of the cytoskeleton)

1. What is an organelle? 2. Which are the organelles listed in the module?

Basic Biology

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Introduction to cells

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.


Introduction to cells

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.


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.

Eukaryotic cell

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

Last edited: 30 August 2020

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Plant Cell – Diagram, Organelles, and Characteristics

Labeled Plant Cell Diagram

A plant cell is the basic building block of a plant. Plant cells, like all eukaryotic cells, contain a nucleus and other organelles , each with its distinct functions. However, plant cells also possess unique components that differentiate them from animal, fungal, and bacterial cells.

Plant Cell Characteristics

Plant cells are eukaryotic. In other words, each cell has a true nucleus and membrane-bound organelles. However, plant cells have characteristics that distinguish them from other eukaryotic cells:

Cell Wall: Unlike animal cells , plant cells have a rigid cell wall outside the plasma membrane. This cell wall is primarily composed of cellulose, a complex carbohydrate, that provides structural support, protection and helps maintain the cell’s shape. A fungal cell has a cell wall, but it has a different chemical composition.

Large Central Vacuole: Plant cells have a large central vacuole that stores water, nutrients, and waste products. The central vacuole can occupy up to 90% of the cell’s volume, maintaining turgor pressure that helps the plant stand upright.

Plastids: These are unique organelles found in plant cells. A chloroplasts is a type of plastid that performs photosynthesis, the process that allows plants to convert sunlight into energy.

Autotrophic: Unlike animal or fungal cells, which are heterotrophic (relying on consuming food for energy), plant cells are autotrophic. They produce their own food through photosynthesis, using light energy, carbon dioxide, and water.

Organelles in a Plant Cell and Their Functions

Plant cells comprise several organelles, each with unique functions vital to the cell’s operation:

  • Cell Wall: The cell wall is a rigid layer that provides support, protection, and shape to the cell. It consists mainly of cellulose.
  • Plasma Membrane: This is a semi-permeable membrane that controls the movement of substances in and out of the cell.
  • Nucleus: The nucleus is the control center of the cell, containing DNA that directs all cell activities. The nuclear membrane is a double-membrane structure with nuclear pores that encloses the nucleus and regulates transport and communication with the cytoplasm.
  • Mitochondria : Often referred to as the powerhouse of the cell, mitochondria produce ATP through a process called cellular respiration.
  • Chloroplasts : These are the sites of photosynthesis, where sunlight, carbon dioxide, and water change into glucose and oxygen. Chloroplasts contain a green pigment called chlorophyll, which captures light energy.
  • Peroxisomes : Plant peroxisomes perform a key role in photorespiration and produce plant hormones.
  • Endoplasmic Reticulum (ER): ER is a network of membranous tubules and sacs where lipid synthesis and protein folding occur. The rough ER, studded with ribosomes, synthesizes proteins, while the smooth ER performs lipid synthesis.
  • Golgi Apparatus: The Golgi apparatus modifies, sorts, and packages proteins and lipids for transport to their final destinations within or outside the cell.
  • Ribosomes: These are the sites of protein synthesis. They are either free in the cytoplasm or bound to the ER.
  • Vacuoles: Plant cells contain a large central vacuole that stores water and helps maintain turgor pressure, supporting the cell’s structure. Druse and raphite crystals occur in some plant vacuoles. These crystals of calcium oxalate and calcium carbonates deter herbivores and also store minerals for the cell.
  • Cytoskeleton: Just like in animal cells, plant cells also contain a cytoskeleton. The cytoskeleton is a network of protein fibers in the cytoplasm that provides structural support and plays a key role in cell division and the transport of materials within the cell.
  • Plasmodesmata: These are small tubes that connect plant cells to each other, allowing direct communication and transport of substances between them. This is a unique feature of plant cells not found in animal cells.

A Closer Look at Plastids

While most people know plants cells contain chloroplasts, they may not realize there are other types of plastids in cells. Plastids are a group of plant cell organelles that perform a variety of essential functions. Like mitochondria, plastids have a double-membrane envelope and their own DNA and ribosomes.

Here are some key plastids:

  • Chloroplasts : Chloroplasts contain chlorophyll and perform photosynthesis.
  • Chromoplasts: These plastids synthesize and sore pigments other than chlorophyll. They give fruits, flowers, and aging leaves their yellow, orange, and red colors. These pigments, such as carotenoids and xanthophylls, play a crucial role in attracting pollinators and seed dispersers.
  • Amyloplasts: These specialize in the synthesis and storage of starch granules. They are particularly abundant in storage tissues such as potato tubers. Amyloplasts also play a role in gravity sensing (gravitropism) in roots and shoots.
  • Elaioplasts: Elaioplasts synthesizeand store lipids. They occur in seeds, which store lipids for germination.
  • Proteinoplasts or Aleuroplasts: These plastids specialize in protein storage. They are common in seeds.
  • Gerontoplasts: These plastids develop from chloroplasts during the aging process of plant tissues. They break down chlorophyll and other cellular components during senescence, the final stage of leaf development before leaf fall.

Each type of plastid has the ability to convert into other types depending on the cell’s needs, a phenomenon known as plastid differentiation. For instance, proplastids (undifferentiated plastids) develop into chloroplasts in the presence of light or amyloplasts in the dark.

Key Differences Between Plant and Animal Cells

While plant and animal cells share many similarities as eukaryotes, they also exhibit notable differences :

Cell Wall: Unlike plant cells, animal cells do not have a cell wall. This absence gives animal cells a flexible shape, allowing them to form structures such as neurons and muscle cells.

Vacuoles: Animal cells contain smaller vacuoles and often more than one per cell. In contrast, plant cells typically have a single, large central vacuole.

Centrosomes: Animal cells have centrosomes that help in cell division . Each centrosome contains a pair of centrioles, which are absent in most plant cells.

Plastids: Plant cells contain plastids, such as chloroplasts for photosynthesis, which are absent in animal cells.

Lysosomes: These are more commonly found and active in animal cells, playing a major role in digestion and waste removal. Vacuoles perform lysosome-like functions in plants.

Plant Cells vs Bacterial Cells

While plant cells are eukaryotic, bacterial cells are prokaryotic . This leads to several key differences between the cell types:

  • Nucleus: Plant cells have a well-defined nucleus that houses their DNA. In contrast, bacterial cells do not contain a nucleus. Instead, their DNA is in a region called the nucleoid.
  • Size: Plant cells are generally much larger than bacterial cells. A typical plant cell is around 10 to 100 micrometers in diameter, while a bacterial cell is usually between 0.5 and 5 micrometers.
  • Cell Wall: Both plant and bacterial cells have cell walls, but their chemical composition is different. Plant cell walls contain cellulose, while bacterial cell walls use peptidoglycan.
  • Organelles: Plant cells contain membrane-bound organelles like mitochondria, chloroplasts, and the endoplasmic reticulum. Bacteria lack these organelles.
  • Reproduction: Plant cells reproduce primarily through mitosis, while bacteria mainly reproduce through binary fission, a simpler form of asexual reproduction.

Plant Cell vs Fungal Cell

While both plant and fungal cells are eukaryotic, there are several key differences between them:

  • Cell Wall: The cell walls of plant cells contain cellulose, while fungal cell walls mainly consist of chitin.
  • Nutrition: Plant cells are autotrophic and produce their own food via photosynthesis. Fungal cells, on the other hand, are heterotrophic and obtain their nutrients through absorption. They secrete enzymes that break down complex substances in their environment into simpler compounds that they can absorb.
  • Vacuoles: Both plant and fungal cells contain vacuoles, but plant cells typically have a single large central vacuole, while fungal cells have several smaller vacuoles.
  • Growth: Plant cells grow by dividing and expanding in all directions. Fungal cells grow apically, meaning they extend at their tips.
  • Plastids: Plant cells contain plastids such as chloroplasts, which are absent in fungal cells.

Types of Plant Cells

There are different types of plant cells, each with specific structures and functions. These cells further organize into tissues that perform coordinated functions.

  • Parenchyma Cells: These are the most common type of plant cell. They are involved in many functions such as photosynthesis, storage, and tissue repair. Parenchyma cells typically have a thin cell wall and large central vacuole.
  • Collenchyma Cells: These cells provide support for the plant, particularly in regions of new growth. They have irregularly thick cell walls and are often found under the epidermis, the outermost layer of the plant.
  • Sclerenchyma Cells: These cells also provide support to the plant. They are characterized by their thick, lignified cell walls. There are two types of sclerenchyma cells: fibres, which are long and slender and provide tensile strength; and sclereids, which are shorter and provide compressive strength.
  • Xylem Cells: These cells are involved in the transport of water and minerals from the roots to the rest of the plant. The two primary types of xylem cells are tracheids and vessel elements, both of which are dead at maturity and serve primarily as conduits.
  • Phloem Cells: Phloem cells transport sugars and other nutrients produced by photosynthesis from the leaves to the rest of the plant. Sieve tube elements and companion cells are the main cell types in phloem tissue.

Types of Plant Tissues

There are three main types of differentiated plant tissue, plus there is undifferentiated tissue:

  • Dermal Tissue: This is the outermost layer of the plant (the “skin”), which serves as a protective layer. It includes epidermal cells, guard cells (which regulate the opening and closing of stomata for gas exchange), and in some cases, specialized cells like trichomes (hair-like structures that can have various functions like defense or water retention).
  • Vascular Tissue: This tissue type transports water, nutrients, and sugars. It includes xylem (for water and mineral transport) and phloem (for sugar and nutrient transport).
  • Ground Tissue: This tissue type makes up the bulk of the plant body and performs functions such as photosynthesis, storage, and support. It includes parenchyma, collenchyma, and sclerenchyma cells. There are three categories of ground tissue: pith (innermost), cortex (between pith and vascular tissue), and pericycle (outermost layer of the central vascular cylinder).
  • Meristematic Tissue: This is the tissue in plants that consists of undifferentiated cells capable of division and growth. Meristematic tissue occurs in the root and shoot tips (apical meristem) and the vascular and cork cambium (lateral meristem).

Understanding these different cell types and tissues is crucial for studying plant growth, development, and function.

  • Keegstra, K. (2010). “Plant cell walls”. Plant Physiology. 154 (2): 483–486. doi: 10.1104/pp.110.161240
  • Lew, Kristi; Fitzpatrick, Brad (2021). Plant Cells (3rd ed.). Infobase Holdings, Inc. ISBN 978-1-64693-728-8.
  • Raven, J.A. (1987). “The role of vacuoles”. New Phytologist . 106 (3): 357–422. doi: 10.1111/j.1469-8137.1987.tb00149.x
  • Raven, P.H.; Evert, R.F.; Eichhorm, S.E. (1999). Biology of Plants (6th ed.). New York: W.H. Freeman. ISBN 9780716762843.

Related Posts

Critical Thinking Questions

Why is it advantageous for the cell membrane to be fluid in nature?

Why do phospholipids tend to spontaneously orient themselves into something resembling a membrane?

How can a cell use an extracellular peripheral protein as the receptor to transmit a signal into the cell?

Which explanation identifies how the following affect the rate of diffusion: molecular size, temperature, solution density, and the distance that must be traveled?

Why does water move through a membrane?

Both of the regular intravenous solutions administered in medicine, normal saline and lactated Ringer’s solution, are isotonic. Why is this important?

Describe two ways that decreasing temperature would affect the rate of diffusion of molecules across a cell’s plasma membrane.

A cell develops a mutation in its potassium channels that prevents the ions from leaving the cell. If the cell’s aquaporins are still active, what will happen to the cell? Be sure to describe the tonicity and osmolarity of the cell.

Where does the cell get energy for active transport processes?

How does the sodium-potassium pump contribute to the net negative charge of the interior of the cell?

Glucose from digested food enters intestinal epithelial cells by active transport. Why would intestinal cells use active transport when most body cells use facilitated diffusion?

The sodium/calcium exchanger (NCX) transports sodium into and calcium out of cardiac muscle cells. Describe why this transporter is classified as secondary active transport.

Why is it important that there are different types of proteins in plasma membranes for the transport of materials into and out of a cell?

Why do ions have a difficult time getting through plasma membranes despite their small size?

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A Closer Look at Eukaryotic and Prokaryotic Cells: what Sets them Apart?

This essay about the fundamental differences between eukaryotic and prokaryotic cells provides a clear, engaging overview of their unique characteristics and functions. It begins by introducing prokaryotic cells, which are simpler, smaller, and lack a nucleus, typically found in bacteria and archaea. The essay contrasts these with eukaryotic cells, which are larger, complex, and compartmentalized with a nucleus and various organelles like mitochondria and the Golgi apparatus, making them capable of more advanced functions. It also discusses the distinct mechanisms of DNA replication, transcription, and cell division in both cell types, and how these differences are pivotal for applications in medicine, biotechnology, and agriculture. Through this comparison, the essay underscores the importance of understanding cellular structures in the broader context of biological science and their practical implications in various fields.

How it works

When we talk about cells, it’s not just about tiny biological structures under a microscope; it’s about understanding the core of life itself. Two main characters in the story of cellular biology are eukaryotic and prokaryotic cells, and their differences are not just fundamental, they’re fascinating.

Let’s start with prokaryotic cells. These guys are the old-timers of the cellular world, mostly comprising bacteria and archaea. They’re typically tiny, about 1 to 5 micrometers in diameter, and they keep things simple.

Unlike their eukaryotic counterparts, prokaryotic cells don’t have a nucleus. Their DNA floats freely within the cell in an area called the nucleoid, which isn’t enclosed by any membrane. It’s kind of like having all your clothes strewn around a room instead of tucked away in a closet. They also lack other membrane-bound organelles, like mitochondria or chloroplasts. Instead, their cell membrane might fold in on itself to handle the cell’s metabolic functions. Their DNA is usually in a single circular strand, and it’s not wrapped around proteins like the DNA in eukaryotic cells.

Eukaryotic cells, on the other hand, are like the upgraded model in the world of cells. These are the cells that make up plants, animals, and fungi, and they’re generally larger, between 10 to 100 micrometers. The biggest difference? They have a nucleus. Their DNA is neatly packed away inside this double-membraned organelle, giving the nucleus the nickname “the brain of the cell.” Eukaryotic cells are also full of other specialized compartments, including mitochondria for power generation, the endoplasmic reticulum for making proteins and fats, and the Golgi apparatus for packaging proteins. This complexity allows eukaryotic cells to engage in more advanced and regulated functions.

The differences don’t stop at structure. Eukaryotic and prokaryotic cells also have unique ways of handling life’s blueprint: DNA. Prokaryotes are straightforward, using just one type of RNA polymerase to transcribe their DNA into RNA. Eukaryotes, however, use three different RNA polymerases, allowing them a more tailored approach to transcribing DNA.

When it comes to reproduction, prokaryotes keep it simple with binary fission, where the cell just splits into two after doubling its DNA. Eukaryotes can choose between mitosis, for creating genetically identical offspring, and meiosis, for producing genetically diverse gametes. It’s like the difference between cloning yourself versus having kids each with a mix of traits from you and your partner.

Prokaryotic cells also have a nifty trick up their sleeve with plasmids, small circular DNA molecules separate from their main chromosome. These can be swapped between cells like trading cards, allowing them to quickly share beneficial genes, like those for antibiotic resistance. Eukaryotes can swap genes too, but it’s a lot less common and a bit more complicated.

Understanding these distinctions isn’t just academic; it has real-world implications in biotechnology, medicine, and agriculture. By harnessing the strengths and understanding the weaknesses of each cell type, scientists can develop better drugs, fight diseases more effectively, and even improve crop resilience.

So, next time you think about cells, remember that these aren’t just tiny specks under a microscope. They’re complex, fascinating, and incredibly diverse entities that are at the very heart of what it means to be alive. They tell the story of life on Earth, from the simplest bacteria to the most complex human tissues. Isn’t that something?


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A Closer Look at Eukaryotic and Prokaryotic Cells: What Sets Them Apart?. (2024, May 12). Retrieved from https://papersowl.com/examples/a-closer-look-at-eukaryotic-and-prokaryotic-cells-what-sets-them-apart/

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PapersOwl.com. (2024). A Closer Look at Eukaryotic and Prokaryotic Cells: What Sets Them Apart? . [Online]. Available at: https://papersowl.com/examples/a-closer-look-at-eukaryotic-and-prokaryotic-cells-what-sets-them-apart/ [Accessed: 16 May. 2024]

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"A Closer Look at Eukaryotic and Prokaryotic Cells: What Sets Them Apart?," PapersOwl.com , 12-May-2024. [Online]. Available: https://papersowl.com/examples/a-closer-look-at-eukaryotic-and-prokaryotic-cells-what-sets-them-apart/. [Accessed: 16-May-2024]

PapersOwl.com. (2024). A Closer Look at Eukaryotic and Prokaryotic Cells: What Sets Them Apart? . [Online]. Available at: https://papersowl.com/examples/a-closer-look-at-eukaryotic-and-prokaryotic-cells-what-sets-them-apart/ [Accessed: 16-May-2024]

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Biology library

Course: biology library   >   unit 32, tissues, organs, & organ systems.

  • Homeostasis
  • Body structure and homeostasis
  • Humans—and other complex multicellular organisms—have systems of organs that work together, carrying out processes that keep us alive.
  • The body has levels of organization that build on each other. Cells make up tissues , tissues make up organs , and organs make up organ systems .
  • The function of an organ system depends on the integrated activity of its organs. For instance, digestive system organs cooperate to process food.
  • The survival of the organism depends on the integrated activity of all the organ systems, often coordinated by the endocrine and nervous systems.


Multicellular organisms need specialized systems, overview of body organization, types of tissues, epithelial tissue, connective tissue, muscle tissue, nervous tissue, organ systems, major organ systems of the human body, organs in a system work together., organ systems work together, too., control and coordination.

  • In the endocrine system, the chemical messengers are hormones released into the blood.
  • In the nervous system, the chemical messengers are neurotransmitters sent straight from one cell to another across a tiny gap.


Works cited.

  • Eva Bianconi et al., "An Estimation of the Number of Cells in the Human Body," Annals of Human Biology 40, no. 6 (2013): 463, http://dx.doi.org/10.3109/03014460.2013.807878 .
  • David E. Sadava, David M. Hillis, H. Craig Heller, and May Berenbaum,"How Do Multicellular Animals Supply the Needs of Their Cells?" in Life: The Science of Biology , 9th ed. (Sunderland: Sinauer Associates, 2009), 837.
  • Carol Guze, "Animal Structure and Function: The Digestive System," Carol’s Classroom: Biology 102: General Biology, accessed June 22, 2016,
  • David E. Sadava, David M. Hillis, H. Craig Heller, and May Berenbaum, "How Does the Vertebrate Gastrointestinal System Work?" in Life: The Science of Biology , 9th ed. (Sunderland: Sinauer Associates, 2009), 1018.
  • William K. Purves, David Sadava, Gordon H. Orians, and H. Craig Heller, "Most Hormones Are Distributed in the Blood," in Life: The Science of Biology , 7th ed. (Sunderland, MA: Sinauer Associates, 2003), 801.

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Good Answer


  1. Cell Organelles

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    The body has levels of organization that build on each other. Cells make up tissues, tissues make up organs, and organs make up organ systems. The function of an organ system depends on the integrated activity of its organs. For instance, digestive system organs cooperate to process food. The survival of the organism depends on the integrated ...

  25. Effects of different cellular and subcellular ...

    Here, Tillandsia usneoides, an indicator plant for atmospheric heavy metals, was treated with an aerosol generation device to analyze Pb contents in different cells (three types of cells in leaf surface scales, epidermal cells, mesophyll cells, vascular bundle cells), subcellular structures (cell wall, cell membrane, vacuoles, and organelles ...