The Cytoskeleton and Cytosol (Cytoplasm)

THE CYTOSKELETON AND CYTOSOL

In this section we will discuss the intracellular components that are not organelles. The cytoskeleton and cytosol are structural elements that help provide the cell with its structure. The cytoskeleton is composed of protein filaments and is found throughout the inside of a eukaryotic cell. The cytosol is the main component of the cytoplasm, the fluid that fills the inside of the cell. The cytoplasm is everything in the cell except for the cytoskeleton and membrane-bound organelles. Both structures, the cytoskeleton and cytosol, are "filler" structures that do not contain essential biological molecules but perform structural functions within a cell.

The Cytosol

The interior of a cell is composed of organelles, the cytoskeleton, and the cytosol. The cytosol often comprises more than 50% of a cell's volume. Beyond providing structural support, the cytosol is the site wherein protein synthesis takes place, and the provides a home for the centrosomes and centrioles. These organelles will be discussed more with the cytoskeleton.

Figure %: Location of the cytosol within a cell.

The Cytoskeleton

The cytoskeleton is similar to the lipid bilayer in that it helps provide the interior structure of the cell the way the lipid bilayer provides the structure of the cell membrane. The cytoskeleton also allows the cell to adapt. Often, a cell will reorganize its intracellular components, leading to a change in its shape. The cytoskeleton is responsible for mediating these changes. By providing "tracks" with its protein filaments, the cytoskeleton allows organelles to move around within the cell. In addition to facilitating intracellular organelle movement, by moving itself the cytoskeleton can move the entire cells in multi-cellular organisms. In this way, the cytoskeleton is involved in intercellular communication.

The cytoskeleton is composed of three different types of protein filaments: actin, microtubules, and intermediate filaments.

Actin

Actin is the main component of actin filaments, which are double-stranded, thin, and flexible structures. They have a diameter of about 5 to 9 nanometers. Actin is the most abundant protein in most eukaryotic cells. Most actin molecules work together to give support and structure to the plasma membrane and are therefore found near the cell membrane.

Microtubules

Microtubules are long, cylindrical structures composed of the protein tubulin and organized around a centrosome, an organelle usually found in the center of the cell near the cell nucleus. Unlike actin molecules, microtubules work separately to provide tracks on which organelles can travel from the center of the cell outward.

Microtubules are much more rigid than actin molecules and have a larger diameter: 25 nanometers. One end of each microtubule is embedded in the centrosome; the microtubule grows outward from there. Microtubules are relatively unstable and go through a process of continuous growth and decay. Centrioles are small arrays of microtubules that are found in the center of a centrosome. Certain proteins will use microtubules as tracks for laying out organelles in a cell.

Intermediate filaments

Intermediate filaments are the final class of proteins that compose the cytoskeleton. These structures are rope-like and fibrous, with a diameter of approximately 10 nanometers. They are not found in all animal cells, but in those in which they are present they form a network surrounding the nucleus often called the nuclear lamina. Other types of intermediate filaments extend through the cytosol. The filaments help to resist stress and increase cellular stability.

Figure %: Organization of actin, microtubules, and intermediate filaments within a cell.

These three types of protein are distinct in their structure and specific function, but all work together to help provide intra-cellular structure. Because they are so diverse, it is very difficult to study the specific functions of the cytoskeletal components.

EUKARYOTIC ORGANELLES: CELL NUCLEUS, MITOCHONDRIA & PEROXISOMES

We will now begin our discussion of intracellular organelles. As we have mentioned, only eukaryotic cells have intracellular sub-divisions, so our discussion will exclude prokaryotic cells. We will also focus on animal cells, since plant cells have a number of further specialized structures. In this section we will discuss the importance of the cell nucleus, mitochondria, peroxisomes, endoplasmic reticulum, golgi apparatus, and lysosome.

The Cell Nucleus

The cell nucleus is one of the largest organelles found in cells and also plays an important biological role. It composes about 10% of the total volume of the cell and is found near the center of eukaryotic cells. Its importance lies in its function as a storage site for DNA, our genetic material. The cell nucleus is composed of two membranes that form a porous nuclear envelope, which allows only select molecules in and out of the cell.

The DNA that is found in the cell nucleus is packaged into structures called chromosomes. Chromosomes contain DNA and proteins and carry all the genetic information of an organism. The nucleus gains support from intermediate filaments that both form the surrounding nuclear lamina and makes direct contact with the endoplasmic reticulum. The nucleus is also the site of DNA and RNA synthesis.

Figure %: Location of the cell nucleus, mitochondria, and peroxisomes in a cell.

Mitochondria

The mitochondria, with its specialized double-membrane structure, generate adenosine triphosphate (ATP), a molecule that provides organisms with energy.

Figure %: Mitochondrial structure

The outer and inner membranes of the mitochondria form two sub-compartments: the internal matrix space and the intermembrane space. Those few proteins found withn the mitochondria are located within the inner membrane. Mitochondria synthesize ATP with energy supplied by the electron transport chain and a process called oxidative phosphorylation.

Peroxisomes

Peroxisomes are single-membrane structures found in all eukaryotic cells. They are small, membrane-bound structures that use molecular oxygen to oxidize organic molecules. The structure is one of the major oxygen utilizing organelles, the other being the mitochondria. Peroxisomes contain oxidative enzymes and other enzymes that help produce and degrade hydrogen peroxide.

Because of their varying enzymatic compositions, peroxisomes are diverse structures. Their main function is to help breakdown fatty acids. They perform specific functions in plant cells, which we will discuss later.

The Endoplasmic Reticulum

The endoplasmic reticulum, or ER, is a very important cellular structure because of its function in protein synthesis and lipid synthesis. For example, the ER is the site of production of all transmembrane proteins. Since nearly all proteins that are secreted from a cell pass through it, the ER is also important in cellular trafficking. In addition to these major roles, the ER plays a role in a number of other biological processes. There are two different types of ER: smooth ER and rough ER (RER).

The rough ER has its name because it is coated with ribosomes, the structures most directly responsible for carrying out protein synthesis. Smooth ER lacks these ribosomes and is more abundant in cells that are specific for lipid synthesis and metabolism.

Figure %: The Endoplasmic Reticulum

In addtion to protein and lipid synthesis, the ER also conducts post-synthesis modifications. One such modification involves the addition of carbohydrate chains to the proteins, though the function of this addition is unknown. Another major modification is called protein folding, whose name is rather self- explanatory. Another role of the ER is to capture calcium for the cell from the cytosol. Finally, the ER can secrete proteins into the cell that are usually destined for the golgi apparatus.

Figure %: The location of the Endoplasmi Reticulum, golgi apparatus, and lysosome in a eukaryotic cell.

The Golgi Apparatus

The golgi apparatus is usually located near the cell nucleus. It is composed of a series of layers called golgi stacks. Proteins from the ER always enter and exit the golgi apparatus from the same location. The cisface of the golgi is where proteins enter. A protein will make its way through the golgi stacks to the other end called the trans face where it is secreted to other parts of the cell.

Figure %: Structure of the Golgi Apparatus

In the golgi apparatus, more carbohydrate chains are added to the protein while other chains are removed. The golgi stacks also sort proteins for secretion. After sorting, the membrane of the golgi buds off, forming secretory vesicles that transport proteins to their specific destination in the cell. A protein's destination is often signaled with a specific amino acid sequence at its end. A protein secretion most often travels back to the ER, to the plasma membrane where it can become a transmembrane protein, or to the next structure we will discuss, the lysosomes.

Lysosomes

Lysosomes are sites of molecular degradation found in all eukaryotic cells. They are small, single-membrane packages of acidic enzymes that digest molecules and are found throughout eukaryotic cells. As such, Lysosomes are a sort of cellular "garbage can," getting rid of cellular debris. Proteins that are not correctly folded or have significant mutations can be secreted to the lysosomes and be degraded instead of taking up space in the cell. Detritus proteins and other molecules can find their way to the lysosome in a variey of ways.

Molecules from outside a cell can be taken in through a process called endocytosis. In this process, the cell membrane invaginates, forming a vesicle containing the transported molecule that will eventually reach a lysosome. The reverse of endocytosis is exocytosis. In this process, molecules within a cell are secreted into an endosome, a membrane-bound structure that delivers the molecule to the lysosome. After reaching the lysosomes, the molecules are secreted from a cell in membrane vesicles. Proteins secreted by the golgi apparatus into the plasma membrane can also be taken back to the lysosome by endosomes.

Terms

Actin  -  A very abundant protein in eukaryotic cells that is the main component of actin filaments.

Actin Filaments  -  Approximately 5-9 nanometers in diameter. Provide structural support to the plasma membrane. As a cytoskeletal protein provides for movement of organelles within cells.

Centromere  -  A round structure that holds together sister chromatids.

Centrosome  -  A region of the cell near the nucleus from which microtubules sprout. Centrosomes are not found in all cells. Centrosomes are comprised of two centrioles.

Chromosome  -  A structure composed of DNA and proteins containing all the genetic material of a cell. Found in the cell nucleus.

Cytoplasm  -  A fluid found in the main compartment of eukaryotic cells. Includes everything outside the cell nucleus but the organelles and the cytoskeleton. The main component is cytosol.

Cytoskeleton  -  A system of protein filaments found throughout the cytoplasm of eukaryotic cells that help provide for cell structure. Composed of actin, intermediate filaments, and microtubules.

Cytosol  -  The main component of the cytoplasm that fills the main compartment of eukaryotic cells.

Endoplasmic reticulum  -  A membrane-bound organelle found in eukaryotic cells. Makes direct contact with the cell nucleus and, since it is dotted with ribosomes, is the site of lipid and protein synthesis. Comes in two forms, smooth and rough.

Endosome  -  A membrane-bound organelle found in eukaryotic cells. Responsible for delivering molecules to the lysosome for digestion.

Eukaryote  -  An organism composed of one or more cells with defined intracellular components including a nucleus and cytosol. Includes all organisms except bacteria and viruses.

Golgi apparatus  -  A membrane-bound organelle found near the cell nucleus in eukaryotic cells. Responsible for sorting and packaging proteins for secretion to various destinations in the cell.

Intermediate filament  -  One of three protein components of the cytoskeleton. A fibrous protein filament approximately 10 nanometers in diameter. Forms the nuclear lamina that helps protect the cell nucleus.

Intermembrane space  -  The space between the outer and inner membrane in a mitochondria.

Lysosome  -  A membrane-bound organelle found in eukaryotic cells. Contain acids and enzymes that degrade unwanted molecules.

Matrix  -  The space inside the inner membrane of mitochondria.

Microtubule  -  One of three protein components of the cytoskeleton. Long, cylindrical structures approximately 25 nanometers in diameter. Extend from the centrosome to all parts of the cell, forming tracks on which organelles can travel within the cell. Microtubules can be either kinetocore microtubules or non-kinetocore microtubules. Kinetocore microtubules bind to sister chromatids during mitosis; non-kinetocore microtubules do not.

Mitochondria  -  An organelle within the cell. Much of cell respiration is carried out within its bounds.

Nucleus  -  A large, double membrane-bound organelle found in eukaryotic cells. Contains DNA and RNA.

Organelle  -  A membrane-bound sub-cellular structure found in eukaryotic cells. The Cell nucleus, mitochondria, ER, and golgi apparatus are all examples.

Peroxisome  -  A small, membrane-bound organelle found in eukaryotic cells. Contains oxidizing enzymes that oxidize organic molecules and process hydrogen peroxide in the cell.

Prokaryote  -  An organism composed of usually one, but occasionally more, cells that lack defined sub-cellular compartments. All essential material is enclosed within the cell membrane. Includes all bacteria and close relatives.

Ribosome  -  A molecule composed of ribosomal RNA* {biology/molecularbiology/translation}* and proteins, and located on the endoplasmic reticulum**. Responsible for mediating protein synthesis.

Rough endoplasmic reticulum  -  Endoplasmic reticulum that is coated with ribosomes and involved in protein synthesis.

Smooth endoplasmic reticulum  -  Naked endoplasmic reticulum that lacks ribosomes and is more involved in lipid synthesis.

Introduction to intracellular components

INTRACELLULAR COMPONENTS

INTRODUCTION TO INTRACELLULAR COMPONENTS

Now that we have discussed the one universal structural element of cells, the cell membrane, we will begin reviewing the specific intracellular components found in eukaryotes, or multi-cellular organisms. Eukaryotes differ from prokaryotes in the level of their structural complexity. Whereas the simpler prokaryotes contain all their genetic material, such as DNA and RNA, within the cell membrane, eukaryotes have intracellular compartments enclosing different structures, called organelles, which contain different molecules and enzymes that perform various functions within and between cells.

In this section, we will discuss the function, structure, and location of intracellular compartments found in eukaryotic cells. With an understanding of these principles we can see how these components are necessary for cell life. While eukaryotic cells have increased structural complexity, prokaryotic cells are able to carry out most of the same processes with their simple structure. We will discuss what significance the different components have in eukaryotic cell life.

We will begin our discussion of eukaryotic intracellular components by discussing the structural roles of the cytoskeleton and cytosol. We will then discuss the biological function of various organelles, including the cell nucleus, mitochondria, peroxisome, endoplasmic reticulum, golgi apparatus, lysosome, endosome, and related structures. The nucleus and mitochondria house DNA and provide the cell with energy, respectively. Peroxisomes and lysosomes are responsible for degrading molecules within the cell. The endoplasmic reticulum, golgi apparatus, and endosomes are involved in cellular transport.

'Semi-autonomous Organelles';

There are two organelles which contain their own DNA (coding for about 50% of the organelle) and reproduce independently of the nucleus. They are said to be 'semi-autonomous organelles'. mitochondrial DNA mutates at a known, constant, rate and is ONLY inherited from the mother, so it can be used to track purely female genetic lines. In the same way, the Y chromosomes is (obviously) only passed on from father to son and so can be used to track the purely male genetic line.

• Mitochondria (sing. Mitochondrion). these are sausage-shaped organelles (2-5 micro.m long) where aerobic respiration takes place in eukrayotic cells.Mitochondria are surrounded by a double membrane: the outer membrane is quite permeable, but the inner membrane is highly folded into cristae, which give it a large surface area. It is studded with ATPase, the enzyme which is the main site of ATP synthesis. This is where the last stage of respiration-the ETC takes place. The space enclosed by the inner membrane is called the mitochondrial matrix and contains small circular strands of DNA and 70S ribosomes. This is the site of the TCA or Kreb's cycle stage of respiration . 
• Chloroplasts. Bigger and fatter than mitochondria ( so settle first when cells are homogenised and centrifuged), chloroplast are the site of photosynthesis, so are only found in photosynthetic cells (plants and algae). Like mitochondria a double membrane enclose them, but chloroplast are contain membranes arranged in disks called thylakoids. Thylakoids contain chlorophyll and other photosynthetic pigments and carry out the light reaction of photosnythesis. The thylakoids are then stacked into piles called grana. The space between the inner membrane and the thylakoid is called the stroma- the site of the light-independent (or 'carbon-fixing') stage of photosynthesis. Chloroplasts also contain starch grains, 70S ribosomes and circular DNA.

Endosymbiosis (= probable evolution of mitochondria and chloroplasts)

Prokaryotic cells are far older and more diverse than eukaryotic cells. Prokaryotic cells have probably been around for 3.5 billion years, while eukaryotic cells arose only about 1 billion years ago. It is thought that eukaryotic cell organelles like mitochondria and chloroplasts are derived from prokaryotic cells that became incorporated inside larger prokaryotic cells. This idea is called endosymbiosis, and is supported by these observations:

• organelles contain circular DNA, like bacteria cells. 
• contain 70S ribosomes, like bacteria cells
• Organelles have double membranes, as though a single-membrane cell had been engulfed and surrounded by a larger cell.
• Organelles reproduce by binary fission, like bacteria. 
• Organelles are very like some bacteria that are alive today
• 
  

More on Prokaryotic And Eukaryotic Cells

All Living Things are made of cells, and cells are the smallest units that can be alive. Life on Earth is classified into five kingdoms, and they each have their own characteristic kind of cell. However the biggest division is between the cells of the Prokaryote kingdom (the bacteria) and those of the other four kingdoms (Animals, Plants, Fungi and Protoctista), which are all eukaryotic cells. Prokaryotic cells are smaller and simpler than eukaryotic cells, and do not have a nucleus.

               Prokaryote = "before carrier bag" i.e without a nucleus
               Eukaryote = "good carrier bag" i.e with a nucleus

We'll examine these two kinds of cell in detail based on structures seen in electron micrographs (= photos taken with an electron microscope). These show the individual organelles inside a cell.

Summary of the Differences between 

Prokaryotic and Eukaryotic Cells. 

Prokaryotic Cells 

• Small cells (< 5 microm.)
• Always unicellular
• No nucleus or any membrane bound organelles, such as mitochondria
• DNA is circular, without proteins 
• Ribosomes are small (70S)
• No cytoskeleton 
• Motility by rigid rotating flagellum (made of flagellin)
• Cell division is by binary fission
• Reproduction is always asexual 
• Huge variety of metabolic pathways 

Eukaryotic Cells.

• Larger cells (> 10 micro.m) 
• Often multicellular
• Always have nucleus and other membrane bound organelles
• DNA is linear and associated with proteins to form chromatin 
• Ribosomes are large (80S)
• Always has a cytoskeleton
• Motility by flexible waving cilia and flagellae (made of tubulin)
• Cell division is by mitosis or meiosis
• Reproduction is asexual or sexual 
• Common metabolic pathways

MORE ON PROKARYOTIC AND EUKARYOTIC CELLS

• Cytoplasm. Contains all the enzymes needed for all metabolic reactions, since there are no organelles 
• Ribosomes. The smaller (70S) type. 
• Nuclear Body. the region of the cytoplasm that contains DNA. it is not surrounded by a nuclear membrane. 
• DNA. Always circular and not associated with any proteins to form chromatin
• Plasmid. Small loops of DNA, used to exchange DNA between bacterial cells. Used in genetic engineering, they often contain genes giving resistance to antibiotics. 
• Cell membrane. Made of phospholipids and proteins, like eukaryotic membranes. 
• Mesosome. A tightly folded region of the cell membrane containing all the membrane-bound proteins required for respiration and photosynthesis. Can also be associated with the nucleoid. 
• Cell wall. Made of murein (not cellulose), which is glycoprotein (i.e a protein/carbohydrate complex, also called peptidoglycan). There are two kinds of cell wall, which can be distinguish by Gram's stain:
• Gram +ve bacteria have a thick cell wall, stain purple, may have spores and are sensitive to penicillin and lysosome (an antibacterial enzyme found in tears and saliva)
• Gram -ve bacteria have a thin cell wall with an outer lipid layer, have no spores and stain pink-these are thought to be more highly evolved.
• Capsule. A thick polysaccharide layer outside the cell wall. Used for sticking cells together, as a food reserve, as protection against desiccation and chemicals, and as protection against phagocytosis. Found only in some Gram +ve bacteria, if a capsule is present, then flagellae are not. 
• Flagellum. A rigid rotating helical-shaped tail used for propulsion. The motor is embedded in the cell membrane and is driven by a H+ (hydrogen) gradient across the membrane. They always rotate clockwise - the only known example of a rotating motor in nature-rather like a propeller on a ship, it has to pass through the 'hull' of the cell via a waterproof seal.

• Cell membrane (or plasma membrane). This is a thin, flexible layer round the outside of all cells made of phopholipids and proteins. It separates the content of the cell from the outside environment, and controls the entry and exit of materials. The membrane is examined in detail later. 
• Cytoplasm (or Cytosol). This is the solution within the cell membrane. It contains enzymes for glycolysis (the first stage of respiration) and other metabolic reactions together with sugars, salts, amino acids, nucleotides and everything else needed for the cell to function. This is where the first stage of respiration (=glysolysis) takes place. 
• Nucleus. This is the largest organelle. Surrounded by a nuclear envelope, which is a double membrane with nuclear pores-large holes containing proteins that control the exit of substances such as mRNA  and ribosomes from the nucleus. The interior is called the nucleoplasm, which is full of chromatin - a DNA/protein complex containing the genes. During cell division the chromatin becomes condensed into discrete observable chromosomes. The nucleolus is a dark region of the nucleus, involved in making ribosomes and 'processing' m-RNA (i.e removing introns)
• 80S Ribosomes. These are the smallest and most numerous of the cell organelles, and are the sites of protein synthesis. They are composed of protein and RNA and are manufactured in the nucleolus of the nuclues. Ribosomes can be free in the cytoplasm, or (more commonly) are attached to the rough endoplasmic reticulum. They are often found in groups called polysomes. NB All eukaryotic ribosomes are of the larger, 80S, type. 
• Endoplasmic Reticulum (ER). This collection of membrane channels forms an important transport 'highway' within the cell, allowing molecules to move from one place to another. It is attached to, and formed from, the outer membrane of the nucleus, and plays an important part in protein synthesis. It comes in two distinct forms:
• Rough Endoplasmic Reticulum (RER). this is studded with numerous 80S ribosomes which give it its rough endoplamic appearance. The ribosomes synthesise proteins which are processed in the SER (e.g by modifying the polypeptide chain, or adding carbohydrates), before being exported from the cell via the Golgi Body. 
• Smooth Endoplasmic Reticulum (SER). Similar to the RER, but without the ribosomes. Series of membrane channels involved in the syntheses and transport of materials, mainly lipids and glycoproteins, needed by the cell.
• Golgi Body (or Apparatus). Another series of flattened membrane vesicles, formed from the endoplasmic reticulum. It's job is to transport proteins from the RER to the cell membrane for export. Parts of the SER containing proteins fuse with one side of the Golgi Body Membranes, while at the other side small vesicles bud off and move towards the cell membrane, where they fuse, releasing their contents by exocytosis.
• Vacuoles. These are membrane-bound sacs containing a dilute solution. most cells have small vacuoles that are formed as required, but plant cells usually have one very large permanent vacuole that fills most of the cell, so that the cyotplasm (and everything else) forms a thin layer round the outside. Plant cell vacuoles are filled with cell sap, and are very important in keeping the cell turgid. Some unicellular protoctists have feeding vacuoles for digesting food, or contractile vacuoles for expelling water (osmoregulation) 
• Lysosomes. these are small membrane-bound vesicles containing a cocktail of digestive enzymes. They are used to break down unwanted chemicals, toxins, organelles or even whole cells, so that the materials may be recycled. They can also fuse with a feeding vacuole to digest its contents. Responsible for cell death - 'autolysis'
• Cytoskeleton. this is a network protein fibres extending throughout all eukaryotic cells, used for support, transport and motility. The cytoskeleton is attached to the cell membrane and gives the cell its shape, as well as holding all the organelles inpositon. There are two types of protein fibres (microfilaments and microtubules); each has corresponding protein that can carry a 'cargo' such as an organelle, chromosome or other cytoskeleton fibres along the fibre. They are responsible for chromosome movement in mitosis and the subsequent division of the cell, cytoplasmic streaming or cycloses (in plants only), cilia and flagella movements and muscle contraction. 
• Centriole. This is a set of short microtubules ('9+2') involved in cell division. before each division the centriole replicates itself and the two centrioles move to opposite ends of the cell, forming the spindle that organises and separates the chromosomes. 
• Cilia and Flagellae (or Undulipodia) these are long flexible 'tails' present in some cells and used for movement. They are surrounded by the cell membrane, and are full of microtubules and motor proteins, so they are capable of complex swimming movements. There are two kinds: 
• Flagellae (no relation of the bacterial flagellum) are longer than the cell, and there are usually only one or two of them, whilst 
• cilia, though identical in structure are much smaller and there are usually very many of them 
• Microvilli These are small finger-like extensions of the cell membrane found in some animal cells (e.g the epithelial cells of the gut and kidney), where they increase the surface area for absorption. They are just visible  under the light microscope as the brush border.
• Cell Wall. this is a thick layer outside the cell membrane used to give a cell strength and rigidity. Cell walls consist of a network of fibres, which give strength but are freely permeable to solutes. (unlike membranes). (A wickerwork basket is a good analogy) Plant cell walls are made mainly of cellulose, but also contain pectin, lignin and other polysaccharides too. It is built up in layers with the middle lamella separating the cell walls called plasmodesmata, which link the cytoplasm of adjacent cells. 
• fungal cell walls are made of chitin 
• animal cells do not have a cell wall 

Prokaryotic Cell

Unlike eukaryotic cells, prokaryote cells lack membrane-bound organelles. However, whereas prokaryote cells are less structurally complex than eukaryotes, they are more chemically complex, since all of the prokaryote cell's biomolecules are floating around together. These biomolecules must interact only with other appropriate molecules to perform biological function.

Prokaryotic cells contain a single compartment enclosed within the cell membrane. In this space reside DNA, RNA, ribosomes and other molecules. Prokaryotes lack a defined nucleus (which is where DNA and RNA are stored in eukaryotic cells), mitochondria, ER, golgi apparatus, and so on. In addition to the lack of organelles, prokaryotic cells also lack a cytoskeleton. Recall that in addition to its role as structural support for the interior of the cell, the cytoskeleton is also involved in intracellular organelle transport. Since there are no organelles to be transported in prokaryotic cells, such a function is unnecessary.

Like the eukaryote cell, the prokaryote cell is filled with cytosol. The prokaryote cytosol is filled with enzymes, which carry out respiratory processes reserved in eukaryotes for the mitochondria. Prokaryote and eukaryote ribosomes also differ slightly, reflect minor differences in prokaryotic versus eukaryotic processing of DNA.

Plant Cell

Figure %: Generalized Plant Cell

Structurally, plant and animal cells are very similar because they are both eukaryotic cells. They both contain membrane-bound organelles such as the nucleus, mitochondria, endoplasmic reticulum, golgi apparatus, lysosomes, and peroxisomes. Both also contain similar membranes, cytosol, and cytoskeletal elements. The functions of these organelles are extremely similar between the two classes of cells (peroxisomes perform additional complex functions in plant cells having to do with cellular respiration). However, the few differences that exist between plant and animals are very significant and reflect a difference in the functions of each cell.

Plant cells can be larger than animal cells. The normal range for an animal cell varies from 10 to 30 micrometers while that for a plant cell stretches from 10 to 100 micrometers. Beyond size, the main structural differences between plant and animal cells lie in a few additional structures found in animal cells. These structures include: chloroplasts, the cell wall, and vacuoles.

Figure %: Plant Cell v. Animal Cell

Chloroplasts

In animal cells, the mitochondria produces the majority of the cells energy from food. It does not have the same function in plant cells. Plant cells use sunlight as their energy source; the sunlight must be converted into energy inside the cell in a process called photosynthesis. Chloroplasts are the structures that perform this function. They are rather large, double membrane-bound structures (about 5 micrometers across) that contain the substance chlorophyll, which absorbs sunlight. Additional membranes within the chloroplast contain the structures that actually carry out photosynthesis.

Chloroplasts carry out energy conversion through a complex set of reactions similar to those performed by mitochondria in animals. The double membrane structure of chloroplasts is also reminiscent of mitochondria. The inner membrane encloses an area called the stoma, which is analogous to the matrix in mitochondria and houses DNA, RNA, ribosomes, and different enzymes. Chloroplasts, however, contain a third membrane and are generally larger than mitochondria.

The Cell Wall

Another structural difference between in plant cells is the presence of a rigid cell wall surrounding the cell membrane. This wall can range from 0.1 to 10 micrometers thick and is composed of fats and sugars. The tough wall gives added stability and protection to the plant cell.

Vacuoles

Vacuoles are large, liquid-filled organelles found only in plant cells. Vacuoles can occupy up to 90% of a cell's volume and have a single membrane. Their main function is as a space-filler in the cell, but they can also fill digestive functions similar to lysosomes (which are also present in plant cells). Vacuoles contain a number of enzymes that perform diverse functions, and their interiors can be used as storage for nutrients or, as mentioned, provide a place to degrade unwanted substances.