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Introducing cells

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Introducing cells

3.1 Introduction

Although cells differ greatly in size and shape, they are the building blocks of the body. Some cells, such as those of skeletal muscle, are very large and may extend for 30 cm along the length of a muscle. In contrast, the red cells of the blood are small disks with a diameter of about 7 μ-m. These two cell types represent some of the more striking variations in cell mor­phology. The structure of a typical mammalian cell is illustrated in Fig. 3.1, which shows it to be bounded by a plasma or cell membrane. The plasma membrane is a continuous sheet that separates the watery phase inside the cell, the cytoplasm, from that outside the cell, the extracellular fluid. The shape of an individual cell is maintained by an array of protein filaments known as the cytoskeleton.

The most prominent structure of most cells is the nucleus, which contains the hereditary material, DNA. Some cells (e.g.

skeletal muscle cells) have more than one nucleus while the red cells of the blood lose their nucleus as they mature. Cells possess other structures that perform specific functions, such as energy production, protein synthesis, and the secretion of various materials. The internal structures of a cell are collectively known as organelles and include the nucleus, the mitochondria, the Golgi apparatus, the endoplasmic reticulum, and various membrane-bound vesicles (see below).

3.2 The structure and functions of the cellular organelles

The plasma membrane

The plasma membrane regulates the movement of substances into and out of a cell and is responsible for regulating a cell's response to a variety of signals, such as hormones and

Lysosomes Gol9i apparatus Fig. 3.1 A diagram of a typical mammalian cell, showing the major organelles.


3 Introducing cells

neurotransmitters. An intact cell membrane is, therefore, essen­tial for the proper function of a cell. When viewed at high power in an electron microscope, the plasma membrane appears as a sandwich-like structure, 5-10 nm thick (i.e. 5-10 X 10-9 meters). The outer surface is covered by a layer of fine filaments which form the glycocalyx or cell coat. The membranes of the intracellular organelles (e.g. endoplasmic reticulum, Golgi appa­ratus, lysosomes, and mitochondria) have a similar three-layered structure to that of the plasma membrane.

^ The plasma membrane is a lipid bilayer containing proteins

Chemical analysis shows that the plasma membrane is made of lipid and protein in approximately equal amounts by weight. The lipids are arranged so that their polar head groups are oriented towards the aqueous phase and the hydrophobic fatty acid chains face inwards to form a central hydrophobic region, as shown in Fig. 3.2. The membrane proteins either span the lipid bilayer or they are anchored to it in various ways.

The principal lipids of the plasma membrane belong to one of three classes: phospholipids, glycolipids, and cholesterol. There is now good evidence that the composition of the outer and inner layers of the plasma membrane differs significantly. The outer leaflet consists of glycolipids, phosphatidylcholine, and sphin­gomyelin. The inner leaflet is richer in negatively charged phos­pholipids, such as phosphatidylinositol. Cholesterol is present in both leaflets of the bilayer. The presence of phosphatidylinositol in

the inner leaflet of the bilayer is important as inositol plays an important role in the transmission of certain signals from the cell membrane to the interior of the cell (see Chapter 5).

Each phospholipid molecule is able to diffuse freely in one plane of the bilayer but phospholipids rarely flip from one leaf of the bilayer to the other. This indicates that the lipid bilayer is an inherently stable structure.

^ Artificial lipid membranes are not very permeable to ions or polar molecules

Artificial lipid membranes are relatively permeable to carbon dioxide, oxygen, and lipid-soluble molecules but they are almost impermeable to polar molecules and ions, such as glucose and sodium. Moreover, such membranes are relatively impermeable to water. Natural cell membranes, however, are permeable to a wide range of polar materials, which cross the hydrophobic barrier formed by the lipid bilayer via specific protein molecules.


The plasma membrane consists of about equal amounts by weight of protein and lipid. The lipid is arranged as a bilayer whose inner and outer leaflets have a different composition. The lipid bilayer forms a barrier to the passage of polar materials so that these substances must enter or leave a cell via specialized transport proteins. The combination of lipid bilayer and transport proteins allows cells to maintain an internal composition that is very different to that of the extracellular fluid.

Fig. 3-2 The structure of the plasma membrane, (a) The basic arrangement of rhe lipid bilayer; nore the presence of glycolipid only in the outer leaflet of the bilayer. (b) A simplified model of rhe plasma membrane, showing the arrangement of some of the membrane proteins.

3.2 The structure and functions of the cellular organelles 25

^ Membrane proteins

The membrane proteins may be divided into two broad groups, intrinsic—those that are embedded in the bilayer itself—and extrinsic—those that are external to the bilayer but linked to it in some way. The proteins that facilitate the movement of ions and other polar materials across the plasma membrane are all intrinsic membrane proteins. Some extrinsic proteins link the cell to its surroundings (the extracellular matrix) or to neigh­boring cells. Others play a role in the transmission of signals from the plasma membrane to the interior of the cell. The phy­siological roles of the membrane proteins are discussed in greater detail in the following chapters.

The nucleus

The nucleus is separated from the rest of the cytoplasm by the nuclear membrane (also known as the nuclear envelope) which con­sists of two lipid bilayers separated by a narrow space. The nuclear membrane is furnished with small holes, known as nuclear pores, which provide a means of communication between the nucleus and the cytoplasm.

The nucleus contains DNA, which associates with proteins called histones to form chromatin fibers. The chromatin may either be very condensed {beterochromatin) or relatively dispersed (euchromatin). When a cell synthesizes a protein it first needs to read the base sequence from its DNA (a process known as tran­scription). Heterochromatin is chromatin that is not taking part in transcription while euchromatin is chromatin that is doing so. From this it is evident that a cell with a small, densely stain­ing nucleus is transcribing very little of its DNA and is therefore not using many of its genes. Conversely, a cell with a large, pale nucleus is involved in wholesale gene transcription. During cell division (mitosis), the chromatin becomes distributed into pairs of chromosomes which attach to a structure known as the mitotic spindle before they separate as the cell divides (see below).

A densely staining round structure is often visible within the nucleus. It is called the nucleolus and is concerned with the manufacture of the ribosomes, which play an important part in the synthesis of proteins.

The organelles of the cytoplasm

The cytoplasm contains many different organelles, a number of which are separated from the rest of the cytoplasm by membranes. Examples are the mitochondria, the endoplasmic reticulum, the Golgi apparatus, and vesicles such as lysosomes. Thus, the cell's internal membranes divide the cytoplasm into various compartments.


As discussed in Chapter 1, living cells continually transform materials. They oxidize glucose and fats to provide energy for other activities such as motility and the synthesis of proteins for growth and repair. The energy is provided as ATP. In most cells, the bulk of ATP synthesis occurs in the mitochondria by a process known as oxidative phosphorylation (see below).

The mitochondria are 2—6 /xm in length and about 0.2 fim in diameter. They have two distinct membranes, an outer mem­brane which is smooth and regular in appearance and an inner membrane which is thrown into a number of folds, known as cristae. It is here, on the inner membrane, that the synthesis of ATP takes place via the tricarboxylic acid cycle and electron transport chain. Cells that have a high demand for ATP (e.g. the muscle cells of the heart) have many mitochondria, which are often located close to the site of ATP utilization. In the case of heart muscle the mitochondria lie close to the contractile elements of the cells, the myofibrils.

The numbers of mitochondria in a cell are regulated accord­ing to metabolic requirements. They may increase substantially if required, for example in skeletal muscle cells subject to pro­longed periods of contractile activity. Mitochondria are not assembled from scratch from their molecular constituents but increase in number by the division of existing mitochondria. This division takes place in the interphase of the cell cycle (the interval between cell divisions) and the new mitochondria are shared out between the daughter cells during cell division.

^ Endoplasmic reticulum

The endoplasmic reticulum is a system of membranes that extends throughout the cytoplasm. These membranes are continuous with the nuclear membrane and enclose a significant space within the cell. The endoplasmic reticulum is involved with the synthesis and transport of membrane proteins and lipids. It is also involved with cell signaling via the inositol lipids of the inner leaflet of the plasma membrane (Chapter 5). In muscle cells the endoplasmic reticulum is called the sarcoplasmic reticulum.

^ Golgi apparatus

The Golgi apparatus is a system of flattened membranous sacs that are involved in the modification and packaging of materials for secretion. Although the Golgi apparatus is effectively an extension of the endoplasmic reticulum, it is not in direct con­tinuity with it. Rather, small vesicles (known as transport vesicles) pinch off from the endoplasmic reticulum and migrate to the Golgi apparatus, with which they fuse. The Golgi appara­tus itself gives rise to specific secretory vesicles that migrate to the plasma membrane. Further details of the mechanism of secretion are discussed in Chapter 4.

^ Membrane-bound vesicles

Cells contain a variety of membrane-bound vesicles that are integral to their function. Secretory vesicles are formed by the Golgi apparatus and may contain dense granules of secretory material. After they have discharged their contents, the secre­tory vesicles are retrieved to form endocytotic vesicles (Chapter 4). Other cytoplasmic vesicles include the lysosomes and the per­oxisomes. The lysosomes contain hydrolytic enzymes that allow cells to digest materials they have taken up, whereas the peroxi­somes contain enzymes that can synthesize and destroy hydrogen peroxide.


3 Introducing cells


Ribosomes consist of proteins and ribonucleic acid (RNA). They are formed in the nucleolus and migrate to the cytoplasm where they may occur free or in groups called polyribosomes. Ribosomes are responsible for making new proteins. Some ribo­somes become attached to the outer membrane of the endoplas­mic reticulum to form the rough endoplasmic reticulum, which is involved in the synthesis of membrane proteins.

^ The cytoskeleton

A cell is not just a bag of enzymes and isolated organelles. Different cell types each have a distinctive and stable mor­phology that is maintained by an internal array of protein filaments known as the cytoskeleton. The protein filaments are of three main kinds: actin filaments, intermediate filaments, and microtubules.

Actin filaments

These play an important role in cell movement, such as the contraction of skeletal muscle. They also help to maintain cell shape in nonmotile cells.

Intermediate filaments

The diameter of these filaments, estimated from electron micro­graphs, lies between those of the thin actin filaments and the thick myosin filaments of skeletal muscle. They play an impor­tant role in the mechanical stability of cells. Those cells that are subject to a large amount of mechanical stress (such as epithelia) are particularly rich in intermediate filaments, which link cells together via specialized junctions.


As their name suggests, microtubules are hollow tubes with an external diameter of about 25 nm and a wall thickness of 5-7 nm. They are formed from a protein called tubulin and play an important role in moving organelles (e.g. secretory vesicles) through the cytoplasm. They also play a major role in the movement of cilia and flagella.

Microtubules originate from a complex structure known as a centrosome. Between cell divisions, the centrosome is located at the center of a cell, near the nucleus. Embedded in the cen­trosome are two centrioles, which are cylindrical structures arranged at right angles to each other. At the beginning of cell division the centrosome divides into two and the daughter centrosomes move to opposite poles of the nucleus to form the mitotic spindle (see below).


These are very small, hair-like projections from certain cells which have a characteristic array of microtubules at their core. In mammals, cilia beat in an orderly, wave-like motion to propel material over the surface of an epithelial layer, such as the lining of the upper respiratory tract. In this case, the cilia beat in such a way as to move the layer of mucus covering the epithelium towards the mouth. The mucus traps dust, cell debris, and invading organisms and the action of the cilia moves these

materials away from the respiratory surface towards the throat where they can be coughed up. Flagella are similar in structure to cilia, but are much longer. While they are common in single-celled organisms, in mammals flagella are only found as the motile part of the sperm (Chapter 19).

3.3 Cell division

During life, animals grow by two processes:

  1. the addition of new material to pre-existing cells; and

  2. by increasing the number of cells by division.

Cell division can occur by one of two processes: mitosis, in which each daughter cell has the same number of chromosomes as the parent cell; and meiosis, in which each daughter cell has half the number of chromosomes of its parent. Most cells that divide, do so by mitosis. Meiosis occurs only in the germinal cells during the formation of the eggs and sperm. It is discussed in Chapter 19-

The process of mitosis can be divided into six phases:


During the early part of prophase the nuclear chromatin con­denses into well-defined chromosomes, each of which consists of two identical chromatids linked by a specific sequence of DNA known as a centromere. As prophase proceeds, the cytoplasmic microtubules disassemble and the mitotic spindle begins to form outside the nucleus, between a pair of separating centrioles.


This begins with the dissolution of the nuclear membrane. It is followed by the movement of the microtubules of the mitotic spindle into the nuclear region. The chromosomes then become attached to the mitotic spindle by their centromeres.


During metaphase the chromosomes become aligned along the central region of the mitotic spindle.


Anaphase begins with the separation of the two chromatids, to form the chromosomes of the daughter cells. The poles of the mitotic spindle move further apart.


In telophase the separated chromosomes reach the poles of the mitotic spindle, which begins to disappear. A new nuclear membrane is formed around each daughter set of chromosomes and mitosis proper is complete.


This is the division of the cytoplasm between the two daughter cells. It begins during anaphase but is not completed until after the end of telophase. The cell membrane invaginates in the centre of the cell at right angles to the long axis of the mitotic spindle, to form a cleavage furrow. The furrow deepens until the two daughter cells are joined only by a narrow neck of cyto-


3 Introducing cells

plasm, which finally breaks and so separates the two daughter cells.

The processes of mitosis are summarized in Fig. 3.3.

3.4 Epithelia

The interior of the body is physically separated from the outside world by the skin, which forms a continuous sheet of cells known as an epithelium. Epithelia also line the gut, lungs, and urinary tract. A cell layer that separates an internal space from the rest of the body is also called an epithelium. The cell layer that lines the blood vessels is, however, generally called the vascular endothelium.

The structure of epithelia varies according to their functional requirements. For example, the epithelium of the skin is thick and tough to resist abrasion and to prevent the loss of water from the body. In contrast, the epithelial lining of the alveoli of the lungs is very delicate and thin to permit free exchange of the respiratory gases.

Despite such differences in form and function, all epithelia share certain features. First, they are composed entirely of cells. Secondly, their cells are joined together via specialized cell—cell junctions. Thirdly, as one surface faces the external world while the other is oriented towards the interior of the body, epithelia show polarity. Fourthly, epithelia lie on a matrix of connective tissue fibers, called the basal lamina, which provides physical support and separates the epithelium from the underlying tissues. Finally, all epithelia undergo continuous cell replacement.

The surface of an epithelial layer that is oriented towards the outside world or central space of a gland or hollow organ is known as the apical surface. The surface that is orientated towards the basal lamina and the interior of the body is called the baso-lateral surface.

All around the sides of an epithelial cell close to the apical surface lies the junctional complex. This consists of three com­ponents: the tight junction (also known as the zona occludens), the intermediate junction (or zona adherens), and the des-mosomes. Within the junctional complexes specialized regions of contact, called gap junctions, are found. These junctions allow small molecules to diffuse between adjacent cells. In this way gap junctions play a role in cell—cell communication (Section 5.8).

The tight junction is a continuous band that runs around the side of the cell near to the apical surface. It links the cell to each of the adjacent cells to seal off the space above the apical surface from that surrounding the basolateral surface. In a tight junction the plasma membranes of the adjacent cells are so closely apposed that the extracellular space is eliminated. The inter­mediate junction lies immediately below the tight junction. It also runs in a continuous band around the sides of each cell and is associated with many microfilaments. It probably serves to stiffen the apical region of the cell and so contributes to the structural strength of the epithelium.

The desmosomes are points of contact between the plasma membranes of adjacent cells. On the cytoplasmic side, there is an accumulation of electron-dense material linked with the microfilaments of the cytoskeleton. This is known as an attach­ment plaque. The desmosomes are distributed in clusters along the lateral surfaces of the epithelial cells and act in much the same way as spot welds to hold the plasma membranes of adjacent cells together.

Unlike cells scattered throughout a tissue, the arrangement of cells into epithelial sheets permits the directional transport of materials either into or out of a compartment. In the gut, kidney, and many glandular tissues this feature of epithelia is of great functional significance.

The classification of epithelia

Epithelia vary considerably in their morphology. They may be divided into covering epithelia and glandular epithelia. Each class then may be further subdivided.

^ Covering epithelia

Covering epithelia may consist of a single cell layer or they may consist of more than one layer. In the first case they are classed as simple epithelia and in the second they are classed as stratified epithelia. Depending on cell shape, a simple epithelium may be squamous, cuboidal, or columnar (Fig. 3.4). The endothelium of the blood vessels is an example of a squamous epithelium, as is

3.5 Energy metabolism in cells 29

that of the alveoli of the lung, while the small intestine is lined by a columnar epithelium.

The best-known stratified epithelium is the epidermis of the skin. In this case, the flattened epithelial cells form many layers and the more superficial cells are filled with a special protein called keratin. This renders the skin almost impervious to water and provides an effective barrier against invading organisms such as bacteria. If these layers are damaged, for example by burns, there is a loss of fluid and a risk of infection. If the area involved is very large, the loss of fluid may become life threatening.

^ Glandular epithelia

These epithelia are specialized for the secretion of fluids. They often synthesize and secrete enzymes or other materials. Some glands secrete material via a specialized duct onto an epithelial surface. These are known as exocrine glands. Examples are rhe pancreas, salivary glands, and sweat glands. Other glandular epithelia lack a duct and secrete material directly into the blood. These are known as endocrine glands. An example is the epithelium that lines the follicles of the thyroid gland.

^ 3.5 Energy metabolism in cells

Animals take in food as carbohydrate, fats, and protein. These complex molecules are broken down in the gut and absorbed. The carbohydrates of the diet are mainly starch, which is broken down to glucose. The fats are broken down to fatty acids and glycerol, while the dietary protein is broken down to its con­stituent amino acids. These breakdown products are then used by the cells of the body to make ATP which provides a convenient way of harnessing chemical energy.

ATP can be synthesized in two ways. First, by the glycolytic breakdown of glucose to pyruvate and, second, by the oxidative metabolism of pyruvate and acetate via the tricarboxylic acid cycle (also known as the citric acid cycle or Krebs cycle). The utilization of glucose, fatty acids, and amino acids for the synthesis of ATP is summarized in Fig. 3.5. In each case, the synthesis of ATP is accompanied by the production of carbon dioxide and water.

The generation of ATP by glycolysis

Of the simple sugars, glucose is the most important in the synthesis of ATP. It is transported into cells where it is phos-phorylated to form glucose 6-phosphate. This is the first stage of its breakdown by the process of glycolysis in which glucose is broken down to form pyruvare. Glycolysis takes place in the cytoplasm of the cell, outside the mitochondria, and does not require the presence of oxygen. For this reason, rhe glycolytic breakdown of glucose is said to be anaerobic. The glycolytic pathway is summarized in Fig. 3.6. Further details can be found in any textbook of biochemistry.

The breakdown of a molecule of glucose by glycolysis yields two molecules of pyruvate and two molecules of ATP. In addirion, two molecules of reduced nicotinamide adenine dinucleotide (NADH) are produced. When oxygen is present, the NADH generated by glycolysis is oxidized by the mito­chondria via the electron transport chain, resulting in the syn­thesis of about three molecules of ATP and the regenerarion of two molecules of nicotinamide adenine dinucleotide (NAD).

In normal circumstances, rhe pyruvate that is generated during glycolysis combines wirh coenzyme A to form acetyl coenzyme A (acetyl CoA), which is oxidized via the tricarboxylic acid cycle to yield ATP. In the absence of sufficient oxygen, however, some of the pyruvate is reduced by NADH in rhe cytosol to generate lactate. This step regenerates the NAD used in the early srages of glycolysis so that it can participate in the breakdown of a furrher molecule of glucose. Thus glycolysis can generate ATP even in the absence of oxygen and becomes an important source of ATP for skele­tal muscle during vigorous exercise (Chapter 25).

The breakdown of pyruvate by the tricarboxylic acid cycle

The two molecules of pyruvate formed by the glycolytic break­down of glucose combine wirh coenzyme A to form acetyl CoA


3 Introducing cells

Fig. 3.6 The principal steps in the glycolytic breakdown of glucose.

generate ATP via an enzyme complex known as the electron trans­port chain. This process requires molecular oxygen and is called aerobic metabolism. Aerobic metabolism is very efficient and yields about 10 molecules of ATP for each molecule of acetyl CoA that enters the tricarboxylic acid cycle. Overall, the complete oxida­tion of one molecule of glucose to carbon dioxide and water yields 30 molecules of ATP. This should be compared with the gen­eration of ATP by anaerobic metabolism where only two molecules of ATP are generated for each molecule of glucose used.

Fatty acids are the body's largest store of food energy. They are stored in fat cells (adipocytes) as triglycerols. Fat cells are widespread but are most abundant in the adipose tissues. Stored fat is broken down to fatty acids and glycerol by lipases in a process known as lipolysis. The glycerol is metabolized by the glycolytic pathway while the fatty acids combine with coenzyme A before being broken down by a process known as /3-oxidation. This results in the formation of acetyl CoA, NADH and FADH2. The acetyl CoA is oxidized via the tricarboxylic acid cycle and the NADH and FADH2 are oxidized via the electron transport chain. Overall, the complete oxidation of a molecule of palmitic acid (which has 16 carbon atoms) yields over 100 molecules of ATP.

Although animals can synthesize fats from carbohydrates via acetyl CoA, they cannot synthesize carbohydrates from fatty acids. When glucose reserves are low, many tissues preferentially utilize fatty acids liberated from the fat reserves. Under these circumstances, the liver converts fatty acids to acetyl CoA from which it synthesizes acetoacetate, D-3-hydroxybutyrate, and acetone. These are known as ketone bodies and are produced in large amounts when the utilization of glucose by the tissues is restricted, as in starvation or in poorly controlled diabetes melli-tus (Chapter 27). Acetoacetate and 3-hydroxybutyrate are utilized for ATP production by the heart and the kidney.

Proteins are the main structural components of cells and the principal use for dietary protein is the synthesis of new protein. Those amino acids that are not required for protein synthesis are deaminated in the liver and the carbon skeleton utilized for the generation of ATP. Like the glycerol liberated by the breakdown of fats, the carbon skeleton of some amino acids can be used to synthesize glucose. This function, which is known as gluconeogene-sis, is also performed by the liver.

before they enter the tricarboxylic acid cycle. This step occurs in the mitochondria and results in the formation of two molecules of carbon dioxide and two molecules of NADH. Each molecule of acetyl CoA combines with a molecule of oxaloacetate to form the tricarboxylic acid, citrate, which then undergoes a series of reactions that result in the complete oxidation of the acetyl CoA to carbon dioxide and water. These are summarized in Fig. 3.7.

During each turn of the tricarboxylic acid cycle three mole­cules of NADH, one of reduced flavine adenine dinucleotide (FADH2) and one molecule of guanosine triphosphate (GTP) are generated. The mitochondria utilize the NADH and FADH2 to

Recommended reading

Alberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K., and Watson, J. D. (1989). Molecular biology of the cell, (3rd edn), Chapters 1,10, 16, and 17. Garland, New York.

Elliott, W. H. and Elliott, D. C. (1997). Biochemistry and cell biology, Chapters 3, 5, 7—9- Oxford University Press, Oxford.

Junquieira, L. C, Carneiro, J., and Kelley, R. O. (1995). Basic histology, (8th edn), Chapters 2-4. Prentice-Hall, London.

Stryer, L. (1995). Biochemistry, (4th edn), Chapters 19, 20 21, and 24. Freeman, New York.


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