Introduction to the chemistry of life HCI→HCl 4→3HPO3 Table 2.2 Examples of normal plasma levels In the second example, three atoms of hydrogen ha Substance Amount in SI units Amount in other units each lost one electron, all of which have been taken up by one unit, the phosphate radical, making a phosphate ion Chloride 97-106mmo 97-106mE with three negative charge 135-143mmo∥l 135-143mEq A large number of compounds present in the body are not ionic and therefore have no electrical propertiesGlucose 3.5-5.5mmol 60-100mg/100ml when dissolved in water, e. g carbohydrates 14-35μmoW 90-196ug/100ml Molecular weigh The molecular weight of a molecule is the sum of the In physiology this system has the advantage of being a atomic weights of the elements which form its molecules, measure of the number of particles(molecules, atoms, ions) of substances present because molar solutions of Water (HOh different substances contain the same number of parti- 2 hydrogen atoms cles. It has the advantage over the measure milliequiva atomic wei ents per litre* because it can be used for non-electrolytes 1 oxygen atom (atomic weight 16) 16 in fact for any substance of known molecular weight Molecular weight =18 Many of the chemical substances present in the body are in very low concentrations so it is more convenient to Sodium bicarbonate(NaHCO) lit 1 sodium atom (atomic weight 23) 23 (mmol/l)or micromoles per litre (umol/I)as a biological 1 hydrogen atom (atomic weight 1) 1 1 carbon atom (atomic weight 12) 12 For substances of unknown molecular we 3 oxygen atoms insulin, concentration may be expressed in International Molecular weight =84 Units per millilitre(IU/ml) ply as a figure until a scale of measurement of weight is applied Acids, alkalis and pH Molar concentration The number of hydrogen ions present in a solution is a measure This is the term recommended in the Systeme Internationale of the acidity of e maint of the normal hydrogen ion concentration(IH'l)within for expressing the concentration of substances present in the body is an important factor in maintaining a stable the body fluids(SI units environment, ie homeostasis The mole(mol) is the molecular weight in grams of a substance(formerly called 1 gram molecule). One mole The ph scale of any substance contains 6. x 10 molecules or A standard scale for the measurement of the hydrogen ion atoms. For example, 1 mole of sodium bicarbonate(the concentration in solution has been developed: the pH example above)is 84 grams A molar solution is a solution in which 1 mole of a sub. scale. Not all acids ionise completely when dissolved in ater. The hydrogen ion concentration is a measure, there- stance is dissolved in 1 litre of solvent. In the human fore, of the amount of dissociated acid(ionised acid)rather body the solvent is water or fat. A molar solution of than of the total amount of acid present. Strong acids dis- sodium bicarbonate is therefore prepared using 84 g of sociate more freely than weak acids, e.g. hydrochloric acid sodium bicarbonate dissolved in 1 litre of solvent Molar concentration may be used to measure quanti- ties of electrolytes, non-electrolytes, ions and atoms, e.g. * Milliequivalents per litre(mEq/) molar solutions of the following substances mean 1 mole of sodium chloride molecules =58.5 g per litre Equivalent weight number of electrical 1 mole of sodium ions(Na =23 g per Concentration is expressed 1 mole of carbon atoms(C) 12 lit 1 mole of atmospheric oxygen(o)) =32 g per litre x number of electrical char
Introduction to the chemistry of life HC1 -> H+ ClH3PO4 3H+ PO4 3 - In the second example, three atoms of hydrogen have each lost one electron, all of which have been taken up by one unit, the phosphate radical, making a phosphate ion with three negative charges. A large number of compounds present in the body are not ionic and therefore have no electrical properties when dissolved in water, e.g. carbohydrates. Molecular weight The molecular weight of a molecule is the sum of the atomic weights of the elements which form its molecules, Water (H.OH) 2 hydrogen atoms 1 oxygen atom (atomic weight 1) 2 (atomic weight 16) 16 Molecular weight = 18 ?Table 2.2 Examples of normal plasma levels fiB Sodium bicarbonate (NaHCO3) 1 sodium atom (atomic weight 23) 23 1 hydrogen atom (atomic weight 1) 1 1 carbon atom (atomic weight 12) 12 3 oxygen atoms (atomic weight 16) 48 Molecular weight = 84 Molecular weight, like atomic weight, is expressed simply as a figure until a scale of measurement of weight is applied. Molar concentration This is the term recommended in the Systeme Internationale for expressing the concentration of substances present in the body fluids (SI units). The mole (mol) is the molecular weight in grams of a substance (formerly called 1 gram molecule). One mole of any substance contains 6.023 x 1023 molecules or atoms. For example, 1 mole of sodium bicarbonate (the example above) is 84 grams. A molar solution is a solution in which 1 mole of a substance is dissolved in 1 litre of solvent. In the human body the solvent is water or fat. A molar solution of sodium bicarbonate is therefore prepared using 84 g of sodium bicarbonate'dissolved in 1 litre of solvent. Molar concentration may be used to measure quantities of electrolytes, non-electrolytes, ions and atoms, e.g. molar solutions of the following substances mean: 1 mole of sodium chloride molecules = 58.5 g per litre (NaCl) 1 mole of sodium ions (Na+ ) = 23 g per litre 1 mole of carbon atoms (C) = 12 g per litre 1 mole of atmospheric oxygen (O2) = 32 g per litre Substance Amount in Si units Amount in other units Chloride Sodium Glucose Iron 97-106 mmol/l 135-143 mmol/l 3.5-5.5 mmol/l 14-35 nmoi/I 97-106mEq/l 135-143 mEq/l 60-100mg/100ml 90-196ng/100 ml In physiology this system has the advantage of being a measure of the number of particles (molecules, atoms, ions) of substances present because molar solutions of different substances contain the same number of particles. It has the advantage over the measure milliequivalents per litre* because it can be used for non-electrolytes, in fact for any substance of known molecular weight. Many of the chemical substances present in the body are in very low concentrations so it is more convenient to use smaller metric measures, e.g. millimoles per litre (mmol/l) or micromoles per litre (umol/1) as a biological measure (Table 2.2). For substances of unknown molecular weight, e.g. insulin, concentration may be expressed in International Units per millilitre (IU/ml). Acids, alkalis and pH The number of hydrogen ions present in a solution is a measure of the acidity of the solution. The maintenance of the normal hydrogen ion concentration ([H+ ]) within the body is an important factor in maintaining a stable environment, i.e. homeostasis. The pH scale A standard scale for the measurement of the hydrogen ion concentration in solution has been developed: the pH scale. Not all acids ionise completely when dissolved in water. The hydrogen ion concentration is a measure, therefore, of the amount of dissociated add (ionised acid) rather than of the total amount of acid present. Strong acids dissociate more freely than weak acids, e.g. hydrochloric acid *Milliequivalents per litre (mEq/1) _ atomic weight Equivalent weight = number of electrical charges Concentration is expressed: mEq/n1/ = --------- x number of electrical charges atomic weight 21
The body and its constituents Neutral Acid Alkaline 2.3 pH values of body fluids 1234567891011121314 Body fluid Blood 7.35to745 Blood Saliva 5.4to7.5 Figure 2. 6 The pH scale Gastric juice 1.5to3.5 dissociates freely into H"and Cl, while carbonic acid dis-Bile 6to8.5 sociate much less freely into H* and HCO The number of free hydrogen tons in a solution is a measure of its acidity Urine 4.5to8.0 rather than an indication of the type of molecule from which the hydrogen ions originate amylase, the enzyme present in saliva which initiates the The alkalinity of a solution depends on the number of digestion of carbohydrates. The action of salivary amy hydroxyl ions(OH). Water is a neutral solution because lase is inhibited when food containing it reaches the every molecule contains one hydrogen ion and one stomach and is mixed with acid gastric juice hydroxyl radical. For every molecule of water(HOH Blood has a pH value between 7.35 and 7. 45. The pH which dissociates, one hydrogen ion (H, )and one range of blood compatible with life is 7.0 to 7. 8. The meta- hydroxyl ion(OH-)are formed, neutralising each other. bolic activity of the body cells produces certain acids and The scale for measurement of pH was developed alkalis which alter the pH of the tissue fluid and blood taking water as the standard To maintain the pH within the normal range, there are oF In a neutral solution such as water, where the number substances present in blood that act as buffers hydrogen ions is balanced by the same number of hydroxyl ions, the pH=7. The range of this scale is from Buffers 0to14. The optimum pH level is maintained by the balance A pH reading below 7 indicates an acid solution, while between acids and bases produced by cells. Bases are readings above 7 indicate alkalinity(Fig. 2.6). A change of substances that accept(or bind)hydrogen ions and when one whole number on the ph scale indicates a tenfold dissolved in water they produce an alkaline solution hange in [H*]. Therefore, a solution of pH 5 contains ten Buffers are substances such as phosphates, bicarbonates times as many hydrogen ions as a solution of pH6 and some proteins that maintain the [H'I within normal, Ordinary litmus paper indicates whether a solution is but narrow, limits. Some buffers'bind' hydrogen ions cid or alkaline by colouring blue for alkaline and red for and others'bind hydroxyl ions, reducing their circulat acid. Other specially treated absorbent papers give an levels and preventing damaging changes. For exam- approximate measure of ph by a colour change. When ple, if there is sodium hydroxide(NaoH) and carbonic accurate measurements of ph are required, sensitive ph acid(H2CO3) present, both will ionise to some extent, but meters are used they will also react together to form sodium bicarbonate aHCO,)and water(H OH). One of the hydrogen pH values of the body fluids from the acid has been 'bound in the formation of the Body fluids have pH values that must be maintained bicarbonate radical and the other by combining with the within relatively narrow limits for normal cell activity. hydroxyl radical to form water The ph values are not the same in all parts of the body e.g. the normal range of pH values of certain body fluids carbonic sodit water are shown in table 2.3 hydroxide ate The pH value in an organ is produced by its secretion of acids or alkalis which establishes the optimum level. Acidosis and alkalosis The highly acid pH of the gastric juice is maintained by The substances in the complex buffer system that ' bind hydrochloric acid secreted by the parietal cells in the hydrogen ions are called the alkali reserve of the blood walls of the gastric glands. The low pH value in the stom- When the pH is below 7.35, and all the reserves of alka ach provides the environment best suited to the function- line buffer are used up, the condition of acidosis exists g of the enzyme pepsin that begins the digestion of When the reverse situation pertains and the pH is above dietary protein. Saliva has a pH of between 5.4 and 7.5 7.45, and the increased alkali uses up all the acid reserve, which is the optimum value for the action of salivary the state of alkalosis exist
The body and its constituents 22 Figure 2.6 The pH scale. Table 2.3 pH values of body fluids Body fluid Blood Saliva Gastric juice Bile Urine pH 7.35 to 7.45 5.4 to 7.5 1.5 to 3.5 6 to 8.5 4.5 to 8.0 dissociates freely into H+ and Cl~, while carbonic acid dissociates much less freely into H+ and HCO3 - . The number of free hydrogen ions in a solution is a measure of its acidity rather than an indication of the type of molecule from which the hydrogen ions originated. The alkalinity of a solution depends on the number of hydroxyl ions (OH-). Water is a neutral solution because every molecule contains one hydrogen ion and one hydroxyl radical. For every molecule of water (H.OH) which dissociates, one hydrogen ion (H+ ) and one hydroxyl ion (OH-) are formed, neutralising each other. The scale for measurement of pH was developed taking water as the standard. In a neutral solution such as water, where the number of hydrogen ions is balanced by the same number of hydroxyl ions, the pH = 7. The range of this scale is from 0 to 14. A pH reading below 7 indicates an acid solution, while readings above 7 indicate alkalinity (Fig. 2.6). A change of one whole number on the pH scale indicates a tenfold change in [H+ ]. Therefore, a solution of pH 5 contains ten times as many hydrogen ions as a solution of pH 6. Ordinary litmus paper indicates whether a solution is acid or alkaline by colouring blue for alkaline and red for acid. Other specially treated absorbent papers give an approximate measure of pH by a colour change. When accurate measurements of pH are required, sensitive pH meters are used. pH values of the body fluids Body fluids have pH values that must be maintained within relatively narrow limits for normal cell activity. The pH values are not the same in all parts of the body; e.g. the normal range of pH values of certain body fluids are shown in Table 2.3. The pH value in an organ is produced by its secretion of acids or alkalis which establishes the optimum level. The highly acid pH of the gastric juice is maintained by hydrochloric acid secreted by the parietal cells in the walls of the gastric glands. The low pH value in the stomach provides the environment best suited to the functioning of the enzyme pepsin that begins the digestion of dietary protein. Saliva has a pH of between 5.4 and 7.5 which is the optimum value for the action of salivary amylase, the enzyme present in saliva which initiates the digestion of carbohydrates. The action of salivary amylase is inhibited when food containing it reaches the stomach and is mixed with acid gastric juice. Blood has a pH value between 7.35 and 7.45. The pH range of blood compatible with life is 7.0 to 7.8. The metabolic activity of the body cells produces certain acids and alkalis which alter the pH of the tissue fluid and blood. To maintain the pH within the normal range, there are substances present in blood that act as buffers. Buffers The optimum pH level is maintained by the balance between acids and bases produced by cells. Bases are substances that accept (or bind) hydrogen ions and when dissolved in water they produce an alkaline solution. Buffers are substances such as phosphates, bicarbonates and some proteins that maintain the [H+ ] within normal, but narrow, limits. Some buffers 'bind' hydrogen ions and others 'bind' hydroxyl ions, reducing their circulating levels and preventing damaging changes. For example, if there is sodium hydroxide (NaOH) and carbonic acid (H2CO3) present, both will ionise to some extent, but they will also react together to form sodium bicarbonate (NaHCO3) and water (H.OH). One of the hydrogen ions from the acid has been 'bound' in the formation of the bicarbonate radical and the other by combining with the hydroxyl radical to form water. NaOH sodium hydroxide H2CO3 carbonic acid NaHCO3 sodium bicarbonate + H.OH water Acidosis and alkalosis The substances in the complex buffer system that 'bind' hydrogen ions are called the alkali reserve of the blood. When the pH is below 7.35, and all the reserves of alkaline buffer are used up, the condition of acidosis exists. When the reverse situation pertains and the pH is above 7.45, and the increased alkali uses up all the acid reserve, the state of alkalosis exists
introduction to the chemistry of life The buffer systems maintain homeostasis by preventing Carbohydrates dramatic changes in the pH values in the blood, but can only function effectively if there is some means by which The carbohydrates are the sugars. Carbohydrates are excess acid or alkali can be excreted from the body. The composed of carbon, oxygen and hydrogen and the organs most active in this way are the lungs and the carbon atoms are normally arranged in a ring, with the kidneys. The lungs are important regulators of blood pH oxygen and hydrogen atoms linked to them. The struc because they excrete carbon dioxide(CO, ) . CO, increases tures of glucose, fructose and sucrose are shown in [H] in body fluids because it combines with water Figure 2.7. When two sugars link up. the reaction occur- to form carbonic acid, which then dissociates into a ring expels a molecule of water and the resulting bond bicarbonate ion and a hydrogen ion. is called a glycosidic linkage Simple sugars, like glucose, can exist as single units, CO2+HQ→H2OO3→H+HCO and are referred to as monosaccharides. Glucose is the carbon water carbonic hydrogen bicarbonate main form in which sugar is used by cells, and blood levels are tightly controlled. Frequently, the monosac. In acidosis, the brain detects the rising [H] in the blood charides are linked together, the resultant molecule and stimulates breathing, causing increased CO, loss and ranging from two sugars or disaccharides, e.g. sucrose a fall in [Hl. Conversely, in alkalosis, the brain can table sugar), to long chains containing many thousands reduce the respiration rate to increase co, levels and of sugars. Such complex carbohydrates are called increase [H'] restoring pH towards normal polysaccharides, e.g. starch The kidneys have the ability to form ammonia, an lucose can be broken down(metabolised) in either alkali,which combines with the acid products of protein the presence(aerobically)or the absence (anaerobically)of metabolism which are then excreted in the urine The buffer and excretory systems of the body together is used. During this process, energy, water and carbon maintain the acid-base balance so that the ph range of the dioxide are released (p. 315)This family of molecules blood remains within normal, but narrow, limits a serves as a ready source of energy to fuel cellular activities(p. 272) IMPORTANT BIOLOGICAL a provides a form of energy storage, e.g. glycogen (P.315) MOLECULES a forms an integral part of the structure of DNA and RNA(P. 25) cell to recognise other molecules and cells8 the as receptors on the cell surface, allow Learning outcomes After studying this section, you should be able to n describe in simple terms the chemical nature of Amino acids and proteins sugars, protein, lipids, nucleotides and enzymes Amino acids always contain carbon, hydrogen, Ox n discuss the biological importance of each of these and nitrogen, and many in addition carry sulpl important groups of molecules In human biochemistry, 20 amino acids are used as principal building blocks of protein, although there are CH2OH CHOH HOCH HOCH HO HO CHOH HO\OH CHOH Glucose Sucrose Water Monosaccharides Disaccharide Figure 2.7 The combination of glucose and fructose to make sucrose
Introduction to the chemistry of life The buffer systems maintain homeostasis by preventing dramatic changes in the pH values in the blood, but can only function effectively if there is some means by which excess acid or alkali can be excreted from the body. The organs most active in this way are the lungs and the kidneys. The lungs are important regulators of blood pH because they excrete carbon dioxide (CO2). CO2 increases [H+ ] in body fluids because it combines with water to form carbonic acid, which then dissociates into a bicarbonate ion and a hydrogen ion. CO2 +H2O ->H2CO3 ->H+ + HCO3- carbon water carbonic hydrogen bicarbonate dioxide acid ion ion In acidosis, the brain detects the rising [H+ ] in the blood and stimulates breathing, causing increased CO2 loss and a fall in [H+ ]. Conversely, in alkalosis, the brain can reduce the respiration rate to increase CO2 levels and increase [H+ ], restoring pH towards normal. The kidneys have the ability to form ammonia, an alkali, which combines with the acid products of protein metabolism which are then excreted in the urine. The buffer and excretory systems of the body together maintain the acid-base balance so that the pH range of the blood remains within normal, but narrow, limits. IMPORTANT BIOLOGICAL MOLECULES Learning outcomes After studying this section, you should be able to: • describe in simple terms the chemical nature of sugars, protein, lipids, nucieotides and enzymes • discuss the biological importance of each of these important groups of molecules. Carbohydrates The carbohydrates are the sugars. Carbohydrates are composed of carbon, oxygen and hydrogen and the carbon atoms are normally arranged in a ring, with the oxygen and hydrogen atoms linked to them. The structures of glucose, fructose and sucrose are shown in Figure 2.7. When two sugars link up, the reaction occurring expels a molecule of water and the resulting bond is called a glycosidic linkage. Simple sugars, like glucose, can exist as single units, and are referred to as monosaccharides. Glucose is the main form in which sugar is used by cells, and blood levels are tightly controlled. Frequently, the monosaccharides are linked together, the resultant molecule ranging from two sugars or disaccharides, e.g. sucrose (table sugar), to long chains containing many thousands of sugars. Such complex carbohydrates are called polysaccharides, e.g. starch. Glucose can be broken down (metabolised) in either the presence (aerobically) or the absence (anaerobically) of oxygen, but the process is much more efficient when O2 is used. During this process, energy, water and carbon dioxide are released (p. 315) This family of molecules: • serves as a ready source of energy to fuel cellular activities (p. 272) • provides a form of energy storage, e.g. glycogen (p. 315) • forms an integral part of the structure of DNA and RNA (p. 25) • can act as receptors on the cell surface, allowing the cell to recognise other molecules and cells. Amino acids and proteins Amino acids always contain carbon, hydrogen, oxygen and nitrogen, and many in addition carry sulphur. In human biochemistry, 20 amino acids are used as the principal building blocks of protein, although there are 23 Figure 2.7 The combination of glucose and fructose to make sucrose
The body and its constituents NH COOH NH2 COOH H—-cooH Figure 2.9 Core structure of the fats c (water hating) and therefore lipids do not mix with water. This is important in their function in the cell membrane(p. 30) Other types of lipids include certain vitamins (e.g. E Figure 2. 8 Amino acid structures: A Common structure. R= variable side chain. B Glycine, the simplest amino acid and K), an important group of hormones called steroids, and the fats, a molecule of fat consists of three fatty acids, C. Alanine. D, Pl each linked to a molecule of glycerol(Fig. 2.9). Fats are a source of energy, and provide a convenient form in which thers; for instance, there are some amino acids used to store excess calorific intake. When fats are broken only in certain proteins, and some seen only in microbial down, they release energy, but the process is less efficient products Of the amino acids used in human protein syn- than when carbohydrates are used, since it requires more thesis, there is a basic common structure, including an energy for the breakdown reaction to take place. They are amino group(NH,), a carboxy group ( COOH) and a used in the body for hydrogen atom. What makes one amino acid different from the next is a variable side chain. The basic structure insulation and three common amino acids are shown in Figure 2.8 protection of body parts As in formation of glycosidic linkages, when two amino energy storage acids join up the reaction expels a molecule of water and the resulting bond is called a peptide bond Proteins are made from amino acids joined together, Nucleotides and are the main family of molecules from which the Nucleic acids human body is built. Protein molecules vary enormously in size, shape, chemical constituents and function. Many These are the largest molecules in the body and are built important groups of biologically active substances are from components called nucleotides, which consist of hree subunits a carrier molecules, e. g haemoglobin(p. 63) a sugar unit many hormones, e.g. insulin(P. 225 one or more phosphate groups linked together antibodies(p. 380 Deoxyribonucleic acid (DNA) Proteins can also be used as an alternative energy source, This is a double strand of nucleotides arranged in a spiral usually in dietary inadequacy, although the process is much less efficient than when carbohydrates or fats are heix) Which resembles a twisted ladder(Fig. 2.10) broken down Chromosomes are clusters of dNa molecules consisting of functional subunits called genes. The nucleotides contain the sugar deoxyribose, phosphate groups and one of four Lipids bases: adenine [Al, thymine [T] guanine [G] and cytosine [C]. A in one chain is paired with T in the other, and Lipids are made up of carbon, hydrogen and oxygen G with C. In this way, nucleotides are arranged in a atoms. One group of lipids, the phospholipids, form an precisely ordered manner in which one chain is com- integral part of the cell membrane. One notable feature of plementary to the other. DNA acts as the templa lipid molecules is that they are strongly hydrophobic protein synthesis and is stored safely in the nucleus
The body and its constituents 24 Figure 2.8 Amino acid structures: A. Common structure, R = variable side chain. B. Glycine, the simplest amino acid. C. Alanine. D. Phenylalanine. others; for instance, there are some amino acids used only in certain proteins, and some seen only in microbial products. Of the amino acids used in human protein synthesis, there is a basic common structure, including an amino group (NH2), a carboxy group (COOH) and a hydrogen atom. What makes one amino acid different from the next is a variable side chain. The basic structure and three common amino acids are shown in Figure 2.8. As in formation of glycosidic linkages, when two amino acids join up the reaction expels a molecule of water and the resulting bond is called a peptide bond. Proteins are made from amino acids joined together, and are the main family of molecules from which the human body is built. Protein molecules vary enormously in size, shape, chemical constituents and function. Many important groups of biologically active substances are proteins, e.g.: • carrier molecules, e.g. haemoglobin (p. 63) • enzymes (p. 26) • many hormones, e.g. insulin (p. 225) • antibodies (p. 380). Proteins can also be used as an alternative energy source, usually in dietary inadequacy, although the process is much less efficient than when carbohydrates or fats are broken down. Lipids Lipids are made up of carbon, hydrogen and oxygen atoms. One group of lipids, the phospholipids, form an integral part of the cell membrane. One notable feature of lipid molecules is that they are strongly hydrophobic Figure 2.9 Core structure of the fats. (water hating) and therefore lipids do not mix with water. This is important in their function in the cell membrane (p. 30). Other types of lipids include certain vitamins (e.g. E and K), an important group of hormones called steroids, and the fats. A molecule of fat consists of three fatty acids, each linked to a molecule of glycerol (Fig. 2.9). Fats are a source of energy, and provide a convenient form in which to store excess calorific intake. When fats are broken down, they release energy, but the process is less efficient than when carbohydrates are used, since it requires more energy for the breakdown reaction to take place. They are used in the body for: • insulation • protection of body parts • energy storage. Nucleotides Nucleic acids These are the largest molecules in the body and are built from components called nucleotides, which consist of three subunits: • a sugar unit • a base • one or more phosphate groups linked together. Deoxyribonucleic acid (DNA) This is a double strand of nucleotides arranged in a spiral (helix) which resembles a twisted ladder (Fig. 2.10). Chromosomes are clusters of DNA molecules consisting of functional subunits called genes. The nucleotides contain the sugar deoxyribose, phosphate groups and one of four bases: adenine [A], thymine [T], guanine [G] and cytosine [C]. A in one chain is paired with T in the other, and G with C. In this way, nucleotides are arranged in a precisely ordered manner in which one chain is complementary to the other. DNA acts as the template for protein synthesis and is stored safely in the nucleus
introduction to the chemistry of life which carries the instructions for the assembly of the new rote in the cytoplasm called ribo Strand 1 somes(p. 32). Ribosomes read the message and, follow ing the instructions, assemble the new protein from trand 2 amino acids in the cell cytoplasm(Fig.2.11).New chains of protein are often large molecules which coil up in a particular way to maintain stability of the molecule Adenosine triphosphate(ATP) ATP is a nucleotide which contains ribose(the sugar O Thymine unit), adenine(the base)and three phosphate groups attached to the ribose(Fig. 2. 12A). It is sometimes known Deoxyribose sugar as the energy currency of the body, which implies that the □ Phosphate group body has to'earn(synthesise)it before it canspend'it Figure 2. 10 Deoxyribonucleic acid(DNA) Many of the bodys huge number of reactions releas energy, e. g. the breakdown of sugars in the presence of O The body captures the energy released by these reac tions, using it to make ATP from adenosine diphosphate his is a single-stranded chain of nucleotides which con- (ADP). When the body needs chemical energy to fuel cel. tains the sugar ribose instead of the deoxyribose found in lular activities, ATP releases its stored energy, water and DNA. It contains no thymine, but uses uracil [U] instead. a phosphate group through the splitting of a high-energy is synthesise phosphate bond, and reverts to ADP(Fig. 2. 12B) and carries the message instructing synthesis of a new The body needs chemical energy to protein from the DNA (which cannot leave the nucleus)to a drive synthetic reactions (i.e. building biological the protein-synthesising apparatus in the cell cytoplasm. Protein synthesis. When cells require new protein, aa transport substances across membranes ingle strand of RNA is made using DNA as the template the RNA leaves the nucleus rna acts as the messen ADENINE A High energy the base) Phospha MUWU DNA the suganF-eXeXP H ADENOSINE- H ADENOSINE DIPHOSPHATE- F ADENOSINE TRIPHOSPHATE Nuclear pore Release of chemical energy PROTEIN DUUUSYNTHESIS 日∥ Free Energy input required acids New protein CYTOPLASM H2o Figure 2. 11 The relat between DNA, RNA and protein Figure 2.12 ATP and ADP: A Structures. B Conversion cycle
Introduction to the chemistry of life Figure 2.10 Deoxyribonucleic acid (DMA). Ribonucleic acid (RNA) This is a single-stranded chain of nucleotides which contains the sugar ribose instead of the deoxyribose found in DNA. It contains no thymine, but uses uracil [U] instead. It is synthesised in the nucleus from the DNA template, and carries the message instructing synthesis of a new protein from the DNA (which cannot leave the nucleus) to the protein-synthesising apparatus in the cell cytoplasm. Protein synthesis. When cells require new protein, a single strand of RNA is made using DNA as the template; the RNA leaves the nucleus. RNA acts as the messenger which carries the instructions for the assembly of the new protein to tiny structures in the cytoplasm called ribosomes (p. 32). Ribosomes read the message and, following the instructions, assemble the new protein from amino acids in the cell cytoplasm (Fig. 2.11). New chains of protein are often large molecules which coil up in a particular way to maintain stability of the molecule. Adenosine triphosphate (ATP) ATP is a nucleotide which contains ribose (the sugar unit), adenine (the base) and three phosphate groups attached to the ribose (Fig. 2.12A). It is sometimes known as the energy currency of the body, which implies that the body has to 'earn' (synthesise) it before it can 'spend' it. Many of the body's huge number of reactions release energy, e.g. the breakdown of sugars in the presence of O2. The body captures the energy released by these reactions, using it to make ATP from adenosine diphosphate (ADP). When the body needs chemical energy to fuel cellular activities, ATP releases its stored energy, water and a phosphate group through the splitting of a high-energy phosphate bond, and reverts to ADP (Fig. 2.12B). The body needs chemical energy to: • drive synthetic reactions (i.e. building biological molecules) • fuel movement • transport substances across membranes. Figure 2.11 The relationship between DNA, RNA and protein synthesis. 25 Figure 2.12 ATP and ADP: A. Structures. B. Conversion cycle