BiomoleculesNEET Zoology · Class 11 · NCERT Chapter 9

You can expect 1 to 2 questions from Biomolecules in NEET 2027. The most reliable scoring areas are: the four levels of protein structure with the bonds involved at each level, enzyme properties (active site, lock-and-key), factors that affect enzyme activity (temperature, pH, substrate concentration), the difference between storage and structural polysaccharides, and the structure of a nucleotide.

When you grind living tissue and treat it with strong acid, the molecules separate into two groups. The acid-soluble pool contains small organic compounds (amino acids, sugars, organic acids, nucleotides) and inorganic ions. These are the micromolecules or small molecules of the cell. The acid-insoluble pool contains the large macromolecules: proteins, polysaccharides, nucleic acids and lipids (lipids are technically not true macromolecules but they collect with the acid-insoluble fraction). In living cells, the macromolecular pool accounts for most of the dry weight.

Primary metabolites are compounds that have a clear, identifiable role in the normal physiological processes of an organism: growth, reproduction and development. Examples include amino acids, sugars, nucleotides, vitamins and fatty acids. Secondary metabolites are compounds whose function in the producing organism is not always obvious. They include alkaloids (like morphine, caffeine), rubber, resins, terpenes, essential oils, pigments and toxins. Secondary metabolites are mostly found in plants and microbes and have huge economic and medicinal importance.

Primary structure is the sequence of amino acids joined by peptide bonds. Secondary structure is the regular folding or coiling of the polypeptide chain: the alpha helix (held by hydrogen bonds within the chain) or the beta-pleated sheet (held by hydrogen bonds between parallel chains). Tertiary structure is the overall 3D shape of the entire polypeptide, held by hydrogen bonds, disulfide bonds, ionic bonds and hydrophobic interactions. Quaternary structure exists when a protein has two or more polypeptide subunits; the subunits are held together by non-covalent interactions. Haemoglobin (4 subunits) and collagen (3 subunits) are quaternary proteins.

The lock-and-key model says the active site of an enzyme has a shape that fits the substrate perfectly, like a key fits a lock. The substrate binds the active site, forms an enzyme-substrate complex, the reaction happens, and the products are released. The enzyme is unchanged and ready to catalyse the next reaction. This model explains specificity: each enzyme acts on only one or a few substrates. The induced-fit model is a refinement: the active site can change shape slightly to wrap around the substrate, giving a better fit.

As temperature rises from 0 degrees C, enzyme activity increases because more molecules have the kinetic energy to collide with the active site and react. Activity reaches a peak at the optimum temperature (about 35 to 40 degrees C for most human enzymes). Above the optimum, the enzyme protein begins to denature: the hydrogen bonds, disulfide bonds and other interactions holding the 3D shape break down. The active site shape changes, the substrate no longer fits, and activity falls sharply to zero. Denaturation above the optimum is usually irreversible.

Both are non-protein parts (cofactors) that are essential for an enzyme to work. A coenzyme is a small organic molecule (usually a vitamin derivative) that binds loosely and temporarily to the enzyme during the reaction, then dissociates. NAD+ and FAD are coenzymes. A prosthetic group is a cofactor that is tightly and permanently bound to the enzyme protein. Haem in cytochromes, FAD in succinate dehydrogenase, and biotin in carboxylases are prosthetic groups. A metal ion activator (like Mg2+, Zn2+) is a third type of cofactor.