Biomolecules — NEET Notes

How chemical composition is analysed

If you grind a leaf, a piece of liver, or a microbial paste in trichloroacetic acid and strain it, the slurry splits into two fractions. The filtrate — the acid-soluble pool — holds thousands of small organic compounds: amino acids, monosaccharides, fatty acids, nucleotides. The retentate — the acid-insoluble pool — holds only four types of compound: proteins, nucleic acids, polysaccharides, and the membrane-bound lipids. Burn the same tissue and what is left is ash — calcium, magnesium, phosphate, sulphate. Living tissues hold the same elements as the earth's crust; what differs is the proportion. Carbon, hydrogen, oxygen and nitrogen are dramatically enriched in life, while silicon and sodium are depleted.

The biological vocabulary used to describe these compounds is different from the chemist's. A chemist sees aldehydes, ketones and aromatic rings; a biologist groups the same compounds into amino acids, fatty acids, sugars and nitrogen bases. The acid-soluble compounds have molecular weights below ~800 daltons and are called micromolecules or simply biomolecules. The acid-insoluble compounds are polymers with weights of ten thousand daltons or more, called macromolecules or biomacromolecules. Lipids are odd: each lipid molecule is small (under 800 Da) but, embedded in cell membranes, they get torn off as vesicles during grinding and end up in the macromolecular fraction.

Primary & secondary metabolites

The biomolecules with identifiable roles in everyday physiology — amino acids, simple sugars, fatty acids, glycerol, nitrogen bases, nucleotides — are called primary metabolites. Animals contain almost exclusively these. Plants, fungi and microbes go further: they make thousands of compounds whose functions are still partly mysterious — alkaloids like morphine and codeine, terpenoids, essential oils, lectins like concanavalin A, toxins like ricin and abrin, drugs like vinblastine and curcumin, pigments like carotenoids and anthocyanins, gums, rubber. These are secondary metabolites. They are not waste; many are ecologically important, and a great many are useful to humans.

NEET 2019 lifted concanavalin A straight off NCERT Table 9.3 and asked what it was — a lectin, a secondary metabolite. The same table is the source of any "morphine = alkaloid", "ricin = toxin" or "rubber = polymeric substance" matching question. Memorise the table; the questions write themselves.

Biomacromolecules — the four classes

Only three true macromolecules exist in living systems: proteins, nucleic acids, and polysaccharides. All three are polymers — long chains of monomers linked by specific covalent bonds. Lipids ride along as an honorary fourth class because membrane fragments separate with the macromolecular pellet. Each polymer has its own characteristic bond, and NEET 2021 tested all four in a single matching question.

Carbohydrates, proteins, lipids and nucleic acids together with water and inorganic ions account for the entire chemical composition of any cell. Water alone makes up 70–90% of the cell mass; proteins 10–15%; nucleic acids, polysaccharides and lipids the rest. Living matter is mostly water with carbon-based polymers suspended in it.

Proteins — diversity & function

A protein is a linear polymer of amino acids — a polypeptide — and a heteropolymer, because the 20 amino acids that build proteins are chemically different. Some amino acids the body can make; others — the essential amino acids — must come from the diet. Amino acids are α-amino acids: amino, carboxyl, hydrogen, and a variable R group all attached to the α-carbon. The R group decides whether the amino acid is acidic (glutamic acid), basic (lysine), neutral (valine), aromatic (tyrosine, phenylalanine, tryptophan) or sulphur-containing (cysteine, methionine). In solution, the simultaneously ionised –NH₃⁺ and –COO⁻ form is the zwitterion.

Functionally, proteins do almost everything in a cell. They transport molecules, fight infection, send hormonal signals, sense the environment, give tissues their shape, and catalyse the reactions that keep the cell alive. NCERT's own Table 9.5 lists the canonical examples, and almost every one of them has been an NEET question.

Other Table-9.5 standards: trypsin (enzyme), antibody / immunoglobulin (defence), and receptors for taste, smell and hormones. NEET 2022's Match-List question pinned glycogen to "storage", globulin to "antibody", steroids to "hormone", and thrombin to "biocatalyst" — every one of those rows comes from this section of NCERT.

Polysaccharides — storage vs structural

Polysaccharides are long chains of sugars linked by glycosidic bonds. Starch, the energy store of plants, is a homopolymer of glucose; so is glycogen, the energy store of animals. Cellulose, the structural polymer of plant cell walls, is also a glucose homopolymer — but the difference in linkage geometry changes everything. Inulin is a polymer of fructose. Chitin, found in arthropod exoskeletons and fungal cell walls, is a polymer of N-acetylglucosamine (an amino-sugar). Plant cell walls, paper and cotton fibre are all cellulosic.

A useful diagnostic: starch forms helical secondary structures whose central cavity traps iodine, producing the familiar deep blue starch–iodine complex. Cellulose, despite being chemically very similar, does not form complex helices and therefore cannot hold iodine — no colour. NEET 2023 used this exact reasoning as a stand-alone question.

NEET 2020 asked which compounds carry glycosidic and peptide bonds respectively — answer: inulin (a polysaccharide, glycosidic) and insulin (a peptide hormone, peptide). The questions look like wordplay; the underlying logic is just the polymer-bond table.

Nucleic acids — DNA and RNA

The third class of true macromolecule in the acid-insoluble pellet is the nucleic acid — a polynucleotide. The monomer, a nucleotide, has three pieces: a nitrogenous base, a five-carbon sugar, and a phosphate group. The bases are purines (adenine, guanine — double-ring) and pyrimidines (cytosine, uracil, thymine — single-ring). The sugar is ribose in RNA and 2′-deoxyribose in DNA. A base attached to a sugar is a nucleoside (adenosine, guanosine, cytidine, uridine, thymidine); add a phosphate and it becomes a nucleotide (adenylic acid, etc.).

Successive nucleotides link through a phosphodiester bond: the phosphate bridges the 3′-OH of one sugar and the 5′-OH of the next. DNA is the genetic material of every cell; it is passed faithfully from parent to offspring. RNA, depending on its type, carries the genetic message (mRNA), brings amino acids to the ribosome (tRNA), or builds the ribosome itself (rRNA). Some RNA molecules even act as enzymes — they are called ribozymes, and they are the exception to the rule that "all enzymes are proteins."

Four levels of protein structure

Biologists describe proteins at four hierarchical levels. The primary structure is the linear sequence of amino acids, like beads on a string, running from the N-terminal (first amino acid) on the left to the C-terminal (last amino acid) on the right. The chain does not stay extended — it folds. Local folds into right-handed α-helices and β-pleated sheets, stabilised by hydrogen bonds between backbone amides, give the secondary structure. The entire chain then folds back on itself, "like a hollow woollen ball", producing the tertiary structure — the three-dimensional shape that is necessary for biological activity. When two or more polypeptide subunits assemble together — as four globin chains do in haemoglobin — the architecture of that assembly is the quaternary structure.

The dynamic state of body constituents — and the living state

Looking at the cell at a single instant — a snapshot of its molecular census — you would conclude that nothing is happening. The same compounds appear at roughly the same concentrations day after day. That stillness is an illusion. Every molecule in that snapshot is being made and broken simultaneously, and at extraordinary rates. The body is in a dynamic state: every constituent is turning over.

The web of reactions that does this turnover is called metabolism. Metabolism has two branches. Anabolism builds — small molecules are stitched into larger ones, energy is invested, water is released (condensation). Amino acids → proteins, glucose → glycogen, nucleotides → nucleic acids: all anabolic. Catabolism breaks down — large molecules are hydrolysed into smaller ones, energy is liberated. Glycogen → glucose, glucose → pyruvate → CO₂ + H₂O in cellular respiration. Each step of every pathway is catalysed by an enzyme; chains of enzyme-catalysed reactions form a metabolic pathway.

Glucose → pyruvate is one such pathway — ten enzyme-catalysed steps, the foundation of cellular respiration. From pyruvate, different cells route the carbon differently: skeletal muscle under anaerobic load makes lactic acid; yeast under fermentation makes ethanol; oxygen-fed tissues feed pyruvate into the Krebs cycle. Same pathway, different terminal product, decided by which enzyme acts at the branch point.

This continuous, enzyme-driven flux is what NCERT calls the living state. Cells maintain metabolite concentrations far above or below their chemical equilibria — they are held away from equilibrium by the constant work of enzyme-catalysed reactions. The moment metabolism stops, concentrations begin drifting to equilibrium, and that drift is death. The living state is not a static composition; it is a regulated disequilibrium.

The living state is a non-equilibrium steady state — maintained, not given.

NCERT Class 11, Chapter 9 — the central idea of metabolism

Enzymes — mechanism of action

Almost every metabolic reaction is catalysed by an enzyme. Almost every enzyme is a protein; a handful are catalytic RNAs called ribozymes. An enzyme is a folded polypeptide whose tertiary structure carves out a pocket — the active site — into which the substrate fits. The active site is not a passive crevice. Substrate binding triggers a small conformational change that pulls the enzyme tighter around the substrate, strains key bonds, and stabilises the unstable transition state on the way from substrate to product.

The catalytic cycle in NCERT's own steps:

1. Substrate (S) binds the active site of enzyme (E) — the ES complex forms.
2. Binding induces the enzyme to alter its shape, fitting tighter around the substrate.
3. The active site, now in close proximity, breaks the substrate's bonds and an EP complex forms.
4. The product is released. The free enzyme is regenerated — unchanged — ready for another substrate molecule.

Symbolically: E + S → ES → EP → E + P. The enzyme is recovered intact at the end. This is why a tiny amount of enzyme catalyses an enormous amount of reaction.

To see how much enzymes accelerate reactions, take carbonic anhydrase. The uncatalysed hydration of CO₂ to H₂CO₃ produces about 200 molecules of acid per hour. With the enzyme, the same reaction yields 600,000 molecules per second. The acceleration is roughly ten million-fold. This is typical, not exceptional.

The mechanism behind this acceleration is energetic. Every reaction crosses a transition state — an unstable, high-energy intermediate between substrate and product. The energy gap from substrate to transition state is the activation energy. The higher this barrier, the slower the reaction. Enzymes do not lower the energy of the substrate or the product — they cannot change those. They lower the height of the barrier itself by binding and stabilising the transition state. Less energy is needed to climb to the top, so vastly more substrate molecules cross over per unit time.

Factors affecting enzyme activity

Enzyme activity depends on the integrity of the protein's tertiary structure — and several environmental factors can perturb that structure.

Temperature. Each enzyme has an optimum temperature at which activity is maximal. Below it, the reaction is slow because molecular motion is reduced; the enzyme is preserved in a temporarily inactive state. Above it, activity collapses because the protein is denatured — heat unfolds the tertiary structure and the active site disappears. Most human enzymes have optima near 37 °C and are destroyed above about 40 °C. Enzymes from thermophilic bacteria living in hot vents or sulphur springs remain stable up to 80–90 °C; this thermal stability is what makes them useful in industrial processes such as PCR.

pH. Each enzyme has an optimum pH. Activity falls off above and below it. Stomach pepsin works best at pH ~2; pancreatic trypsin works best at pH ~8. Changes in pH alter the ionisation of side chains in and around the active site and ultimately disrupt the fold.

Substrate concentration. At low substrate concentration the reaction velocity rises linearly. As more substrate is added, velocity climbs to a maximum (V_max) beyond which further increases do nothing — every enzyme molecule is saturated. The substrate concentration at which velocity is half of V_max is K_m, an inverse measure of substrate affinity.

Inhibitors. A chemical that binds the enzyme and shuts off activity is an inhibitor. A competitive inhibitor resembles the substrate closely enough to occupy the active site without being acted upon — it competes with the substrate for binding. Malonate inhibits succinic dehydrogenase because it looks like succinate; this is the textbook example. Competitive inhibitors are widely used as drugs — many antibiotics work by mimicking an essential bacterial substrate.

Six classes of enzymes

Of the thousands of enzymes identified, every one falls into one of six classes defined by the type of reaction catalysed. Each class is given a digit and each enzyme a four-digit EC number. Memorise the six in order — the question almost always involves matching a named reaction to its class.

A mnemonic that survives the exam: "Over The Hill, Like It's Light" — Oxidoreductase, Transferase, Hydrolase, Lyase, Isomerase, Ligase.

Cofactors — the non-protein helpers

Many enzymes need a non-protein partner to function. The protein part on its own is the apoenzyme — catalytically inactive. Combined with its non-protein partner — the cofactor — it becomes the holoenzyme, fully active. Holoenzyme = Apoenzyme + Cofactor. Strip the cofactor away and catalytic activity is lost. Cofactors come in three flavours.

Vitamins matter here because the essential chemical components of many coenzymes are vitamins. NAD and NADP both carry niacin (vitamin B₃). FAD carries riboflavin (B₂). Coenzyme A carries pantothenic acid (B₅). A vitamin deficiency is, biochemically, a coenzyme deficiency — and that is why so many vitamin-deficiency syndromes look like metabolic collapse.