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SECTION 1 The body and its constituents The human body develops from a single cell called the zygote, which results from the fusion of the ovum (female egg cell) and the spermatozoon (male sex cell). Cell division follows and, as the fetus grows, cells with different structural and functional spec...

SECTION 1 The body and its constituents The human body develops from a single cell called the zygote, which results from the fusion of the ovum (female egg cell) and the spermatozoon (male sex cell). Cell division follows and, as the fetus grows, cells with different structural and functional specialisations develop, all with the same genetic make-up as the zygote. Individual cells are too small to be seen with the naked eye. However, they can be seen when thin slices of tissue are stained in the laboratory and magnified using a microscope. A cell consists of a plasma membrane enclosing a number of organelles suspended in a watery fluid called cytosol (Fig.3.1). The cell contents, excluding the nucleus,are made up of the cytoplasm,i.e. the cytosol and other organelles. Plasma membrane The structure and functions of membranes are fundamental to cell survival, as they control passage of substances into and out of it, regulating the intracellular environment. Structure The plasma membrane (Fig. 3.2) consists of two layers of phospholipids (p. 33) with proteins and sugars embedded in them. In addition to phospholipids, the lipid cholesterol is also present. The phospholipid molecules have a head,which is electrically charged and hydrophilic (meaning 'water-loving'), and a tail, which has no charge and is hydrophobic (meaning 'water-hating', Fig. 3.2A). The phospholipid bilayer is arranged like a sandwich with the hydrophilic heads aligned on the outer surfaces of the membrane and the hydrophobic tails forming a central water-repelling layer. These differences influence the transfer of substances across the membrane. Membrane proteins Those proteins that extend all the way through the membrane provide channels that allow the passage of,for example, electrolytes and non-lipid soluble substances.Protein molecules on the surface of the plasma membrane are shown in Fig. 3.2B. The membrane proteins perform several functions: ·Some membrane protein molecules have branched carbohydrate molecules attached to the outside of the cell,giving the cell its immunological identity-'self'markers (p. 412). They can act as receptors (specific recognition sites) for hormones and other chemical messengers. Figure 3.1 The simple cell. Some are enzymes(p.34). APhospholipid bilayer B Figure 3.2 The plasma membrane. (A) Diagram showing structure. (B) Coloured atomic force micrograph of the surface showing plasma proteins. (B, Hermann Schillers, Prof. Dr H Oberleithner,University Hospital of Münster/Science Photo Library, reproduced with permission.) Cells and tissues CHAPTER 3 Figure 3.3 Role of the cell membrane in regulating the composition of intracellular fluid. (A) Particle size. (B) Specific pores and channels. (C)Pumps and carriers. ·Transmembrane proteins form channels that are filled with water and allow very small, water-soluble ions to cross the membrane. ·Some are involved in pumps that transport substances across the membrane. Transport of substances across cell membranes Each cell is enclosed by its plasma membrane, which provides a selective barrier to substances entering or leaving.This property,called selective permeability, allows the cell (plasma) membrane to control the entry or exit of many substances,thereby regulating the composition of its internal environment. Particle size is important, as many small molecules,e.g.water, can pass freely across the membrane by simple diffusion, while large molecules cannot and may therefore be confined to either the interstitial fluid or the intracellular fluid: in Fig. 3.3A, the pink and orange particles are too big to diffuse through the membrane pores, so the pink particles are trapped inside the cell and the orange particles are excluded. Pores or specific channels in the plasma membrane admit certain substances but not others. In Fig. 3.3B, although the green and yellow particles are the same size, the membrane has channels only for the green ones, so the yellow particles are excluded. The membrane is also studded with specialised pumps or carriers that import or export specific substances.In Fig.3.3C the membrane has pumps that actively import the pink particles and other pumps that actively export the blue particles; pink particles therefore concentrate within the cell and blue particles concentrate outside the cell. Selective permeability ensures that the chemical composition of the fluid inside cells is different from the interstitial fluid that bathes them. Transport mechanisms are explained in the next section. Passive transport This occurs when substances can cross the semipermeable plasma and organelle membranes, and move down Figure 3.4 Specialised membrane protein carrier molecules involved in facilitated diffusion and active transport. the concentration gradient (downhill) without using energy. Diffusion This was described on page 36. Small molecules diffuse down their concentration gradient: Lipid-soluble materials, e.g. oxygen,carbon dioxide,fatty acids and steroids, cross the membrane by dissolving in the lipid part of the membrane. ·Water-soluble materials, e.g. sodium,potassium and calcium,cross the membrane by passing through water-filled channels. Facilitated diffusion This passive process is used by some substances that are unable to diffuse through the semipermeable membrane unaided, e.g. glucose, amino acids. Specialised protein carrier molecules in the membrane have specific sites that attract and bind substances to be transferred, like a lock and key mechanism. The carrier then changesits shape and deposits the substance on the other side of the membrane (Fig.3.4).The carrier sites are specific and can be used by only one substance. As there are a finite number of carriers,there is a limit to the amount of a substance that can be transported at any time. This is known as the transport maximum. SECTION 1 The body and its constituents Osmosis Osmosis is passive movement of water down its concentration gradient towards equilibrium across a semipermeable membrane. The process is explained on page 39. Extrusion of waste material by the reverse process through the plasma membrane is called exocytosis.Vesicles formed by the Golgi apparatus (see later) usually leave the cell in this way, as do any indigestibleresidues of phagocytosis. Active transport 3.2Organelles 3.3 This is the transport of substances up their concentration gradient (uphill), i.e. from a lower to a higher concentration.Chemical energy in the form of adenosine triphosphate (ATP,p.37) drives specialised protein carrier molecules that transport substances across the membrane in either direction (Fig.3.4).The carrier sites are specific and can be used by only one substance; therefore the rate at which a substance is transferred depends on the number of sites available. The sodium-potassium pump All cells possess this pump, which indirectly supports other transport mechanisms such as glucose uptake, and is essential in maintaining the electrical gradient needed to generate action potentials in nerve and muscle cells. This active transport mechanism maintains the unequal concentrations of sodium (Na+) and potassium (K+)ions on either side of the plasma membrane. It may use up to 30% of cellular ATP (energy) requirements. Potassium levels are much higher inside the cell than outside - it is the principal intracellular cation. Sodium levels are much higher outside the cell than inside-it is the principal extracellular cation. These ions tend to diffuse down their concentration gradients, K+ outwards and Na+ into the cell. In order to maintain their concentration gradients, excess Na+ is constantly pumped out across the cell membrane in exchange for K+. Bulk transport Transfer of particles too large to cross cell membranes occurs by pinocytosis ('cell-drinking') or phagocytosis ('cell-eating').These particles are engulfed by extensions of the cytoplasm (Fig. 3.5; see also Fig. 15.1), which enclose them,forming a membrane-bound vacuole. Pinocytosis allows the cell to bring in fluid. In phagocytosis, larger particles (e.g. cell fragments,foreign materials, microbes) are taken into the cell. Lysosomes (see later) then adhere to the vacuole membrane, releasing enzymes that digest the contents. Organelles (see Fig.3.1), literally meaning 'small organs',have individual and highly specialised functions, and are often enclosed by their own membrane within the cytosol.They include the nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes and cytoskeleton. Nucleus All body cells have a nucleus, with theexception of mature erythrocytes (red blood cells). Skeletal muscle fibres and some other cells contain several nuclei. The nucleus is the largest organelle and is contained within the nuclear envelope,a membrane similar to the plasma membrane but with tiny pores through which some substances can pass between it and the cytoplasm. The nucleus contains the body's genetic material,in the form of deoxyribonucleic acid (DNA, p. 476);this directs all its metabolic activities. In a non-dividing cell,DNA is present as a fine network of threads called chromatin, but when the cell prepares to divide, the chromatin forms distinct structures called chromosomes (see Fig. 17.1). A related substance,ribonucleic acid (RNA), is also found in the nucleus. There are different typesof RNA, not all found in the nucleus,but which are, in general, involved in protein synthesis. Within the nucleus there is a roughly spherical structure called the nucleolus, which is involved in synthesis (manufacture) and assembly of the components of ribosomes. Mitochondria Mitochondria are membranous, sausage-shaped structures in the cytoplasm, sometimes described as the 'power house'of the cell (Fig. 3.6).They are central to aerobic respiration,the processes by which chemical energy is made available in the cell.This is in the form of ATP,which releases energy when the cell breaks it down (see Fig. 2.10). Synthesis of ATP is most efficient in the final stages of aerobic respiration, a process that requires oxygen.The most active cell types have Nucleus Lysosomes Particle engulfed by plasma membrane Formation of a vacuole Fusion of lysosomes with vacuole Digestion of the particle by lysosomal enzymes Exocytosis Figure 3.5 Bulk transport across plasma membranes. (A-E) Phagocytosis. (F) Exocytosis. Figure 3.6 Mitochondrion and rough endoplasmic reticulum. False colour transmission electron micrograph showing mitochondrion (orange) and rough endoplasmic reticulum (turquoise) studded with ribosomes (dots). (Bill Longcore/Science Photo Library,reproduced with permission.) the greatest number of mitochondria, e.g. liver, muscle and spermatozoa. Ribosomes These are tiny granules composed of RNA and protein.They synthesise proteins from amino acids, using RNA as the template (see Fig. 17.5). When present in free units or in small clusters in the cytoplasm, the ribosomes make proteins for use within the cell. These include the enzymes required for metabolism. Metabolic pathways consist of a series of steps, each driven by a specific enzyme. Ribosomes are also found on the outer surface of the nuclear envelope and rough endoplasmic reticulum (see Fig.3.6 and next section),where they manufacture proteins for export from the cell. Endoplasmic reticulum Endoplasmic reticulum (ER) is an extensive series of interconnecting membranous canals in the cytoplasm (Fig. 3.6). There are two types: smooth and rough. Smooth ER synthesises lipids and steroid hormones, and is also associated with the detoxification of some drugs.Some of the lipids are used to replace and repair the plasma membrane and membranes of organelles. Rough ER is studded with ribosomes.These are the site of synthesis of proteins,some of which are 'exported' from cells, i.e. enzymes and hormones that leave the parent cell by exocytosis (Fig. 3.5F) to be used by cells elsewhere. Golgi apparatus The Golgi apparatus consists of stacks of closely folded flattened membranous sacs (Fig. 3.7). It is present in all cells Cells and tissues CHAPTER 3 Figure 3.7 Coloured transmission electron micrograph showing the Golgi apparatus (green). (Science Photo Library, reproduced with permission.) but is larger in those that synthesise and export proteins.The proteins move from the ER to the Golgi apparats,where they are 'packaged' into membrane-bound vesicles.The vesicles are stored and, when needed,they move to the plasma membrane and fuse with it, expelling the contents from the cell. This process is called exocytosis (Fig. 3.5F). Lysosomes Lysosomes are small membranous vesicles pinched off from the Golgi apparatus. They contain a variety of enzymes involved in breaking down fragments of organelles and large molecules (e.g. RNA, DNA,carbohydrates,proteins)inside the cell into smaller particles that are either recycled,or exported from the cell as waste material. Lysosomes in white blood cells contain enzymes that digest foreign material such as microbes. Cytoskeleton This consists of an extensive network of tiny protein fibres (Fig. 3.8). The cytoskeleton provides an internal support system for the cell, as well as guiding the movement of materials around the cell interior. Microfilaments These tiny fibres, made of actin,are anchored to the inside of the cell membrane and give the cell support and shape.Actin is also involved in the contractile process in muscle cells(p.456). Microtubules These are large, rigid proteins that give the cell mechanical support and also provide the guidance tracking for internal movement of, for example: ·organelles ·chromosomes during cell division. SECTION 1 The body and its constituents Figure 3.8 Fibroblasts. Fluorescent light micrograph showing nuclei (purple) and cytoskeletons (yellow and blue). (R. Torsten Wittman/Science Photo Library, reproduced with permission.) Centrosome This directs organisation of microtubules within the cell.It consists of a pair of centrioles (small clusters of microtubules)and plays an important role in cell division. Cell extensions These project from the plasma membrane in some types of cell. Their main components are microtubules, which allow movement.They include: ·Microvilli-tiny projections that contain microfilaments.They cover the exposed surface of certain types of cell,e.g. absorptive cells that line the small intestine (Fig.3.9).By greatly increasing the surface area, microvilli make the structure of these cells ideal for their function, maximising absorption of nutrients from the small intestine. ·Cilia- microscopic hair-like projections containing microtubules that lie along the free borders of some cells (see Fig. 10.12). They beat in unison, moving substances along the surface, e.g. mucus moves upwards in the respiratory tract. Flagella-single,long, whip-like projections containing microtubules,which form the 'tails' of spermatozoa (see Fig. 1.15) that propel them through the female reproductive tract. The cell cycle Damaged,dead and worn-out cells, depending on their type,can often by replaced by cell division, maintaining tissue integrity and function. The frequency with which cell division occurs varies with different types of tissue (p. 55).Normally,this is carefully regulated to allow effective maintenance and repai of body tissues. At the end of their natural lifespan, Figure 3.9 Microvilli in small intestine. Coloured scanning electron micrograph.(Eye of Science/Science Photo Library,reproduced with permission.) ageing cells are programmed to 'self-destruct' and their components are removed by phagocytosis in a process known as apoptosis (p.56). Cells with nuclei have 46 chromosomes and divide by mitosis, a process that results in two new,genetically identical daughter cells. The only exception to this is the formation of gametes (sex cells), i.e. ova and spermatozoa,which takes place by meiosis (p.480). The period between two cell divisions is known as the cell cycle. This has two phases that can be seen on light microscopy: mitosis (M phase) and interphase(Fig.3.10). Interphase This is the longer phase and three separate stages are recognised: First gap phase (G1) - the cellgrows in size and volume. This is usually the longest phase and most variable in length. Sometimes cells do not continue round the cell cycle but enter a resting phase (Go);despite being called the resting phase, cells at this stage are usually highly active, carrying out their specific functions.The cell may stay in Go for the rest of its life,but may also re-enter the cell cycle and start dividing again if need be. Synthesis of DNA (S phase) - the chromosomes replicate,forming two identical copies of DNA (p.476).Therefore,following the S phase, the cell now has 92 chromosomes,i.e.enough DNA for two cells, and is nearly ready to divide by mitosis. Second gap phase (G2) - there is further growth and preparation for cell division.