The cells, tissues and organisation of the body hydrophobic tails forming a central water-repelling layer These differences influence the transfer of substances across the membrane Chromosomes The membrane proteins perform several functions Secules attached to the outside of some membrane protein molecules give the cell its immunological identity ey can act as specific receptors for horn other chemical messengers some are enzymes some are involved in transport across the membrane Organelles Nucleus Every cell in the body has a nucleus, with the exception of mature erythrocytes(red blood cells). Skeletal muscle and some other cells contain several nuclei. The nucleus the largest organelle and is contained within a mem brane similar to the plasma membrane but it has tiny pores through which some substances can pass between it and the cyte i.e. the cell contents excluding the nucleus The nucleus contains the bodys genetic material, which directs the activities of the cell. this is built from Chromatin DNA (p. 24)and proteins called histones coiled together forming a fine network of threads called chromatin Chromatin resembles tiny strings of beads. During ce 象 31 division the chromatin replicates and becomes more tightly coiled forming chromosomes(Fig 3. 3) The functional subunits of chromosomes are called Histones(proteins) genes. Each cell contains the total complement of genes required to synthesise all the proteins in the body but most cells synthesise only the defined range of proteins that are appropriate to their own specialised functions This means that only part of the genome or genetic code is sed by each cell. Metabolic processes occur in a series of steps, each of which is catalysed by a specific enzyme (p. 26)and each enzyme can be produced only if the controlling gene is present. This is the 'one gene,one enzymeconcept. Therefore, when a gene is missing the ssociated enzyme is also missing and the chemical change it should catalyse does not occur(Fig. 3.4). This C Guanin ans that the intermediate metabolite upon which theOAdenine enzyme should act accumulates In physiological quanti Ct Thymine ties such metabolites are harmless but when they accu- Deoxyribose sugar mulate they may become toxic. There are a number of Phosphate group iseases caused by such inborn errors of metabolism e.g. phenylketonuria, abnormal haemoglobin and some immune deficiencies(see later chapters)
The cells, tissues and organisation of the body hydrophobic tails forming a central water-repelling layer. These differences influence the transfer of substances across the membrane. The membrane proteins perform several functions: • branched carbohydrate molecules attached to the outside of some membrane protein molecules give the cell its immunological identity • they can act as specific receptors for hormones and other chemical messengers • some are enzymes • some are involved in transport across the membrane. Organelles Nucleus Every cell in the body has a nucleus, with the exception of mature erythrocytes (red blood cells). Skeletal muscle and some other cells contain several nuclei. The nucleus is the largest organelle and is contained within a membrane similar to the plasma membrane but it has tiny pores through which some substances can pass between it and the cytoplasm, i.e. the cell contents excluding the nucleus. The nucleus contains the body's genetic material, which directs the activities of the cell. This is built from DNA (p. 24) and proteins called histones coiled together forming a fine network of threads called chromatin. Chromatin resembles tiny strings of beads. During cell division the chromatin replicates and becomes more tightly coiled forming chromosomes (Fig. 3.3). The functional subunits of chromosomes are called genes. Each cell contains the total complement of genes required to synthesise all the proteins in the body but most cells synthesise only the defined range of proteins that are appropriate to their own specialised functions. This means that only part of the genome or genetic code is used by each cell. Metabolic processes occur in a series of steps, each of which is catalysed by a specific enzyme (p. 26) and each enzyme can be produced only if the controlling gene is present. This is the 'one gene, one enzyme' concept. Therefore, when a gene is missing the associated enzyme is also missing and the chemical change it should catalyse does not occur (Fig. 3.4). This means that the intermediate metabolite upon which the enzyme should act accumulates. In physiological quantities such metabolites are harmless but when they accumulate they may become toxic. There are a number of diseases caused by such inborn errors of metabolism, e.g. phenylketonuria, abnormal haemoglobin and some immune deficiencies (see later chapters). 31 Figure 3.3 The structural relationship between DNA, chromatin and chromosomes
The body and its constituents The Golgi apparatus consists of stacks of closely folded synthesis of flattened me IS in all cells Enzyme catalyst larger in those that synthesise and export proteins. The proteins move from the endoplasmic reticulum to the Golgi apparatus where they are 'packaged into mem- brane-bound vesicles called secretory granules. The vesi to the pla Gene absent membrane, through which the proteins are exported Enzyme catalyst absent Intermediate somes metabolite No protein Lysosomes are one type of secretory vesicle formed by the Golgi apparatus. They contain a variety of enzymes involved in breaking down fragments of organelles and Intermediate large molecules(e.g. RNA, DNA, carbohydrates,pro- metabolite teins) inside the cell into smaller particles that are either accumulates recycled, or extruded from the cell as waste material ysosomes in white blood cells contain enzymes that Figure 3.4 The relationship between genes, enzymes and protein digest foreign material such as microbes synthesis: A Enzyme synthesised. B Effect when enzyme not synthesised Microfilaments and microtubules Mitochondria Microfilaments. These are tiny strands of protein that Mitochondria are sausage-shaped structures in the cyto- rovide structural support and maintain the characteristic plasm, sometimes described as the power house of the shape of the cell espiration, the processes by which chemical energy is made available in Microtubules. These are contractile protein structures in the cytoplasm involved in the movement of the cell an the cell. This is in the form of ATe, which releases energy of organelles within the cell, the movement of cilia(small when the cell breaks it down(see Fig. 2.12, p. 25) Synthesis of ATP is most efficient in the final stages of projections from the free border of some cells)and possi- aerobic respiration, a process requiring oxygen(p. 315) bly the organisation of proteins in the plasma membrane Ribosomes are tiny composed of RNA and protein Cell division (Fig 3. 5) They synthesise from amino acids, using RNa There are two types of cell division: mitosis and meiosis as the template 2.11, P. 25). When present in free units or in small clusters in the cytoplasm, the ribosomes Mitosis make proteins for use within the cell. Ribosomes are also Beginning with the fertilised egg, or zygote, cell division found on the outer surface of rough endoplasmic reticu- is an ongoing process. As the fetus develops in the lum(see below) mother's uterus, its cells multiply and grow into all the specialities that provide the sum total of the bodys phys. Endoplasmic reticulum(ER) iological functions. The life span of most individual cel Endoplasmic reticulum is a series of interconnecting is limited. Many become worn out and die, and are membranous canals in the cytoplasm. There are two replaced by identical cells by the process of mitosis types: smooth and rough. Smooth ER synthesises lipids Mitosis occurs in two stages: replication of DNA, in and steroid hormones, and is also associated with the the form of 23 pairs of chromosomes, then division of the detoxification of some drugs. Rough ER is studded with cytoplasm. DNA is the only type of molecule capable of ribosomes. These are the site of synthesis of proteins that independently forming a duplicate of itself. when the are 'exported(extruded) from cells, i. e, enzymes and two identical sets of chromosomes have moved to the hormones that pass out of their parent cell to be used by opposite poles of the parent cell, awaist' forms in the other cells in the body cytoplasm, and the cell divides. There is then a complete
The body and its constituents 32 Figure 3.4 The relationship between genes, enzymes and protein synthesis: A. Enzyme synthesised. B. Effect when enzyme not synthesised. Mitochondria Mitochondria are sausage-shaped structures in the cytoplasm, sometimes described as the 'power house' of the cell. They are involved in 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. 2.12, p. 25). Synthesis of ATP is most efficient in the final stages of aerobic respiration, a process requiring oxygen (p. 315). Ribosomes These are tiny granules composed of RNA and protein. They synthesise proteins from amino acids, using RNA as the template (see Fig. 2.11, p. 25). When present in free units or in small clusters in the cytoplasm, the ribosomes make proteins for use within the cell. Ribosomes are also found on the outer surface of rough endoplasmic reticulum (see below). Endoplasmic reticulum (ER) Endoplasmic reticulum is a series of interconnecting membranous canals in the cytoplasm. There are two types: smooth and rough. Smooth ER synthesises lipids and steroid hormones, and is also associated with the detoxification of some drugs. Rough ER is studded with ribosomes. These are the site of synthesis of proteins that are 'exported' (extruded) from cells, i.e. enzymes and hormones that pass out of their parent cell to be used by other cells in the body. Golgi apparatus The Golgi apparatus consists of stacks of closely folded flattened membranous sacs. It is present in all cells but is larger in those that synthesise and export proteins. The proteins move from the endoplasmic reticulum to the Golgi apparatus where they are 'packaged' into membrane-bound vesicles called secretory granules. The vesicles are stored and, when needed, move to the plasma membrane, through which the proteins are exported. Lysosomes Lysosomes are one type of secretory vesicle formed by 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 extruded from the cell as waste material. Lysosomes in white blood cells contain enzymes that digest foreign material such as microbes. Microfilaments and microtubules Microfilaments. These are tiny strands of protein that provide structural support and maintain the characteristic shape of the cell. Microtubules. These are contractile protein structures in the cytoplasm involved in the movement of the cell and of organelles within the cell, the movement of cilia (small projections from the free border of some cells) and possibly the organisation of proteins in the plasma membrane. Cell division (Fig. 3.5) There are two types of cell division: mitosis and meiosis. Mitosis Beginning with the fertilised egg, or zygote, cell division is an ongoing process. As the fetus develops in the mother's uterus, its cells multiply and grow into all the specialities that provide the sum total of the body's physiological functions. The life span of most individual cells is limited. Many become worn out and die, and are replaced by identical cells by the process of mitosis. Mitosis occurs in two stages: replication of DNA, in the form of 23 pairs of chromosomes, then division of the cytoplasm. DNA is the only type of molecule capable of independently forming a duplicate of itself. When the two identical sets of chromosomes have moved to the opposite poles of the parent cell, a 'waist' forms in the cytoplasm, and the cell divides. There is then a complete
The cells, tissues and organisation of the body Sperm X+ ovum x-> child XX= female Mother cell Sperm Y ovum X- child XY=male (before chromosomes have replicated one division) ( two divisions) ells are said to mutate when their genetic make-up is altered in any chromosomes no significant change in cell function modification of cell function that may cause End of physiological abnormality but does not prevent cell growth and multiplication, e. g inborn errors of metabolism, defective blood clotting End of he death of the cell division Some mutations occur by chance, which may be wo daughter cells Four daughter cells accounted for by the countless millions of cell divisions and dNa replications that occur in the body throughout chromosomes chromosomes life, Others may be caused by extraneous factors, such as X-rays, ultraviolet rays or some chemicals Figure 3.5 Cell division. Simplified diagram of mitosis and meiosis The most important mutations are those that occur in he ova and spermatozoa. Genetic changes in these cells set of chromosomes in each daughter cell. The organelles are passed on to subsequent generations although they in the cytoplasm of the daughter cells are incomplete at do not affect the parent. vision but they develop as the cell grows to maturity. The frequency with which cell division occurs varies with different types of cell(p 42 Transport of substances across cell membranes Meiosis his is the process of cell division that occurs in the for- Passive transport mation of reproductive cells(gammetes-the ova and sper- This occurs when substances can cross plasma and female and the spermatozoa in the testes of the male. In organelle(semipermeable)membranes and move down meiosis four daughter cells are formed after two divi- the concentration gradient(downhill) without using enel During meiosis the pairs of chromosomes separate and one from each pair moves to opposite poles of the Diffusion has only 23 chromosomes, called the haploid number. This page 26 means that when the ovum is fertilised the resultant down the concentration gradient crossing membranes by zygote has the full complement of 46 chomosomes(the dissolving in the lipid part of the membrane, e. g diploid number), half from the father and half from the lipid-soluble substances: oxygen, carbon dioxide, mother. Thus the child has some characteristics inherited fatty acids, steroids from the mother and some from the father, such as colour passing through water- filled channels, or pores in the of hair and eyes, height, facial features, and some diseases membrane, e. g. small water-soluble substances Determination of sex depends upon one particular sodium, potassium, calcium pair of chromosomes: the sex chromosomes. In the female both sex chromosomes are the same size and shape and Facilitated diffusion are called X chromosomes. In the male there is one x This passive process is utilised by some substances chromosome and a slightly smaller Y chromosome. are unable to diffuse through the semipermeable m When the ovum is fertilised by an X-bearing sperma- brane unaided, e.g. glucose, amino acids. Special tozoon the child is female and when it is fertilised by a protein carrier molecules in the membrane have specific Y-bearing spermatozoon the child is male sites that attract and bind substances to be transferred
The cells, tissues and organisation of the body Figure 3.5 Cell division. Simplified diagram of mitosis and meiosis. set of chromosomes in each daughter cell. The organelles in the cytoplasm of the daughter cells are incomplete at cell division but they develop as the cell grows to maturity. The frequency with which cell division occurs varies with different types of cell (p. 42). Meiosis This is the process of cell division that occurs in the formation of reproductive cells (gametes — the ova and spermatozoa). The ova grow to maturity in the ovaries of the female and the spermatozoa in the testes of the male. In meiosis four daughter cells are formed after two divisions. During meiosis the pairs of chromosomes separate and one from each pair moves to opposite poles of the 'parent' cell. When it divides, each of the 'daughter' cells has only 23 chromosomes, called the haploid number. This means that when the ovum is fertilised the resultant zygote has the full complement of 46 chomosomes (the diploid number), half from the father and half from the mother. Thus the child has some characteristics inherited from the mother and some from the father, such as colour of hair and eyes, height, facial features, and some diseases. Determination of sex depends upon one particular pair of chromosomes: the sex chromosomes. In the female both sex chromosomes are the same size and shape and are called X chromosomes. In the male there is one X chromosome and a slightly smaller Y chromosome. When the ovum is fertilised by an X-bearing spermatozoon the child is female and when it is fertilised by a Y-bearing spermatozoon the child is male. Sperm X + ovum X —> child XX = female Sperm Y + ovum X —> child XY = male Mutation Cells are said to mutate when their genetic make-up is altered in any way. Mutation may cause: • no significant change in cell function • modification of cell function that may cause physiological abnormality but does not prevent cell growth and multiplication, e.g. inborn errors of metabolism, defective blood clotting • the death of the cell. Some mutations occur by chance, which may be accounted for by the countless millions of cell divisions and DNA replications that occur in the body throughout life. Others may be caused by extraneous factors, such as X-rays, ultraviolet rays or some chemicals. The most important mutations are those that occur in the ova and spermatozoa. Genetic changes in these cells are passed on to subsequent generations although they do not affect the parent. Transport of substances across cell membranes Passive transport This occurs when substances can cross plasma and organelle (semipermeable) membranes and move down the concentration gradient (downhill) without using energy. Diffusion This was described on page 26. Small substances diffuse down the concentration gradient crossing membranes by: • dissolving in the lipid part of the membrane, e.g. lipid-soluble substances: oxygen, carbon dioxide, fatty acids, steroids • passing through water-filled channels, or pores in the membrane, e.g. small water-soluble substances: sodium, potassium, calcium. Facilitated diffusion This passive process is utilised 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, 33
The body and its constituents like a lock and key mechanism. The carrier then changes its shape and deposits the substance on the other side of cell membrane the membrane(Fig. 3.6). 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 sub stance which can be transported at any time. This known as the transport maximum osmosis Osmosis is passive movement of water down its concentra- tion gradient towards equilibrium across a semipermeable Inside surface of membrane and is explained on page 27 Carrier protein Active transport This is the transport of substances up their concentration gradient (uphill), i.e. from a lower to a higher concentra tion Chemical energy in the form of ATP (p. 25)drives specialised protein carrier molecules that transport sub stances across the membrane in either direction(see Fig 3.6). 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 Figure 3.6 Specialised protein carrier molecules involved in The sodium pump facilitated diffusion and active transport This active tra t mechanism maintains homeostasis of the electrolytes sodium(Na )and potassium(K).It may utilise up to 30% of the ATP required for cellular metabolism engulfed by extensions of the cytoplasm which enclose Q, The principal cations are: K intracellularly and Na* them, forming a membrane-bound vacuole. When the tracellularly. There is a tendency for these ions to dif- vacuole is small, pinocytosis occurs. In phagocytosis fuse down their concentration gradients, K- outwards larger particles, e. g. cell fragments, foreign materials and Na* into the cell. Homeostasis is maintained as microbes, are taken into the cell Lysosomes then adhere excess Na is pumped out across the cell membrane in to the vacuole membrane, releasing enzymes which ange for K digest the content Extrusion of waste material by the reverse process Bulk transport(Fig 3. 7) through the plasma membrane is called exocytosis Secretory granules formed by the Golgi apparatus Transfer of particles too large to cross cell membranes ally leave the cell in this way, as do any indigestible occurs by pinocytosis or phagocytosis. These particles are residues of phagocytose ○⊙②◎ B Particle engulfed Formation Adhesion of Digestion of Exocytosis of a vacuole lysosomes the particle by membrane enzymes Figure 3.7 Bulk transport across plasma membranes: A-E Phagocytosis. F. Exocytosis
The body and its constituents 34 like a lock and key mechanism. The carrier then changes its shape and deposits the substance on the other side of the membrane (Fig. 3.6). 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 which can be transported at any time. This is known as the transport maximum. Osmosis Osmosis is passive movement of water down its concentration gradient towards equilibrium across a semipermeable membrane and is explained on page 27. Active transport 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 ATP (p. 25) drives specialised protein carrier molecules that transport substances across the membrane in either direction (see Fig. 3.6). 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 pump This active transport mechanism maintains homeostasis of the electrolytes sodium (Na+ ) and potassium (K+ ). It may utilise up to 30% of the ATP required for cellular metabolism. The principal cations are: K+ intracellularly and Na+ extracellularly. There is a tendency for these ions to diffuse down their concentration gradients, K+ outwards and Na+ into the cell. Homeostasis is maintained as excess Na+ is pumped out across the cell membrane in exchange for K+ . Bulk transport (Fig. 3.7) Transfer of particles too large to cross cell membranes occurs by pinocytosis or phagocytosis. These particles are Figure 3.6 Specialised protein carrier molecules involved in facilitated diffusion and active transport. engulfed by extensions of the cytoplasm which enclose them, forming a membrane-bound vacuole. When the vacuole is small, pinocytosis occurs. In phagocytosis larger particles, e.g. cell fragments, foreign materials, microbes, are taken into the cell. Lysosomes then adhere to the vacuole membrane, releasing enzymes which digest the contents. Extrusion of waste material by the reverse process through the plasma membrane is called exocytosis. Secretory granules formed by the Golgi apparatus usually leave the cell in this way, as do any indigestible residues of phagocytosis. Figure 3.7 Bulk transport across plasma membranes: A-E. Phagocytosis. F. Exocytosis
The cells, tissues and organisation of the bod TISSUES m含 Learning outcomes Basement membrane After studying this section you should be able to Figure 3. 8 Squamous epithelium a describe the structure and functions of these helial, connective, muscle, nervous absorptive or secretory surfaces, where the single layer m explain the capacity of different types of tissue to enhances these processes, and not usually on surfaces subject to stress. The types are named according to th a outline the structure and functions of membranes shape of the cells, which differs according to their func tions. The more active the tissue the taller are the cells a compare and contrast the structure and functions of exocrine and endocrine glands Squamous (pavement)epithelium This is composed of a single layer of flattened cells(Fig 8) ther like flat stones, fo The tissues of the body consist of large numbers of cells thin and very smooth membra Diffusion takes place freely through this thin, smooth, functions of these cells. There are four main types of inactive lining of the following structures tissue, each of which has subdivisions where it is blood vessels also known as epithelial tissue or epithelium thelium connective tissue ■ alveoli of the lungs nuscle tissu Cuboidal (cubical)epithelium nervous tiss This consists of cube-shaped cells fitting closely together lying on a basement membrane(Fig 3. 9). It forms the 35 Epithelial tissue tubules of the kidneys and is found in some glands Cuboidal epithelium is actively involved in secretion, This group of tissues is found covering the body and lin- absorption and excretion ng cavities and tubes. It is also found in glands. The structure of epithelium is closely related to its functions Columnar epithelium which include This is formed by a single layer of cells, rectangular in a protection of underlying structures from, for shape on a basement membrane( fig 3. 10). It is found example, dehydration, chemical and mechanical Cuboidal epithelial cells Basement membrane The cells are very closely packed and the intercellular substance, called the matrix, is minimal. The cells usually Figure 3.9 Cuboidal epithelium lie on a basement membrane which is an inert connective tissue Epithelial tissue may be Columnar epithelial cells a simple: a single layer of cells stratified: several layers of cells Simple epithelium Simple epithelium consists of a single layer of identical Basement membrane ells and is divided into four types. It is usually found on Figure 3.10 Columnar epithelium
The cells, tissues and organisation of the body TISSUES Learning outcomes After studying this section you should be able to: • describe the structure and functions of these tissues: epithelial, connective, muscle, nervous • explain the capacity of different types of tissue to regenerate • outline the structure and functions of membranes • compare and contrast the structure and functions of exocrine and endocrine glands. The tissues of the body consist of large numbers of cells and they are classified according to the size, shape and functions of these cells. There are four main types of tissue, each of which has subdivisions. They are: • epithelial tissue or epithelium • connective tissue • muscle tissue • nervous tissue. Epithelial tissue This group of tissues is found covering the body and lining cavities and tubes. It is also found in glands. The structure of epithelium is closely related to its functions which include: • protection of underlying structures from, for example, dehydration, chemical and mechanical damage • secretion • absorption. The cells are very closely packed and the intercellular substance, called the matrix, is minimal. The cells usually lie on a basement membrane, which is an inert connective tissue. Epithelial tissue may be: • simple: a single layer of cells • stratified: several layers of cells. Simple epithelium Simple epithelium consists of a single layer of identical cells and is divided into four types. It is usually found on Figure 3.8 Squamous epithelium. absorptive or secretory surfaces, where the single layer enhances these processes, and not usually on surfaces subject to stress. The types are named according to the shape of the cells, which differs according to their functions. The more active the tissue, the taller are the cells. Squamous (pavement) epithelium This is composed of a single layer of flattened cells (Fig. 3.8). The cells fit closely together like flat stones, forming a thin and very smooth membrane. Diffusion takes place freely through this thin, smooth, inactive lining of the following structures: heart blood vessels lymph vessels alveoli of the lungs. where it is also known as endothelium Cuboidal (cubical) epithelium This consists of cube-shaped cells fitting closely together lying on a basement membrane (Fig. 3.9). It forms the tubules of the kidneys and is found in some glands. Cuboidal epithelium is actively involved in secretion, absorption and excretion. Columnar epithelium This is formed by a single layer of cells, rectangular in shape, on a basement membrane (Fig. 3.10). It is found 35 Figure 3.9 Cuboidal epithelium. Figure 3.10 Columnar epithelium