NCERT Solutions · Class 12 Chemistry Biomolecules

"Essential" and "non-essential" describe where the amino acid comes from, not how important it is to the body -- every one of the twenty is needed; only some of them the body can build for itself.An amino acid is a molecule with an amino group \(\displaystyle -NH_2 \) and a carboxyl group \(\displaystyle -COOH \) attached to the same carbon (the \(\displaystyle \alpha \)-carbon), along with a side chain (R group) that differs from one amino acid to the next. Proteins are built by joining amino acids through peptide bonds, and the human body needs a supply of all twenty standard amino acids to keep making its own proteins.Non-essential amino acids. These are the amino acids the human body can synthesise on its own, inside its cells, starting from simpler precursors (other amino acids, or intermediates of carbohydrate and fat metabolism). Because the body can make as much as it needs, these amino acids do not have to be supplied through food -- a diet completely lacking one of them still does not cause a deficiency, since the body compensates by synthesising it.Two examples of non-essential amino acids: (Aspartic acid and glutamic acid are two more valid examples; the textbook accepts any correctly identified pair.)Essential amino acids. These are the amino acids the human body cannot synthesise at all, or cannot synthesise fast enough to meet its needs, because it lacks the enzymes for the necessary biosynthetic steps. They must therefore be obtained ready-made from the diet -- from proteins in food that are digested down to free amino acids and absorbed. Leaving an essential amino acid out of the diet for a sustained period causes a genuine deficiency, because the body has no internal route to replace it.Two examples of essential amino acids: (Isoleucine, lysine, threonine, tryptophan, phenylalanine, and methionine are the other essential amino acids in the standard list; any correctly identified pair is acceptable.)The practical consequence of this distinction is nutritional: a diet must supply the essential amino acids directly, because no amount of the other nineteen amino acids or of general protein intake lets the body build valine or leucine from scratch -- only non-essential amino acids can be substituted for by the body's own biosynthesis.Answer: Non-essential amino acids (e.g., glycine, alanine) can be synthesised by the human body itself and so need not come from the diet; essential amino acids (e.g., valine, leucine) cannot be synthesised by the body and must be obtained from food.

Each of these three terms describes proteins at a different level — the bond that links two amino acids, the sequence that results, and what happens when the folded chain is disturbed.(i) Peptide linkageA peptide linkage is the amide bond, \(\displaystyle -\text{CO}-\text{NH}- \), formed between the \(\displaystyle -\text{COOH} \) group of one amino acid and the \(\displaystyle -\text{NH}_2 \) group of the next, with the elimination of one molecule of water.Take two amino acids, glycine, \(\displaystyle \text{H}_2\text{N}-\text{CH}_2-\text{COOH} \), and alanine, \(\displaystyle \text{H}_2\text{N}-\text{CH}(\text{CH}_3)-\text{COOH} \). The \(\displaystyle -\text{OH} \) of glycine's carboxyl group and one \(\displaystyle -\text{H} \) of alanine's amino group leave together as \(\displaystyle \text{H}_2\text{O} \), and the carbon of the former \(\displaystyle -\text{COOH} \) bonds directly to the nitrogen of the former \(\displaystyle -\text{NH}_2 \):\[\text{H}_2\text{N}-\text{CH}_2-\text{COOH} + \text{H}_2\text{N}-\text{CH}(\text{CH}_3)-\text{COOH} \longrightarrow \text{H}_2\text{N}-\text{CH}_2-\text{CO}-\text{NH}-\text{CH}(\text{CH}_3)-\text{COOH} + \text{H}_2\text{O} \]The product, \(\displaystyle \text{H}_2\text{N}-\text{CH}_2-\text{CO}-\text{NH}-\text{CH}(\text{CH}_3)-\text{COOH} \), is the dipeptide glycylalanine (Gly-Ala); the \(\displaystyle -\text{CO}-\text{NH}- \) group joining the two residues is the peptide linkage. This is a condensation reaction — the small molecule lost is water — and it is the only kind of bond that links amino acid residues along a protein chain. A protein of \(\displaystyle n \) amino acid residues therefore contains \(\displaystyle (n-1) \) peptide linkages, since one water molecule is lost at each junction and the two end residues keep one free \(\displaystyle -\text{NH}_2 \) and one free \(\displaystyle -\text{COOH} \) group respectively.(ii) Primary structureA protein may be built from one or several polypeptide chains. Within any one chain, the amino acid residues are joined, residue after residue by peptide linkages, in one specific linear sequence — this exact sequence, read from the free-amino (N-terminal) end to the free-carboxyl (C-terminal) end, is the primary structure of the protein.For example, if a chain has the sequence glycine–alanine–valine–…, its primary structure is written Gly-Ala-Val-…, and this order is fixed for a given protein: even one residue out of place is a different protein. The primary structure carries no information about how the chain twists or folds in space — it is only the "letters in order," analogous to the sequence of words in a sentence before any grammar (folding) is applied. Every higher level of protein architecture (the coiled or pleated secondary structure, the $\displaystyle 3$-D folded tertiary structure, and the multi-chain quaternary structure) is a consequence of this one underlying sequence.(iii) DenaturationDenaturation is the loss of a protein's biological (native) activity that occurs when its secondary and tertiary structures are disrupted by a physical change (such as heating) or a chemical change (such as a change in pH), while the primary structure — the sequence of peptide-linked residues — remains intact.In its native state a protein exists as one of two structural forms: a fibrous form (chains held parallel by hydrogen bonds and cross-links) or a globular form (chains coiled around themselves and folded into a compact, roughly spherical shape). This native, folded shape is what allows a globular protein such as an enzyme to carry out its function. When the protein is heated or treated with acid or base, the hydrogen bonds that hold the coiled/folded (secondary and tertiary) structure in place are broken. The globules unfold and the chains become disorganised — the protein is denatured. A familiar example is boiling an egg: the soft, soluble albumin (egg white protein) unfolds on heating and becomes a solid, insoluble mass. Because only the folding is destroyed and the peptide-bonded sequence of the chain is unaffected, the primary structure of a denatured protein is the same as that of the native protein, but its biological activity is lost.Answer: (i) Peptide linkage — the amide bond \(\displaystyle -\text{CO}-\text{NH}- \) formed between \(\displaystyle -\text{COOH} \) of one amino acid and \(\displaystyle -\text{NH}_2 \) of another with loss of \(\displaystyle \text{H}_2\text{O} \) (e.g., Gly + Ala → Gly-Ala + \(\displaystyle \text{H}_2\text{O} \)). (ii) Primary structure — the fixed, linear sequence in which amino acid residues are joined by peptide linkages in a polypeptide chain. (iii) Denaturation — loss of a protein's biological activity when heat, acid, or base disrupts the hydrogen bonds holding its secondary/tertiary (folded) structure, unfolding globules into disorganised chains, without breaking the primary (peptide-bonded) sequence.

Secondary structure is not about which amino acids are present — it is about how the backbone folds, held together only by hydrogen bonds between the peptide -C=O and -N-H groups. There are two common patterns: the alpha-helix and the beta-pleated sheet.Start from what a peptide bond actually offers for hydrogen bonding. Every residue in the chain contributes one C=O (from its carbonyl carbon) and one N-H (from its amide nitrogen). These two groups are strongly polarised — the carbonyl oxygen carries a partial negative charge and the amide hydrogen a partial positive charge — so a C=O on one part of the chain can hydrogen-bond to an N-H elsewhere on the chain (or on a neighbouring chain). Secondary structure is just the geometric pattern this hydrogen bonding forces the backbone into.Alpha-helix. The polypeptide chain twists into a right-handed coil, like a spring. As the chain winds round, the C=O group of one amino acid residue lines up directly below the N-H group of the amino acid four residues further along the chain — that is, residue \(\displaystyle i\) hydrogen-bonds to residue \(\displaystyle i+4\) — because the geometry of a $\displaystyle 3.6$-residues-per-turn helix places these two groups close together and pointing at each other. This C=O···H-N hydrogen bond is repeated at every residue all the way down the chain, so the helix is held rigid by a continuous, regularly repeating "staircase" of hydrogen bonds running parallel to the helix axis, with the amino acid side chains projecting outward from the coil.Beta-pleated sheet. Here the chain is not coiled but stretched out almost fully, running in a zig-zag ("pleated") path. Two or more such extended chains (or two extended stretches of the same chain folded back on itself) are laid side by side, and hydrogen bonds form between the C=O of one chain and the N-H of the adjacent chain, linking the strands together edge-to-edge into a flat, sheet-like arrangement. Depending on whether the neighbouring strands run in the same N-terminus-to-C-terminus direction or in opposite directions, the sheet is called parallel or antiparallel — but in both cases the stabilising interaction is the same inter-chain C=O···H-N hydrogen bonding.So the two forms differ in where the hydrogen-bonding partner comes from: in the alpha-helix it is found further down the same chain (an intra-chain bond, giving a coiled rod), while in the beta-pleated sheet it is found on a neighbouring chain or strand (an inter-chain bond, giving a flat sheet).Answer: The two common secondary structures of proteins are the alpha-helix, in which the polypeptide chain coils into a right-handed helix stabilised by hydrogen bonds between the C=O of one amino acid residue and the N-H of the residue four places ahead in the same chain, and the beta-pleated sheet, in which extended, zig-zag polypeptide chains lie side by side (parallel or antiparallel) and are held together by hydrogen bonds between the C=O and N-H groups of neighbouring chains.

Hydrogen bonding between the backbone \(\displaystyle \text{C=O} \) and \(\displaystyle \text{N-H} \) groups of the peptide chain holds the α-helix together — nothing else in the structure is doing the job.A protein's polypeptide backbone is a repeating chain of peptide bonds:\[\cdots -\text{CO}-\text{NH}-\text{CHR}-\text{CO}-\text{NH}-\text{CHR}-\cdots \]Each peptide linkage contributes one carbonyl group, \(\displaystyle \text{C=O} \), and one amide group, \(\displaystyle \text{N-H} \), to the backbone. In the α-helix, this chain is coiled into a right-handed spiral. The coiling is not random — it is held in place because the backbone twists in exactly the way needed to bring the \(\displaystyle \text{C=O} \) of one peptide bond close to the \(\displaystyle \text{N-H} \) of a peptide bond four residues further along the same chain (the \(\displaystyle n \) and \(\displaystyle n+4 \) residues).The mechanism, step by step:1. The oxygen of a \(\displaystyle \text{C=O} \) group carries a partial negative charge (it is the more electronegative atom, and it also bears lone pairs), while the hydrogen of an \(\displaystyle \text{N-H} \) group carries a partial positive charge (bonded to the electronegative nitrogen). 2. When the chain coils into the helical geometry, the lone pair on the carbonyl oxygen of residue \(\displaystyle n \) lines up with the \(\displaystyle \text{N-H} \) hydrogen of residue \(\displaystyle n+4 \). 3. This pulls the two groups together into a hydrogen bond, \(\displaystyle \text{C=O}\cdots\text{H-N} \), running roughly parallel to the axis of the helix. 4. This pattern repeats at every peptide bond along the chain, so every \(\displaystyle \text{C=O} \) is hydrogen-bonded to an \(\displaystyle \text{N-H} \) four residues ahead. The result is a continuous ladder of intramolecular hydrogen bonds that runs the whole length of the helix, and it is this repeating set of bonds — not any bond involving the side chain \(\displaystyle R \) — that locks the coil into its helical shape and keeps every turn held tightly against the next.Because each individual hydrogen bond is weak, no single one of them could hold the shape on its own; it is the large number of them, all pointing the same way along the helix axis, that makes the α-helix a stable, rigid secondary structure.**Answer: Hydrogen bonds — specifically, intramolecular hydrogen bonding between the \(\displaystyle \text{C=O} \) group of one peptide bond and the \(\displaystyle \text{N-H} \) group of a peptide bond four amino acid residues further along the same polypeptide chain — stabilise the α-helix structure of proteins.

Shape is the root cause: a protein either coils tightly into a compact ball, or it lies out straight and packs side by side into a fibre — every other difference follows from that one geometric choice.A protein's final shape (its tertiary/quaternary structure) comes from how the polypeptide chain folds on itself. Two folding patterns exist, and they give two structurally and functionally distinct classes.Globular proteins Fibrous proteins Summarised point by point: **Answer: Globular proteins have their polypeptide chains coiled and folded into a compact, spherical shape, are soluble in water, and perform functional roles (e.g., insulin, haemoglobin, pepsin, antibodies). Fibrous proteins have their polypeptide chains lying parallel to one another, held together by hydrogen and disulphide bonds into long, thread-like fibres, are insoluble in water, and perform structural roles (e.g., keratin, myosin, collagen).

An amino acid carries both an acidic group (\(\displaystyle -COOH\)) and a basic group (\(\displaystyle -NH_2\)) on the same molecule, and in solution these two groups react with each other rather than staying neutral — that internal proton transfer is what lets the molecule act as either an acid or a base depending on what it meets.Take glycine, \(\displaystyle H_2N-CH_2-COOH\) (systematic name $\displaystyle 2$-aminoacetic acid), as the working example — the same argument holds for every amino acid, only the side chain \(\displaystyle R\) changes.Step $\displaystyle 1$ — the internal proton transfer (zwitterion formation). The \(\displaystyle -COOH\) group is a proton donor and the \(\displaystyle -NH_2\) group is a proton acceptor, and they are close enough on the same molecule for the acid group to hand its proton straight to the amine group: \[H_2N-CH_2-COOH \;\longrightarrow\; {}^{+}H_3N-CH_2-COO^{-} \] The species on the right, \(\displaystyle {}^{+}H_3N-CH_2-COO^{-}\), is called a zwitterion ("dipolar ion") — it carries a full positive charge on the nitrogen and a full negative charge on the oxygen at the same time, with no net charge on the molecule overall. This zwitterion, not the neutral-looking \(\displaystyle H_2N-CH_2-COOH\) written on paper, is the form amino acids actually exist in — as a solid and in neutral aqueous solution.Step $\displaystyle 2$ — behaviour in acidic medium (the amino acid acts as a base). When the zwitterion is placed in excess acid (a solution rich in \(\displaystyle H^+\)), the negatively charged carboxylate oxygen, \(\displaystyle -COO^{-}\), is the site with a lone pair available to accept a proton. It picks up an \(\displaystyle H^+\) from the medium: \[{}^{+}H_3N-CH_2-COO^{-} \; + \; H^{+} \;\longrightarrow\; {}^{+}H_3N-CH_2-COOH \] Here the amino acid has consumed a proton — that is exactly what a base does. The product, \(\displaystyle {}^{+}H_3N-CH_2-COOH\), is a cation (net charge \(\displaystyle +1\)); this is the form glycine takes in strongly acidic solution.Step $\displaystyle 3$ — behaviour in basic medium (the amino acid acts as an acid). When the zwitterion is placed in excess base (a solution rich in \(\displaystyle OH^{-}\)), the site with a proton to give up is the ammonium group, \(\displaystyle -NH_3^{+}\). The hydroxide ion pulls this proton off: \[{}^{+}H_3N-CH_2-COO^{-} \; + \; OH^{-} \;\longrightarrow\; H_2N-CH_2-COO^{-} \; + \; H_2O \] Here the amino acid has donated a proton (to \(\displaystyle OH^-\), forming water) — that is exactly what an acid does. The product, \(\displaystyle H_2N-CH_2-COO^{-}\), is an anion (net charge \(\displaystyle -1\)); this is the form glycine takes in strongly basic solution.Why this counts as amphoteric. A substance is amphoteric when the same species can act as a proton donor in one setting and a proton acceptor in another. Steps $\displaystyle 2$ and $\displaystyle 3$ show precisely that: the zwitterion \(\displaystyle {}^{+}H_3N-CH_2-COO^{-}\) Because it can go either way depending on the \(\displaystyle pH\) of the medium, the amino acid is called an amphoteric electrolyte, or ampholyte. (The one detail students often get backwards: it is the carboxylate oxygen, already negative, that grabs the proton in acid; it is the ammonium nitrogen, already holding the extra proton, that gives it up in base — the charges on the zwitterion tell you which group reacts, not the neutral-looking textbook formula \(\displaystyle H_2N-CH_2-COOH\).)**Answer: Amino acids exist as zwitterions, \(\displaystyle {}^{+}H_3N-CH_2-COO^{-}\), formed by an internal proton transfer from the \(\displaystyle -COOH\) group to the \(\displaystyle -NH_2\) group. In acidic solution the \(\displaystyle -COO^{-}\) end accepts a proton, giving the cation \(\displaystyle {}^{+}H_3N-CH_2-COOH\) (the amino acid acting as a base); in basic solution the \(\displaystyle -NH_3^{+}\) end donates a proton, giving the anion \(\displaystyle H_2N-CH_2-COO^{-}\) (the amino acid acting as an acid). Because the same molecule can donate or accept a proton depending on the medium, amino acids are amphoteric electrolytes (ampholytes).

Enzymes are biological catalysts — proteins that speed up the specific biochemical reactions of a living cell without themselves being consumed in the reaction.Almost every enzyme is a globular protein (a few catalytic RNA molecules, called ribozymes, are the rare exception, but at the Class $\displaystyle 12$ level "enzyme" means protein). A protein catalyst folds into a compact three-dimensional shape, and on its surface there is a small pocket or cleft called the active site. The active site is built from the side chains of a handful of amino acid residues that are far apart along the polypeptide chain but are brought close together in space by the folding of the chain — so its shape and its pattern of charges/hydrogen-bond donors and acceptors are unique to that one enzyme.The molecule the enzyme acts on is called the substrate. Because the active site's shape and chemical environment match only one substrate (or a small family of closely related substrates), an enzyme shows very high specificity — the same feature that in ordinary chemistry we call a "lock and key" fit:1. The substrate diffuses into the active site and binds it, forming an enzyme-substrate complex \(\displaystyle \mathrm{E + S \rightleftharpoons ES} \). 2. Binding strains or polarizes particular bonds in the substrate and holds the reacting groups in exactly the orientation needed for reaction, so the transition state is reached with a far lower activation energy than the uncatalysed path. 3. The bound substrate reacts to give product while still on the enzyme, and the product — no longer a good fit for the site — is released, regenerating the free enzyme: \(\displaystyle \mathrm{ES \rightarrow E + P} \).Because the enzyme comes out unchanged at the end of step $\displaystyle 3$, a very small amount of enzyme can process a very large amount of substrate — enzymes are effective in extremely low concentration.Two features follow directly from this mechanism and are worth naming because they are what make enzymes different from ordinary inorganic catalysts: A everyday example: the enzyme invertase (sucrase) hydrolyses the disaccharide sucrose into an equal mixture of glucose and fructose, \[\mathrm{C_{12}H_{22}O_{11}\ (sucrose) + H_2O \xrightarrow{\text{invertase}} C_6H_{12}O_6\ (glucose) + C_6H_{12}O_6\ (fructose)} \] and the enzyme maltase similarly hydrolyses maltose specifically to two units of glucose. Digestion, respiration, and essentially every metabolic transformation inside a living cell proceed through chains of such enzyme-catalysed steps, each enzyme handling one specific reaction in the sequence.**Answer: Enzymes are protein biocatalysts produced by living cells. Each enzyme has an active site whose shape and chemistry match a particular substrate; the substrate binds there to form an enzyme-substrate complex, which lowers the activation energy and converts substrate to product, after which the unchanged enzyme is released to act again. This lock-and-key type binding gives enzymes their two defining properties — very high catalytic efficiency at very low concentration, and very high specificity for a particular reaction/substrate (e.g., invertase converts only sucrose to glucose and fructose; maltase converts only maltose to glucose).

Denaturation strips away every level of folding above the amino-acid sequence itself — the secondary and tertiary structure collapse — while the primary structure, the exact order in which amino acids are joined by peptide bonds, is left completely intact.To see what "collapse" means here, first name the structure that is being destroyed.A protein's shape is built up in stages: What denaturation does, step by step:When a protein is subjected to a physical change (heating, e.g. boiling an egg, or agitation) or a chemical change (a change in pH, addition of a strong acid/base or certain salts, or exposure to alcohol), the weak, non-covalent interactions that hold the secondary and tertiary structure together are the first to give way:1. Heat or a change in pH disrupts the hydrogen bonds between peptide \(\displaystyle \text{C}=\text{O}\) and \(\displaystyle \text{N}-\text{H}\) groups that hold the \(\displaystyle \alpha\)-helix/\(\displaystyle \beta\)-sheet in place, so the regular coiling of the secondary structure is lost. 2. The same disturbance breaks the hydrogen bonds, ionic interactions, and hydrophobic packing that hold the tertiary globule folded, so the compact $\displaystyle 3$-D globule unfolds and the polypeptide chain uncoils. 3. The peptide bonds of the backbone themselves are not broken — no covalent bond in the primary sequence is touched — so the amino-acid sequence stays exactly as it was.Because the tertiary shape (and with it, the biological activity) is lost while the primary sequence survives, the net effect is described as: the globular protein unfolds and is converted into a shapeless, extended, fibrous form, and it loses its biological (e.g. enzymatic) activity.Everyday examples of this exact process: Answer: Denaturation destroys a protein's secondary and tertiary structure — the hydrogen bonds, disulphide bridges, ionic and hydrophobic interactions that hold the \(\displaystyle \alpha\)-helix/\(\displaystyle \beta\)-sheet and the folded $\displaystyle 3$-D globule together are broken by heat or a change in pH — causing the globular protein to unfold into a random, fibrous shape and lose its biological activity. The primary structure (the amino-acid sequence held by peptide bonds) is not affected. Coagulation of egg white on boiling and curdling of milk on souring are common examples of protein denaturation.

Vitamins are grouped by one physical property: whether they dissolve in fat or in water — and that single property also decides how the body handles them.Vitamins are organic compounds the body cannot synthesize (or cannot synthesize in sufficient amount) and must obtain from food in small quantities. They are classified into two groups based on their solubility.1. Fat-soluble vitaminsThese dissolve in fats and oils but not in water: vitamins A (retinol), D (calciferol), E (tocopherol), and K (phylloquinone/menaquinone).Because they are lipid-soluble, they are absorbed along with dietary fat, transported in the blood bound to lipoproteins, and — critically — stored in the liver and in adipose (fatty) tissue. This storage means the body keeps a reserve, so a missed day's intake does not immediately cause deficiency; it also means excess intake can accumulate to toxic levels (hypervitaminosis), which is not a concern with the water-soluble group.2. Water-soluble vitaminsThese dissolve in water: vitamin C (ascorbic acid) and the members of the vitamin B complex (B1 thiamine, \(\displaystyle \mathrm{B_{2}}\) riboflavin, \(\displaystyle \mathrm{B_{6}}\) pyridoxine, \(\displaystyle \mathrm{B_{12}}\) cyanocobalamin, niacin, folic acid, etc.).Being water-soluble, they are not stored in the body to any significant extent — any excess is simply excreted in urine. This is the step people get wrong: because there is no depot to draw on, these vitamins must be supplied continuously through the diet, and their deficiency symptoms appear much sooner than for the fat-soluble group.Which vitamin controls blood coagulationVitamin K is the one responsible for the coagulation (clotting) of blood. It functions as a cofactor for an enzyme that carboxylates glutamate residues on several clotting factors (including prothrombin, i.e., clotting factor II), and this carboxylation is essential for those factors to bind calcium ions and become active in the clotting cascade. Without adequate vitamin K, blood clotting is impaired and even minor injuries can bleed excessively — this is why vitamin K deficiency shows up clinically as increased blood-clotting time.Answer: Vitamins are classified as fat-soluble (A, D, E, K — stored in the liver and adipose tissue) and water-soluble (B complex and C — not stored, excreted if in excess). Vitamin K is responsible for the coagulation of blood.

Vitamins are organic compounds the body cannot manufacture in adequate amounts, so each one that is missing shows up as a specific deficiency disease — the "importance" of a vitamin is best read off what fails when it is absent.Vitamin A (retinol)Vitamin A is a fat-soluble vitamin built on a beta-ionone ring with a long isoprenoid side chain ending in an alcohol group; in the retina it is oxidised to retinal, which combines with the protein opsin to form rhodopsin (visual purple), the pigment that makes vision in dim light possible. Vitamin C (ascorbic acid)Vitamin C is water-soluble, so unlike vitamin A it cannot be stored in the body in significant amounts and must be supplied continuously in the diet. Answer: Vitamin A (retinol) is essential because it forms the visual pigment rhodopsin needed for eyesight and keeps epithelial tissue healthy — its deficiency causes night blindness and xerophthalmia; its sources are fish liver oil, milk, butter, eggs, carrots and leafy vegetables. Vitamin C (ascorbic acid) is essential because it maintains the collagen that cements cells together in skin, blood vessels and gums — its deficiency causes scurvy (bleeding gums, loose teeth); its sources are citrus fruits (amla, orange, lemon) and fresh green vegetables.