Biomolecules Class 11 Notes

When living tissue is ground and filtered, the filtrate contains small molecules — amino acids, sugars, fatty acids, nucleotides, and ions — collectively called micromolecules. The residue contains large macromolecules. This analysis shows that living tissue is built from a handful of chemical elements: C, H, O, N, P and S, organized into a surprisingly small number of molecular types.

• Micromolecules: water, mineral ions, monosaccharides, amino acids, nucleotides, fatty acids, vitamins.
• Biomacromolecules: proteins, polysaccharides, nucleic acids, lipids (large but non-polymeric).
• Primary metabolites have direct roles in growth and reproduction — amino acids, sugars, lipids, proteins, nucleotides.
• Secondary metabolites (alkaloids, flavonoids, terpenoids, essential oils) have ecological roles like defence but are not directly involved in basic metabolism.

Proteins are the most abundant organic molecules in living cells and serve as enzymes, structural components, transport molecules, hormones and antibodies. Proteins are polymers of 20 different types of amino acids linked by peptide bonds. Each amino acid has an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain (R group) attached to the same α-carbon.

Levels of protein structure

• Primary structure: the linear sequence of amino acids in the polypeptide chain. It determines all higher structures.
• Secondary structure: local folding into α-helices (hydrogen bonds between C=O and N–H of the backbone) and β-pleated sheets (hydrogen bonds between adjacent strands).
• Tertiary structure: overall three-dimensional folding of a single polypeptide driven by hydrophobic interactions, hydrogen bonds, ionic bonds, disulfide bridges (–S–S–) and van der Waals forces. Functional proteins have this compact globular shape.
• Quaternary structure: arrangement of multiple polypeptide subunits. Haemoglobin has four subunits (two α and two β chains). Not all proteins have quaternary structure.

Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds. They may be linear or branched. The monomeric unit in most plant polysaccharides is glucose, but the glycosidic linkage differs, giving each polysaccharide distinct properties.

• Starch: the storage polysaccharide in plants; consists of amylose (linear, α-1,4-glycosidic bonds) and amylopectin (branched, α-1,4 and α-1,6-glycosidic bonds).
• Glycogen: the storage polysaccharide in animals; similar to amylopectin but more highly branched — found in liver and muscle cells.
• Cellulose: the structural polysaccharide in plant cell walls; linear chain of glucose with β-1,4-glycosidic bonds. Humans cannot digest cellulose (lack cellulase enzyme) — it is dietary fibre.
• Chitin: a structural polysaccharide of N-acetylglucosamine units with β-1,4 linkages; forms the exoskeleton of arthropods and fungal cell walls.

• DNA bases: adenine (A), guanine (G), cytosine (C), thymine (T). Double-stranded; base pairing: A = T (two hydrogen bonds), G ≡ C (three hydrogen bonds).
• RNA bases: adenine, guanine, cytosine, uracil (U replaces T). Usually single-stranded.
• Types of RNA: mRNA (messenger), tRNA (transfer, clover-leaf shape), rRNA (ribosomal).
• Nucleotides also serve as energy carriers (ATP), coenzymes (NAD⁺, FAD), and signalling molecules (cAMP).

Lipids are a heterogeneous group of molecules that are insoluble in water but soluble in organic solvents such as chloroform and ether. Unlike true macromolecules, lipids are not polymeric. Lipids serve as structural components of cell membranes, energy reserves, insulation, and signalling molecules.

• Fatty acids: long hydrocarbon chains with a terminal carboxyl group. Saturated fatty acids (no double bonds, e.g. palmitic acid, stearic acid) and unsaturated fatty acids (one or more cis double bonds, e.g. oleic acid, linoleic acid).
• Triglycerides (simple lipids): one glycerol esterified with three fatty acids. They are the main storage lipids in adipose tissue.
• Phospholipids: glycerol with two fatty acids and one phosphate group; form the lipid bilayer of cell membranes. The phosphate head is hydrophilic; the fatty acid tails are hydrophobic.
• Steroids: four fused carbon rings; cholesterol is a precursor for steroid hormones (testosterone, oestrogen, cortisol) and bile salts.

Mechanism of enzyme action

The lock-and-key model explains specificity: the substrate fits into the active site like a key into a lock. The induced-fit model refines this — the active site changes shape slightly upon substrate binding, ensuring a tighter fit and stabilising the transition state. Enzymes lower the activation energy (Ea), making the reaction faster without changing the equilibrium constant.

Factors affecting enzyme activity

• Temperature: enzyme activity increases with temperature up to an optimum (usually 37–40 °C in humans) and then drops sharply as the protein denatures.
• pH: each enzyme has an optimum pH (e.g. pepsin at pH 2, trypsin at pH 8). Extreme pH disrupts ionic bonds in the protein.
• Substrate concentration: at low [S], rate increases linearly; at high [S], rate plateaus at Vmax when all active sites are occupied (Michaelis–Menten kinetics).
• Enzyme inhibition: competitive inhibitors compete for the active site; non-competitive inhibitors bind elsewhere (allosteric site) and change the enzyme's shape. Irreversible inhibitors permanently inactivate the enzyme.

Cofactors, coenzymes and prosthetic groups

Many enzymes require non-protein helpers called cofactors to be active. Cofactors can be inorganic metal ions (Zn²⁺, Mg²⁺, Fe²⁺) or organic molecules called coenzymes (derived from vitamins, e.g. NAD⁺ from niacin, FAD from riboflavin, coenzyme A from pantothenic acid). Prosthetic groups are coenzymes tightly bound to the enzyme (e.g. haem in catalase).

Metabolism is the sum of all biochemical reactions in an organism. It is divided into catabolism (breakdown of complex molecules into simpler ones, releasing energy) and anabolism (synthesis of complex molecules from simpler ones, consuming energy). ATP is the universal energy currency. Living cells maintain a dynamic steady state — the concentrations of metabolites are kept constant through feedback regulation, not by equilibrium.

Example: How many peptide bonds are present in a polypeptide chain of 120 amino acids?

Solution: A chain of n amino acids contains (n − 1) peptide bonds. For 120 amino acids: 120 − 1 = 119 peptide bonds. Each peptide bond is formed by a condensation reaction releasing one water molecule, so 119 water molecules are released during synthesis.

Example: Why does the rate of an enzyme-catalysed reaction stop increasing after a certain substrate concentration?

Solution: At high substrate concentrations, all active sites of the enzyme molecules are occupied — the enzyme is saturated. Adding more substrate cannot increase the rate further because there are no free active sites. The reaction has reached Vmax. Increasing enzyme concentration would raise Vmax.

Exercises (NCERT Q1–Q11)

• 1) What are macromolecules? Give examples.
• 2) What is meant by tertiary structure of proteins?
• 3) Find and write down structures of 10 interesting small molecular weight biomolecules. Find if there is any industry which manufactures the compounds by isolation. Find out who are the buyers.
• 4) Find out and make a list of proteins used as therapeutic agents. Find other applications of proteins (e.g., Cosmetics etc.)
• 5) Explain the composition of triglyceride.
• 6) Can you attempt building models of biomolecules using commercially available atomic models (Ball and Stick models).
• 7) Draw the structure of the amino acid, alanine.
• 8) What are gums made of? Is Fevicol different?
• 9) Find out a qualitative test for proteins, fats and oils, amino acids and test any fruit juice, saliva, sweat and urine for them.
• 10) Find out how much cellulose is made by all the plants in the biosphere and compare it with how much of paper is manufactured by man and hence what is the consumption of plant material by man annually. What a loss of vegetation!
• 11) Describe the important properties of enzymes.

Answers at a glance

1. 1
   Q1 → Macromolecules are the large molecules of the cell, and they are the compounds with a molecular weight of more than one thousand daltons. They occur in the cell as two fractions, the acid soluble fraction, which is made of the smaller molecules and can be extracted by treatment with dilute acid, and the acid insoluble fraction, which is the macromolecular fraction and requires stronger treatment to break down. The macromolecules fall into three groups, and the first is the polysaccharides, which are polymers of monosaccharides, and the examples are starch, glycogen, cellulose, chitin and inulin. The second group is the proteins, which are polypeptides, and the examples are the enzymes, the structural proteins such as the collagen of the connective tissue, the hormone insulin, the antibodies, and also keratin and myosin. The third group is the nucleic acids, which are polynucleotides, and these are DNA and RNA. The lipids form a separate and important case: they have a molecular weight of less than 800 daltons and are therefore not true macromolecules but micromolecules, and yet they are a part of the acid insoluble fraction, and the reason is that the lipids are not polymers, since a triglyceride is formed by the esterification of glycerol with fatty acids and not by the repetition of a monomer unit. Apart from the lipids, therefore, every macromolecule of the cell is a polymer built by the repetition of a small monomer, and this is what makes the macromolecules both a distinctive class and a problem of structure, since the properties of the whole depend on the arrangement of the units as much as on their identity.
2. 2
   Q2 → The tertiary structure of a protein is its three-dimensional shape, that is the definite and stable compact or globular conformation assumed by a single polypeptide chain in the environment in which it is placed. It is a level of organisation above the secondary structure, and the distinction matters, because the secondary structure, comprising the alpha helix and the beta pleated sheet, is stabilised only by hydrogen bonds between the backbone groups and is a local and comparatively loose arrangement, whereas the tertiary structure is stabilised by the interactions between the side chains or R groups of the amino acids. These are the hydrophobic bonds, formed because the non-polar side chains are driven together away from water and the most important of them, the disulphide bonds, which are covalent sulphur-to-sulphur linkages and the strongest of all the interactions, the hydrogen bonds, the ionic or electrostatic bonds, and the van der Waals forces, which are individually weak but very numerous. It is these additional side-chain interactions, absent at the secondary level, that convert a long polypeptide into a compact, definite and reproducible three-dimensional shape, and since a protein can function only when it is folded into that shape, the tertiary structure and the biological activity of the protein are the same thing viewed from two sides, which is also why a change in the medium, as in an acid or an alkaline solution, that disrupts the side-chain interactions denatures the protein and destroys its activity.
3. 3
   Q3 → Ten small molecular weight biomolecules, with their structures, are these, two from each of the five classes. Glycine, an amino acid, is NH2-CH2-COOH, and is the simplest of the twenty and the only one without a chiral carbon. Alanine is NH2-CH(CH3)-COOH. Glucose, a monosaccharide, is a six-carbon aldose written as CHO-(CHOH)4-CH2OH, and is the sugar that is polymerised into the starch, the glycogen and the cellulose. Ribose is a five-carbon aldose, CHO-(CHOH)3-CH2OH, and is the pentose of every nucleotide. Adenosine monophosphate, a nucleotide, consists of the base adenine joined to the pentose ribose joined to a phosphate. Guanosine monophosphate is the same three-part unit with guanine as the base. A triglyceride, a lipid, is glycerol esterified with three fatty acids. A phospholipid is glycerol esterified with two fatty acids and one phosphate group. Vitamin A, the retinol, is a fat-soluble vitamin with a long isoprenoid chain attached to a beta-ionone ring. Vitamin C, the ascorbic acid, is a water-soluble vitamin with a five-membered lactone ring bearing an enediol group. Several of these are manufactured by isolation. The amino acids are the clearest case, since glutamic acid is isolated from a microbial broth and sold as monosodium glutamate, a flavouring agent whose buyers are the food-processing manufacturers of soups, snack foods, seasoning powders and instant noodles, while cysteine is isolated and used by the baking industry to improve dough. The sugars are manufactured on a much larger scale, since sucrose is isolated from sugar cane and sugar beet and glucose is obtained from starch by acid or enzyme hydrolysis, and the buyers are the confectionery, soft-drink, bakery and pharmaceutical industries. The nucleotides and nucleosides are isolated and sold to the biochemical laboratories that make culture media and reagents and to the pharmaceutical manufacturers that make nucleotide drugs such as the antivirals. The commercial point that the exercise brings out is that these industries exist only because the human body cannot synthesise all of these small biomolecules and is dependent on dietary intake, so the isolation industry, like the food and the pharmaceutical industries that buy its products, rests directly on the biochemical fact of that dependence.
4. 4
   Q4 → Proteins used as therapeutic agents include the following. Among the enzymes, pepsin and trypsin are used in digestion, bromelain, obtained from the pineapple, treats bruises and inflammation, and papain, from the papaya, is used for indigestion. The hormones are the largest group, and among them insulin is used in the treatment of diabetes, oxytocin in disorders of childbirth, the growth hormone or somatotropin in growth-related disorders, and thyroxine, from the thyroid, in goitre and in hypothyroidism, while cortisone and adrenaline with its derivatives are used for bronchial asthma and for cardiac disorders, cortisone being a steroid rather than a protein. Antibodies and antitoxins are used against infections, and these include the cobra antitoxin, the antitetanus serum and the anti-Rh serum used in erythroblastosis foetalis. Albumin and the plasma proteins are used to restore the blood volume in severe blood loss, and gelatin is used in some surgical and pharmaceutical preparations. The proteins have a wide range of other applications as well. In cosmetics, collagen is added to face creams and to anti-wrinkle preparations to give elasticity and smoothness, elastin keeps the skin supple, and keratin is used in hair creams and polishes. In nutrition, protein hydrolysates and casein are used in dietetic and infant foods. In industry, casein from milk is used in waterproofing and in plastics and in the manufacture of adhesives and buttons, gelatin is used in photographic film, and enzymes are used as biological detergents. Albumin is used in the laboratory as a molecular weight marker in electrophoresis, and the microbial single-cell proteins are used as a food source. The two points to note are that most of these uses depend on the specificity of the protein, since it is the specificity of the enzyme that makes the digestive applications possible at all, and that the commercial demand is now met increasingly by the recombinant production of human insulin, human growth hormone and the interferons in bacteria, which is why the genetic engineering of proteins has direct medical importance.
5. 5
   Q5 → A triglyceride is a simple lipid formed by the esterification of one molecule of glycerol with three molecules of fatty acid, so it contains one glycerol and three fatty acid chains in a 1:3 ratio. Glycerol is a trihydric alcohol with the structure CH2OH-CHOH-CH2OH, having three hydroxyl groups, and a fatty acid is a monocarboxylic acid with a long even-numbered hydrocarbon chain. The three components are joined by three ester bonds, each formed by a condensation or dehydration reaction between a hydroxyl group of the glycerol and a carboxyl group of a fatty acid, with the loss of one molecule of water per linkage, and the compound that results is therefore an ester. The three fatty acid chains are long and unbranched, and because the hydrocarbon chains dominate the molecule and there is no charged or strongly polar group, the whole molecule is non-polar and hydrophobic, being insoluble in water and soluble in organic solvents such as ether and chloroform. The long unbranched chains pack closely together, which is what gives the fat its semisolid or solid consistency. A triglyceride is largely solid at ordinary temperature and is called a fat, while one that is largely liquid is called an oil, and the difference lies wholly in the length and the degree of saturation of the fatty acid chains, because a shorter or a saturated chain packs more tightly and has a higher melting point whereas an unsaturated chain, with its double bond producing a kink, packs loosely and melts at a lower temperature; the natural unsaturated chains are almost all cis, and it is this same bent geometry that keeps an oil liquid. Finally, a triglyceride has no charged or polar group and cannot form hydrogen bonds, so it cannot by itself form a membrane, and this is the reason the phospholipid, which keeps the phosphate head group and only two fatty acid chains, is the molecule that builds the biological membrane.
6. 6
   Q6 → Yes. The models used are the commercially available ball-and-stick molecular model sets, which supply spheres representing the atoms of each element, all the spheres of one element being of the same size and colour, together with short rods representing the bonds, and an instruction sheet giving the valency of each element, hydrogen being one, oxygen two, nitrogen three and carbon four. The models can be assembled in order of increasing difficulty. Alanine is built first, from three carbons in which the central alpha carbon bears an amino group, a carboxyl group, a methyl group and a hydrogen, which introduces the tetrahedral carbon and its four valencies. A saturated fatty acid is built next, and then a triglyceride, in which the three long chains must be fixed pointing away from the small glycerol backbone, so that the model shows for the first time a real three-dimensional arrangement rather than a flat chain. A molecule of glucose is then closed into its ring, and from it a short polysaccharide chain is built, which shows the geometry of the glycosidic linkage. A short polypeptide is then assembled from amino acid units and wound into the alpha helix, so that the repeating peptide bond and the hydrogen bonds that hold the helix are both visible. Finally a short fragment of DNA is built, as two antiparallel strands of paired bases joined by hydrogen bonds, in the form of a twisted ladder, which makes the double helical structure of DNA directly visible instead of merely stated. What the exercise establishes is three things. First, that the valency of an atom fixes its geometry, so that a carbon with four valencies is tetrahedral and therefore a chain of carbons must zig-zag, and sulphur and phosphorus are pyramidal, and this single fact accounts for the shapes of most biomolecules. Second, that the monomers are joined in a definite sequence and a definite geometry, and that therefore the same atoms in the same numbers can make quite different substances, since a starch and a cellulose are both glucose polymers that differ only in the geometry of the glycosidic bond, and a triglyceride and a phospholipid are both glycerol-fatty acid lipids that differ only in the third group attached to the glycerol. Third, that the three-dimensional folding of a macromolecule is what makes it functional, since only a correctly folded protein is active. The limitation of the exercise must also be stated: the ball-and-stick model is a rigid model, so it shows the valencies, the bond angles and the stereochemistry correctly but does not represent the mobility of the atoms, the electron distribution or the real flexibility of the molecules, and it is therefore a teaching model rather than a copy of the living molecule.
7. 7
   Q7 → Alanine is the amino acid whose side chain is a methyl group, and it is the compound used to show the general structure of an alpha-amino acid. The general structure to be drawn and labelled is the central alpha carbon, shown as C, bearing four substituents: the amino group, NH2, the carboxyl group, COOH, a hydrogen atom, H, and the variable side chain or R group, so the whole formula is written with the alpha carbon in the centre as CH(NH2)(COOH) bonded to the R group. The drawing should be made with the carboxyl group on one side, the amino group on the adjacent side above or below, the hydrogen on the same carbon and the R group on the other side, and the four labels to be written in are the alpha or central carbon, the amino group, the carboxyl group and the R or side chain, with the hydrogen shown as the fourth bond. In alanine the R group is a methyl group, CH3, so the specific structure of alanine is CH3-CH(NH2)-COOH, which when fully displayed is H3C-CH(NH2)-C(=O)-OH, and in this molecule the carbon bearing the amino group and the carboxyl group is the alpha carbon and the carbon of the methyl group is the beta carbon. Alanine is thus a non-polar aliphatic amino acid, one of the twenty that the cell uses to build proteins, and it is also the amino acid from which pyruvate is formed, and from which glucose is therefore made, so the structure has a direct metabolic significance as well as a structural one.
8. 8
   Q8 → The gums are polysaccharides, that is complex carbohydrates, and they are formed mainly by plants as the dried exudates of the stems and trunks of trees, so the gum of an Acacia or of a Sterculia is the usual example. Chemically the gums are polymers of monosaccharides joined by glycosidic bonds, and they may be homopolysaccharides or heteropolysaccharides, being non-reducing in the usual case, and in the plant the exudate serves to protect the wound and to check the loss of water, hardening as it dries in the air. Fevicol is quite different. It is a synthetic adhesive and is not a gum in the biological sense at all, because it is not a carbohydrate. Fevicol is a polyvinyl acetate emulsion, a synthetic polymer made from the petroleum-derived monomer vinyl acetate and sold as an aqueous emulsion of white latex, so its molecular nature is a vinyl polymer and not a polysaccharide of monosaccharides. There are two points of difference. Chemically, the gum is a natural complex carbohydrate of plant origin, a polymer of pentose or hexose units joined by glycosidic bonds, whereas Fevicol contains no monosaccharide units and no carbohydrate of any kind, being a synthetic polyvinyl polymer. Biologically and practically, the gum is a product of the living plant, formed as an exudate for the plant's own protection, and it is a recognised food material, whereas Fevicol is a factory product of a synthetic monomer, is used solely as an adhesive and is not fit for food. The two do resemble each other in that both are used as adhesives and as thickeners, and it is that similarity of use, not any similarity of composition, that accounts for the comparison.
9. 9
   Q9 → Three tests are required. For proteins, the Biuret test: a small quantity of the sample is taken in a test tube, a few drops of freshly prepared ten per cent sodium hydroxide solution are added to make it alkaline, and then two or three drops of dilute copper sulphate solution are added drop by drop with shaking. A violet or purple colour is a positive result, and no violet, only the blue of the excess reagent, is negative. In the alkaline medium the copper ions form a coordination complex with the peptide bonds, so the test detects peptide linkages and any polypeptide gives it. For fats and oils, the grease spot test: a little of the sample is placed on a piece of paper or cotton and dried, and a permanent translucent oily spot that water will not remove is a positive result, the principle being that the non-polar fat molecules cannot be wetted by water and so impregnate the cellulose; the accompanying emulsion test shakes the sample with water, when a milky emulsion forms and settles on standing, which confirms that the substance is insoluble in water and was held apart by an emulsifier. For amino acids, the ninhydrin test: the sample is boiled with ninhydrin solution and, on cooling, a deep blue or violet colour is positive, the blue product being Ruhemann's purple, which forms with any free alpha-amino group, so this test detects free amino groups and is complementary to the Biuret, which detects peptide bonds. On testing the fluids, fruit juice should give a strong positive ninhydrin test for the free amino acids and a positive grease spot for the waxes and oils of the cuticle and the seed, and a Biuret test only if it is a protein-rich juice; saliva should give a positive Biuret test because it contains the protein enzyme ptyalin or salivary amylase, together with a positive ninhydrin test; sweat should give a positive ninhydrin test for the free amino acids and a positive result for the inorganic salts, but only a very weak or a negative Biuret test, since the protein of the sweat is present in trace amounts only; and urine should give a positive ninhydrin test for the free amino acids and a negative grease spot, since a normal urine contains no appreciable fat, while it should normally also give a negative Biuret test, since the plasma proteins are too large to pass the glomerular filter, so that a positive Biuret test in urine is a pathological finding used clinically to detect damage to the glomerulus, and the fat appears in the urine only in the nephrotic condition, as the lipiduria and the fatty casts. Every test must be controlled, with egg white, glycine and vegetable oil as the positive controls and distilled water as the negative, the Biuret requires the alkaline medium and too much copper sulphate gives a blue precipitate that masks the violet, the ninhydrin needs boiling and is interfered with by the ammonia of the urine, and the grease spot is insensitive at low concentration, so that a negative result means below the limit of detection and not absent. The essential limitation is that these are tests for chemical groups, and a positive result shows the presence of a peptide bond, a free amino group or a lipid, and never identifies which particular protein or amino acid is present.
10. 10
    Q10 → Cellulose is the most abundant of all the organic compounds on the earth, because it is the structural polysaccharide of the cell wall of every green plant, and the green plants are the producers of the entire biosphere. The annual synthesis of cellulose by all the plants of the world is of the order of 10^11 to 10^12 tonnes of organic matter a year, and the standing stock is of the same very large order, since the total biomass of the world's forests alone is commonly estimated at about 400 to 450 billion tonnes of carbon, a large fraction of which is cellulose, so that the cellulose held in the world's forests runs into hundreds of billions of tonnes. The paper made by man is small by comparison. The world production of paper and paperboard is about 400 to 500 million tonnes a year, that is of the order of 10^9 tonnes, and it is made almost entirely from the wood of trees, chiefly the pulping species such as the eucalyptus, the pine, the spruce and the bamboo, with a smaller quantity made from the straw, the bagasse and the waste paper of recycling. The comparison is therefore that the cellulose produced by the vegetation of the biosphere each year, of the order of 10^11 to 10^12 tonnes, exceeds the cellulose converted into paper by man, of the order of 10^9 tonnes, by a factor of roughly a hundred to a thousand, so that the whole paper industry of the world consumes only about one part in a thousand of the cellulose that the world's plants produce in a single year. The consumption of plant material by man in all forms is of course far greater than the paper figure, because it includes the timber used for fuel, for building and for furniture, the hay and the fodder of the livestock, the grain and the food of man himself, and the wood of the pulp, and it is of the order of 10^10 tonnes, that is tens of billions of tonnes, a year, which is still only a small fraction of the annual primary production of the biosphere. The conclusion is therefore double. Taken one way, the disparity shows that the total removal of plant material by man, enormous though it is, remains small beside the productive capacity of the biosphere, which could in principle sustain a considerably larger human population at the present level of consumption. Taken the other way, and this is the warning in the question, the seriousness of the loss of vegetation is not measured by the rate of consumption at all but by the rate of destruction, for what is lost is the area of the forest and of the natural ecosystem, and if the trees are felled faster than the seedlings replace them, which is the normal case, then a mature forest that needs a hundred years or more to regenerate is gone in a decade. Since the forests are also the sinks for the carbon dioxide of the atmosphere, their destruction raises atmospheric carbon dioxide and methane and so increases the greenhouse effect and the global warming. The amount man consumes is thus small; the rate at which he destroys the vegetation that produces it, set against the very slow rate at which that vegetation can be regenerated, is what makes the loss of vegetation so grave.
11. 11
    Q11 → The important properties of enzymes are these. Enzymes are catalysts, so they speed up the reactions they catalyse without being consumed, a small amount of enzyme acting on a large amount of substrate, and the enhancement is of the order of a million to a hundred million times; crucially, the enzyme acts by lowering the activation energy of the reaction and does not alter the equilibrium or the direction of the reaction, so it makes a reaction fast but does not make an otherwise impossible reaction possible. Enzymes are highly specific, and they show three levels of specificity: reaction specificity, in which an enzyme catalyses one particular type of reaction; substrate specificity, in which the enzyme acts on a particular substrate, as urease acts only on urea; and group specificity, in which one enzyme acts on a whole group of chemically similar substrates, as hexokinase acts on the hexoses glucose, fructose and mannose, and this last is what allows an enzyme to be used industrially on a natural mixture. The specificity has a structural basis in the active site, a small well-defined region of a few amino acid residues into which the substrate fits complementarily in shape and in chemical groups, the lock-and-key idea, refined in the induced-fit model in which the substrate slightly alters the shape of the enzyme. Enzymes have a pH optimum at which they act best, and it differs from enzyme to enzyme: pepsin of the stomach is optimal at about pH 1.8 to 2, trypsin of the intestine at about pH 7.8 to 8, and salivary amylase at about pH 6.8. Below or above the optimum the activity falls, because the charge on the active site is altered and the substrate no longer fits, and the extremes of pH, that is both the strong acid and the strong alkali, denature the enzyme and break its tertiary structure irreversibly, whereas a return to the optimum after a small change of pH restores the activity. Enzymes likewise have a temperature optimum, which for the enzymes of the human body is the normal body temperature of 37 degrees Celsius, one of the reasons the body temperature is maintained constant; below the optimum the activity falls reversibly as a mere slowing, whereas above it the enzyme is denatured by heat and the loss is permanent, which is the basis of cooking, pasteurisation and boiling as methods of preservation and of the survival of the enzymes of thermophilic bacteria at temperatures that would denature ours. Enzymes can be inhibited, and the inhibitors are of two kinds. Competitive inhibitors compete with the substrate for the active site and their inhibition is overcome by raising the substrate concentration, as with the malonate ion inhibiting succinic dehydrogenase of the Krebs cycle; non-competitive inhibitors bind at a site other than the active site, the allosteric site, and alter the shape of the enzyme so that the active site no longer functions, and their inhibition cannot be overcome by an increase of substrate, as with the cyanide ion inhibiting cytochrome oxidase. The metabolic importance of allosteric inhibition is that a natural end product acting as an inhibitor of an early step of its own pathway switches that pathway off when the end product accumulates, which is feedback inhibition and is how metabolism is regulated. Finally, enzymes are denatured by heat, by the extremes of pH and by the heavy metals, which is why the salts of mercury, lead and silver are toxic, since they inhibit the enzymes of the cell, and enzymes require only the mild aqueous near-neutral conditions of the living cell, whereas the same reactions would be hopelessly slow without them at a temperature of 37 degrees, a near-neutral pH and the low substrate concentrations found in the cell.

MCQ Questions

1 Mark Questions

1. 1
   What is denaturation of proteins? (Ans: loss of biological activity due to disruption of the native structure by heat/pH)
2. 2
   Give two examples of disaccharides. (Ans: sucrose, maltose, lactose)
3. 3
   What are essential amino acids? (Ans: amino acids that the human body cannot synthesise and must get from diet)
4. 4
   Name a coenzyme that carries hydrogen in metabolism. (Ans: NAD⁺/NADP⁺/FAD)

2 Mark Questions

1. 1
   Classify carbohydrates into monosaccharides, disaccharides and polysaccharides with one example of each.
2. 2
   Differentiate between DNA and RNA on any two points.
3. 3
   Name two factors that affect enzyme activity and explain how they act. (Ans: temperature (optimum ~37°C), pH (optimum limits), also substrate/enzyme concentration)

3 Mark Questions

1. 1
   Describe the four levels of protein structure. (Ans: primary (amino acid sequence), secondary (α-helix/β-pleated), tertiary (3D folding), quaternary (two or more chains))
2. 2
   Explain the specificity of enzymes and the lock-and-key model, and mention the effect of temperature on enzyme action.
3. 3
   Describe the structure of a nucleotide and how nucleotides polymerise into a nucleic acid. (Ans: phosphate + pentose sugar + nitrogen base; linked by phosphodiester bonds)

5 Mark Questions

1. 1
   Describe the structure and functions of proteins, and explain the types of bonds that stabilise protein structure with examples. (Ans: pepsin-digestion, actin-contraction; peptide bonds, hydrogen bonds, disulphide bonds, hydrophobic interactions)
2. 2
   Differentiate between anabolic and catabolic pathways, and give a detailed account of enzyme action including inhibition. (Ans: synthesis vs breakdown; competitive/non-competitive inhibitors, feedback regulation and allosteric sites)