Gutter=16> 1 Introduction
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Зміст 2.2 Body water2.4 The lipids 2.5 The amino acids and proteins |
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The chemical constitution of the body  2.1 IntroductionThe human body consists largely of four elements: oxygen, carbon, hydrogen, and nitrogen. These are combined in many different ways to make a huge variety of chemical compounds. The major organic (i.e. carbon-containing) constituents of mammalian cells are the carbohydrates, fats, proteins, and nucleic acids, which are built from small molecules belonging to four principal groups: the sugars, the fatty acids, the amino acids, and the nucleotides. About 60 per cent of the body of an adult male is water, the remaining 40 per cent being made up of fats, proteins, carbohydrates, and minerals. Females have a higher proportion of body fat and so total body water is about 50 per cent of their body weight. In both males and females, the lean body tissues are about 70 per cent water, 15 per cent fats, 12 per cent protein, 2 per cent nucleic acids, and 0.5 per cent carbohydrate (sugars). The remainder is made up of minerals of various kinds such as Table 2.1 The approximate chemical constitution of the body (all values are expressed as percentage body weight)
T  he body contains trace amounts of other elements in addition to those listed above.calcium, iron, magnesium, phosphate, potassium, and sodium. The chemical composition of the body given in Table 2.1 is an approximate average of all the tissues of an adult. The proportions of the various constituents vary between tissues and change during development. ^ Water is the principal constituent of the human body and is essential for life. It is the chief solvent in living cells. Molecules of biological interest can be divided into those that dissolve readily in water and those that do not. Substances that dissolve readily in water are called polar or bydrophilic while those that are insoluble in water are called nonpolar or hydrophobic. Examples of polar substances are sodium chloride, glucose, and ethanol, while examples of nonpolar materials are the paraffin waxes and benzene. Many molecules have mixed properties so that one part is polar while another is nonpolar. These are known as amphiphilic substances. Examples of amphiphilic substances are the long-chain fatty acids, cholesterol, and the bile salts. The intracellular and extracellular fluids Body water can be divided into that within the cells, the intracellular water, and that which lies outside the cells, the extracellular water. The extracellular water is further subdivided into the plasma and the interstitial water (Fig. 2.1). The interstitial water lies outside the blood vessels and bathes the cells. In addition, a small percentage of body water is found in the lymphatic fluid. The mechanisms by which body water is kept in balance are considered in Chapter 28. Body water contains many different substances in solution and the solutes and water of the space outside the cells is called the extracellular or interstitial fluid while that inside the cells is the intracellular fluid. The intracellular fluid is separated from the extracellular fluid by the plasma membrane of the individual cells, which is composed of lipids (fats) and has a nonpolar core (see Chapter 3). Consequently, polar molecules cannot cross readily from the extracellular fluid to the intracellular fluid. Indeed, this barrier is used to create concentration gradients that the cells exploit to perform various functions.    Diffusion When a substance (the solute) is dissolved in a solvent to form a solution, the individual solute molecules become dispersed within the solvent and are free to move in a random way. In an aqueous solution, the molecules of both water and solute are in continuous random motion with frequent collisions between them. This process leads to diffusion, the random dispersion of molecules in solution. Diffusion is not confined to aqueous solutions and occurs through cell membranes which are largely made of lipids (see below). When a drop of a concentrated solution (e.g. 5 per cent w/v glucose) is added to a volume of pure water, the random motion of the glucose molecules results in their slow dispersion throughout the whole volume. If the 5 per cent solution had been added to a 1 per cent solution of glucose, the same process of dispersion of the glucose molecules would occur until the whole solution was of uniform concentration. There is a tendency for the glucose (or any other solute) to diffuse from a region of high concentration to one of a lower concentration (i.e. down a concentration gradient). The rate of diffusion in a solvent depends on temperature (it is faster at higher temperatures), the magnitude of the concentration gradient, and the area over which diffusion can occur. In general, large molecules diffuse more slowly than small ones. The molecular characteristics of the solute and solvent also affect the rate of diffusion. These characteristics are reflected in a physical constant known as the diffusion coefficient. The osmotic pressure of the body fluids When an aqueous solution is separated from pure water by a membrane that is permeable to water but not to the solute, water moves across the membrane into the solution by a process known as osmosis. This movement can be opposed by applying a hydrostatic pressure to the solution. The pressure that is just sufficient to prevent the uptake of water is known as the osmotic pressure of the solution. The osmotic pressure of a solution is expressed as the osmolality and is related to the number of particles present per kilogram of solvent, independent of their chemical nature. One gram mole of a nondissociating substance in one kilogram of solvent exerts an osmotic pressure of 1 osmole. Thus the osmotic pressure exerted by a mole of glucose (relative molecular mass (Afr) 180) is the same as that exerted by a mole of albumin (Mc 67 000). Aqueous salt solutions are an important exception to this rule: the salts separate into their constituent ions so that a solution of sodium chloride will exert an osmotic pressure double that of its molal concentration, i.e. a lOOmmolkg-1 solution of sodium chloride in water will have an osmotic pressure of 200 mosmol kg"1 of which a half is due to the sodium ions and half to the chloride ions. The total osmolality of a solution is the sum of the osmolality due to each of the constituents. The extracellular fluid and plasma have an osmolality of around 290 mosmol kg-1. The principal ions (Na+, K+, CI , etc.) contribute 280 mosmol kg-1 (about 96 per cent) while glucose, amino acids, and other small nonionic substances contribute approximately 10 mosmol kg-1. Proteins contribute only around 0.5 per cent to the total osmolality of plasma and still less to the osmolality of the extracellular fluid (which has little protein). This is made clear by the following calculations: the plasma of the blood contains about 8.2 g of sodium chloride and 45 g of albumin per liter. The osmotic pressure exerted by 8.2 g of sodium chloride (Mr 58) is: (2 X 8.2) -^ 58 = 0.276 osmol g-1 or 276 mosmol kg-1. The osmotic pressure exerted by 45 g of albumin is: 45 -*■ 67 000 = 6.7 X lO^1 osmol kg"1 or 0.67 mosmol kg"1. Thus the osmotic pressure exerted by 45 g of albumin is only 0.2 per cent that of 8 g of sodium chloride. This makes clear that the osmotic pressure exerted by proteins is far less than that exerted by the principal ions of the biological fluids. Nevertheless, the small osmotic pressure that the proteins do exert (known as the colloid osmotic pressure or oncotic pressure) plays an important role in the exchange of fluids between body compartments. Although lipid membranes are hydrophobic, they are more permeable to water than they are to ions so that the osmolality of the intracellular fluid is the same as that of the extracellular fluid (i.e. the two fluids have an osmolality of about 290 mosmol kg-1 2.3 The sugars 11  and are iso-osmotic). If the osmotic pressure in one compartment is higher than the other, water will move from the region of low osmotic pressure to that of the higher osmotic pressure until the two become equalized.The tonicity of solutions If a suspension of red blood cells is placed in a solution of sodium chloride that has an osmolality of 260 mosmol kg"1, the cells will swell as water is drawn into them to equalize the osmotic pressure across the cell membrane. This concenrration of sodium chloride is said to be hypotonic with respect to the cells. Conversely, if the cells are placed in a solution of sodium chloride that has an osmolality of 320 mosmol kg-1 they will shrink as water is drawn from the cells. In this case the fluid is hypertonic. Cells placed in a solution of 0.9 per cent sodium chloride in water (i.e. 0.9 g sodium chloride in 100 ml of water) neither swell nor shrink. This concentration has an osmolality of approximately 290 mosmol kg-1 and is said to be isotonic with the cells. (This solution is sometimes referred to as 'normal saline' but would be better called isotonic saline.) Not all solutions that are iso-osmotic with respect to the intracellular fluid are isotonic with cells. A solution containing 290 mosmol kg-1 of urea is iso-osmotic with both normal saline and the intracellular fluid but it is not isotonic as cells placed in such a solution would swell and burst (a process called lysis). This behavior is explained by the fact that urea can penetrare the cell membrane relatively freely. When it does so, it diffuses down its concentration gradient and warer will follow (otherwise the osmolality of the intracellular fluid would increase and become hyperosmotic). As a result the cells swell until they burst. Filtration When a fluid passes through a permeable membrane, it leaves behind those particles that are larger in diameter than the pores of the membrane. This process is filtration and is driven by the pressure gradient between the two sides of the membrane. When filtration separates large solutes, such as proteins, from small ones, such as glucose and inorganic ions (Na+, K+, Cl~, etc.), the process is called ultrafiltration. The walls of the capillaries are not normally permeable to plasma proteins (e.g. albumin) but are permeable ro small solutes. The pumping action of the heart causes a pressure gradient across the walls of the capillaries so tending to force fluid from the capillaries into the interstitial space. This process occurs in all vascular beds but is particularly important in the glomerular capillaries of the kidney which filter large volumes of plasma each day. Summary
2  .3 The sugarsThe sugars are the principal source of energy for cellular reactions. They have the general formula Cn(H2O)m and some examples are shown in Fig. 2.2. Sugars containing three carbon atoms are known as trioses, those with five carbons are pentoses, and those containing six are bexoses. Examples are glyceraldehyde (a triose), ribose (a pentose), fructose and glucose (both hexoses). When two sugar molecules are joined together with the elimination of one molecule of water, they form a disaccharide. Fructose and glucose combine to form sucrose, while glucose and galactose (another hexose) form lactose, the principal sugar of milk. When many sugar molecules are joined together they form a polysaccharide. Examples of polysaccharides are starch, which is an important constituent of the diet, and glycogen, which is the main store of carbohydrate within the muscles and liver. Although sugars are the major source of energy for cells, they are also constituents of a number of important molecules. The nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) contain the pentose sugars 2-deoxyribose and ribose. Ribose is also one of the components of the purine nucleotides which play a central role in cellular metabolism. (The structure of the nucleotides is given below in Section 2.6.) Some hexoses have an amino group in place of one of the hydroxyl groups. These are known as the amino sugars or hexosamines. The amino sugars are found in the glycoproteins {= sugar + prorein) and the glycolipids (= sugar + lipid). In the glycoproteins, a polysaccharide chain is linked to a protein by a covalent bond. The glycoproteins are important constituents of bone and connective tissue. The glycolipids consist of a polysaccharide chain linked to the glycerol residue of a sphingosine lipid (see below). Glycolipids are found in the cell membranes, particularly those of the white matter of the brain and spinal cord.    Fig. 2.2 The structure of representative members of the carbohydrates. The polysaccharide glycogen consists of many glucose molecules joined together by 1-4 linkages, known as glycosidk bonds, to form a long chain. A number of glucose chains are joined together by 1-6 linkages to form a single glycogen molecule. Summaryl  .The carbohydrates, especially glucose (a hexose sugar), are broken down to provide energy for cellular reactions. The body stores carbohydrate for energy metabolism as glycogen, which is a polysaccharide.2. While sugars are the major source of energy for cells, they are also constituents of a large number of molecules of biological importance, such as the purine nucleotides and the nucleic acids. ^ 2 .4 The lipidsThe lipids are a chemically diverse group of substances that share the property of being insoluble in water but soluble in organic solvents such as ether and chloroform. They serve a wide variety of functions: they are the main structural element of cell membranes (Chapter 3); they are an important reserve of energy; some act as chemical signals (e.g. the steroid hormones and prostaglandins); they provide a layer of heat insulation beneath the skin; and some provide electrical insulation for the conduction of nerve impulses. The triglycerides or triacylglycerols are the body's main store of energy and can be laid down in adipose tissue in virtually unlimited amounts. They consist of three fatty acids joined by ester linkages to glycerol, as shown in Fig. 2.3. Diglycerides have two fatty acids linked to glycerol, while monoglycerides have only one. The fatty acids have the general formula CH3(CH2)nCOOH. Typical fatty acids are acetic acid (with two carbon atoms), butyric acid (with four carbon atoms), palmitic acid (with 16 carbon atoms), and stearic acid (with 18 carbon atoms). Triglycerides generally contain fatty acids with many carbon atoms, e.g. palmitic and stearic acids, and the middle fatty-acid chain frequently has an unsaturated fatty acid such as linoleic acid (18 carbon atoms with two double bonds) and aracbidonic acid (20 carbon atoms with four double bonds). Although mammals, including humans, are unable to synthesize these unsaturated fatty acids, they play an important role in cellular metabolism. Consequently they must be provided by the diet and are known as the essential fatty acids. The essential fatty acids are precursors for an important group of lipids known as the prostaglandins (see below). The structural lipids are the main component of cell membranes. They fall into three main groups: phospholipids, glycolipids and cholesterol. The basic chemical structures of these key constituents can be seen in Fig. 2.4. The phospholipids fall into two groups: those based on glycerol and those based on sphingosine. The glycero-phospholipids are the most abundant in the mammalian plasma membranes and are classified on the basis of the polar groups attached to the phosphate. Phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phospha-tidylinositol are examples of glycero-phospholipids. The glycerophosphate head groups are linked to long-chain fatty-acid residues via ester linkages. There is, however, another class of phospholipid, the plasmalogens, in which one hydrocarbon chain is linked to the glycerol of the head group via an ether linkage. The fatty-acid residues vary in chain length from 14 to 24   Fig. 2.4 The structure of some of the structural lipids (lipids that form the cell membranes). Note that they have a polar head-group region and a long hydrophobic tail. carbons and may contain one or more double bonds. Commonly, one fatty-acid chain is fully saturated and one possesses a cis double bond, for example oleic acid has a double bond between carbons 9 and 10.The glycolipids of mammalian membranes are based on sphin-gosine, with one hydroxyl group of glycerol linked to a sugar residue, as shown in Fig. 2.4. As with the phospholipids, there are very many glycolipids and the carbohydrate chains may be straight or branched. The steroids are lipids with a structure based on four carbon rings, known as the steroid nucleus. The most abundant steroid is cholesterol (Fig. 2.3), which is a major constituent of cell membranes and which acts as the precursor for the synthesis of many steroid hormones. The prostaglandins are lipids that are derived from the unsaturated fatty acid arachidonic acid (Fig. 2.5). Their biosynthesis and physiological roles are discussed in Chapter 5. The long-chain fatty acids and steroids are insoluble in water but they naturally form micelles in which the polar head groups face outwards towards the water (the aqueous phase) and the long hydrophobic chains associate together. They are transported in the blood and body fluids in association with proteins, as lipoprotein particles. Each particle consists of a lipid micelle protected by a coat of proteins known as apoproteins.  Fig. 2.5 The chemical structures of some of the prostaglandins. In cell membranes, the lipids form bilayers which are arranged so that their polar head groups are orientated towards the aqueous phase while the hydrophobic fatty-acid chains face inwards to form a central hydrophobic region. This provides a barrier to the diffusion of polar molecules (e.g. glucose) and ions but not to small, nonpolar molecules such as urea. The cell membranes divide the cell into discrete compartments which provide the means of storage of various materials and permit the segregation of different metabolic processes. This compart-mentalization of cells by lipid membranes is discussed in greater detail in Chapter 3.Summary T  he lipids are a chemically diverse group of substances that are insoluble in water but soluble in organic solvents such as ether and chloroform. They serve a "wide variety of functions: the phospholipids form the main structural element of cell membranes; the triglycerides are an important reserve of energy; while many steroids and prostaglandins act as chemical signals.^ Proteins serve an extraordinarily wide variety of functions in the body. They form the enzymes that catalyze the chemical reactions of living things. They are involved in the transport of molecules and ions around the body. Proteins bind ions and small molecules for storage and are responsible for the transport of molecules and ions across cell membranes. Proteins such as tubulin form the cytoskeleton, which provides the structural strength of cells, and they form the motile components of muscle and of cilia. The immunoglobulins are proteins that play an important part in the body's defense against infection. As if all this were not enough, some proteins act as signaling molecules—the hormone insulin is one example of this type of protein. Proteins are assembled from a set of twenty α-amino acids The basic structural units of proteins are the α-amino acids. An α-amino acid is a carboxylic ^cid that has an amine group and a side-chain attached to the carbon atom next to the carboxyl group (the α-carbon atom), as shown in Fig. 2.6. The α-carbon atom is thus attached to four different groups and exhibits optical asymmetry, with an L-form and a D-form. All naturally occurring amino acids belong to the L-series.   Proteins are built from 20 different L—amino acids which may be grouped into five different classes:
Amino acids can be combined together by linking the amine group of one with the carboxyl group of another and eliminating water, to form a dipeptide as shown in Fig. 2.7. The linkage between two amino acids joined in this way is known as a peptide bond. The addition of a third amino acid would give a tripeptide, a fourth a tetrapeptide, and so on. Peptides with large numbers of amino acids linked together are known as polypeptides. Proteins are large polypeptides. By convention, the structure of a peptide begins at the end with the free amine group (the amino terminus) on the left and ends with the free carboxyl group on the right, and the order in which the amino acids are arranged is known as the peptide sequence. Since proteins and most peptides are large structures, the sequence of amino acids would be tedious to write out in full so that a single letter or three-letter code is used, as shown in Table 2.2. Since proteins are made from 20 L-amino acids and there is no specific limit to the number of amino acids that can be linked together, the number of possible protein structures is essentially Table 2.2 The α-amino acids of proteins and their customary abbreviations
T he amino acids are arranged in alphabetical order of their single-letter codes. Asparagine and glutamine are amides of aspartic and glutamic acids. Fig. 2.7 The formation of a peptide bond and the structure of the amino-terminus region of a polypeptide, showing the peptide bonds. R1; R2, etc. represent different amino-acid side-chains. 2.6 The nucleosides, nucleotides, and nucleic acids 17 infinite. This is why they are so versatile. Different proteins have different shapes and different physical properties. The fact that some amino-acid side-chains are hydrophilic while others are hydrophobic results in different proteins having differing degrees of hydrophobicity. As a result, some are soluble in water while others are not but are associated with the lipid membranes of cells.Many cellular structures consist of protein assemblies, i.e. units made up of several different kinds of protein. Examples are the myofilaments of the skeletal muscle fibers. These contain the proteins actin, myosin, troponin, and tropomyosin. Actin molecules also assemble together to form microfibrils. Enzymes are frequently arranged so that the product of one enzyme can be passed directly to another, and so on. These multi-enzyme assemblies increase the efficiency of cell metabolism. Some important amino acids are not found in proteins Some amino acids of physiological importance are not found in proteins but have other important functions. Coenzyme A contains an isomer of alanine called /3-alanme. The amino acid γ-aminobutyric acid (GABA) plays a major role as a neurotransmitter in the brain and spinal cord. Creatine is phos-phorylated in muscle to form creatine phosphate which is an important source of energy in muscle contraction. Ornithine is an intermediate in the urea cycle. Summary
2  .6 The nucleosides, nucleotides, and nucleic acidsThe genetic information of the body resides in its DNA which is stored in the chromosomes of the nucleus. DNA is made by assembling smaller components known as nucleotides into a long chain. RNA has a similar primary structure. Each nucleotide consists of a base linked to a pentose sugar, which is in turn linked to a phosphate group, as shown in Fig. 2.8. The bases in the nucleic acids are cytosine, thymine, and uracil, which are based on the structure of pyrimidine (the pyrimidine bases), and adenine and guanine, which are based on the structure of purine (the purine bases). Other bases of importance are nicotinamide and dimethylisoalloxazine. These form the nicotinamide and flavine nucleotides that play an important role in cellular metabolism (Chapter 3). Nucleosides When a base combines with" a pentose sugar it forms a nucleoside. Thus, the combination of adenine and ribose forms adenosine. The combination of thymine with ribose forms thymidine, and so on. Nucleotides A nucleotide is formed when a nucleoside becomes linked to one or more phosphate groups. Thus adenosine may become linked to one phosphate to form adenosine monophosphate, uridine will form uridine monophosphate, and so on. The nucleotides are the basic building blocks of the nucleic acids. The nucleotide coenzymes Nucleotides can be combined together or with other molecules to form coenzymes. Adenosine monophosphate (AMP) may become linked to a further phosphate group to form adenosine diphosphate (ADP) or to two further phosphate groups to form adenosine triphosphate (ATP) (Fig. 2.8). Similarly, guanosine may form guanosine mono-, di-, and triphosphate and uridine can form uridine mono-, di-, and triphosphate. The higher phosphates of the nucleotides play a vital role in cellular energy metabolism. The phosphate group of nucleotides is attached to the 5' position of the ribose residue and has two negative charges. It can link with the hydroxyl of the 3' position to form 3',5' cyclic adenosine monophosphate, or cyclic AMP, which plays an important role as an intracellular messenger. The nucleic acids In nature there are two main types of nucleic acid: DNA and RNA. In DNA the sugar of the nucleotides is deoxyribose and the bases are adenine, guanine, cytosine, and thymine (abbreviated A, G, C, and T). In RNA the sugar is ribose and the bases are adenine, guanine, cytosine, and uracil (A, G, C, and U). In both DNA and RNA the nucleotides are joined by phosphate linkages between the 5' position of one nucleotide and the 3 position of the next, as shown in Fig. 2.9. A molecule of DNA consists of a pair of nucleotide chains linked together by hydrogen bonds in such a way that adenine links with thymine and guanine links with cytosine. The hydrogen bonding between the two chains is precise, so that if the sequence of bases on one chain is given, that of the second is automatically determined. The pair of chains is twisted to form a double helix in which the complementary strands run in opposite directions (Fig. 2.9). The discovery of this base pairing was    crucial to the understanding of the three-dimensional structure of DNA and to the subsequent unraveling of the genetic code. For their work in this area J. D. Watson, F. Crick, and M. Wilkins were awarded the Nobel prize in 1962. A sequence of DNA bases that codes for a specific protein is known as a gene. A strand of DNA is too large to leave the nucleus, so the DNA sequence is passed to the cellular machinery for making proteins by messenger RNA, which is one of the three main forms of RNA. Messenger RNA makes up only 1 per cent of the total RNA of cell but it plays a crucial role in carrying the genetic information from the nucleus to the cytoplasm where protein synthesis takes place. The other forms of RNA are transfer RNA and ribosomal RNA. Ribosomal RNA is found mainly in association with small subcellular particles  Fig. 2.9 The structure of DNA: (a) represents a short length of one of the strands of DNA; (b) is a diagrammatic representation of the two complementary strands. Note that the sequence of one strand runs in the opposite sense to the other. known as ribosomes, which are the sites of protein synthesis. The amino acids required for protein synthesis are carried to the ribosomes by transfer RNA. Unlike DNA, each RNA molecule has only one polynucleotide chain. Summary
R  ecommended readingAlberts, B., Bray, D., Lewis, J., Raff, M., Roberts, K., and Watson, J. D. (1994). Molecular biology of the cell, (3rd edn), Chapters 2 and 3. Garland, New York. Elliott, W. H. and Elliott, D. C. (1997). Biochemistry and cell biology, Chapters 2, and 18-22. Oxford University Press, Oxford. Stryer, L. (1995). Biochemistry, (4th edn), Chapters 1-4. Freeman, New York. |
roteins are assembled from a set of 20 -amino acids which are linked
ucleotides are made of a base, a pentose sugar, and a phosphate