BiomoleculesNEET Chemistry · Class 12 · NCERT Chapter 10

A reducing sugar is one that has a free aldehyde (-CHO) or ketone (-CO-) group that can be oxidised by Tollens reagent, Fehling solution, or Benedict reagent. The free anomeric carbon (C-1 for aldoses, C-2 for ketoses) must be unsubstituted — not involved in a glycosidic bond. Reducing sugars: glucose, fructose, galactose, maltose (its C-1 is free), and lactose (its C-1 of the glucose unit is free). Non-reducing sugars: sucrose — in sucrose, both the C-1 of glucose AND the C-2 of fructose are involved in the glycosidic bond, leaving no free anomeric carbon. Sucrose cannot reduce Fehling solution. NEET tests this directly: "Which of the following is a non-reducing sugar? Answer: Sucrose."

Essential amino acids are those that cannot be synthesised by the human body at all, or not in adequate amounts, and must therefore be obtained from the diet. There are 20 standard amino acids in human proteins, of which 9 are essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. A helpful mnemonic: PVT TIM HaLL (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, arginine (conditionally), Leucine, Lysine). The remaining amino acids are non-essential — the body can synthesise them from metabolic intermediates. NEET often asks to identify essential vs non-essential amino acids from a list.

Primary structure: the sequence of amino acids in the polypeptide chain. It is determined by the gene (DNA sequence). Peptide bonds (-CO-NH-) link the amino acids. Secondary structure: the local folding of the polypeptide chain into regular repeating patterns. The two common patterns are alpha-helix (hydrogen bonds within one chain, along the backbone) and beta-pleated sheet (hydrogen bonds between two adjacent parallel or antiparallel chains). Both are held by hydrogen bonds between the C=O of one peptide bond and the N-H of another. Tertiary structure: the overall 3D shape of a single polypeptide chain. Maintained by interactions between R groups: disulfide bonds (cysteine-cysteine), hydrophobic interactions, hydrogen bonds, ionic interactions. This gives the functional protein shape with an active site. Quaternary structure: the arrangement of multiple polypeptide subunits (chains). Example: haemoglobin has 4 subunits (2 alpha + 2 beta). Held by the same forces as tertiary structure. Not all proteins have quaternary structure.

Denaturation is the loss of the three-dimensional (3D) shape of an enzyme (or any protein) without breaking the primary structure (peptide bonds). The active site is disrupted, so the enzyme loses catalytic activity. Conditions that cause denaturation: (1) High temperature: breaks weak non-covalent bonds (H-bonds, hydrophobic interactions). Above the optimum temperature (~37°C for most human enzymes), activity drops sharply. (2) Extreme pH: changes the ionisation state of amino acid R groups in and around the active site, altering the shape. Optimum pH is usually 6–8 but varies (pepsin: pH 2, trypsin: pH 8). (3) Heavy metal ions: e.g., Hg²⁺, Pb²⁺ bind to the protein and alter the shape. (4) Organic solvents and detergents: disrupt hydrophobic interactions. Denaturation is usually irreversible for severe conditions (hard-boiled egg) but can be reversible if mild (some enzymes refold when the denaturing agent is removed).

Chargaff's rules: in any double-stranded DNA, the molar ratio of adenine (A) equals thymine (T), and the ratio of guanine (G) equals cytosine (C). In other words: [A] = [T] and [G] = [C], so [A]+[G] = [C]+[T] (purines = pyrimidines). Also, [A]+[T] / [G]+[C] is constant for a given species. These rules were crucial evidence for the Watson-Crick double helix model: A pairs with T (2 hydrogen bonds) and G pairs with C (3 hydrogen bonds). NEET application: if %A = 30%, then %T = 30%, and %G + %C = 40%, so %G = %C = 20%. These calculations appear directly in NEET.

Fat-soluble vitamins are stored in body fat and liver; toxicity is possible with excess. Water-soluble vitamins are not stored (excreted in urine); must be taken regularly. Fat-soluble vitamins (A, D, E, K): A (retinol) — night blindness, xerophthalmia. D (calciferol) — rickets (children), osteomalacia (adults). E (tocopherol) — haemolysis of RBCs. K (phylloquinone) — impaired blood clotting. Water-soluble vitamins (C and B group): C (ascorbic acid) — scurvy. B1 (thiamine) — beriberi. B2 (riboflavin) — cheilosis, glossitis. B3 (niacin) — pellagra. B5 (pantothenic acid) — burning feet syndrome. B6 (pyridoxine) — convulsions. B7 (biotin) — dermatitis, hair loss. B9 (folic acid) — megaloblastic anaemia. B12 (cobalamin) — pernicious anaemia. NEET questions typically test which vitamin and which disease are correctly paired.

DNA (deoxyribonucleic acid): double-stranded helix, sugar = deoxyribose (no -OH at 2' carbon), bases = A, T, G, C, base pair T with A. Stable. Found mainly in the nucleus. Carries genetic information. RNA (ribonucleic acid): usually single-stranded, sugar = ribose (-OH at 2' carbon), bases = A, U (uracil, not T), G, C. Uracil pairs with A. Less stable than DNA. Found in nucleus and cytoplasm. Three types: mRNA (carries genetic code from DNA to ribosome), tRNA (carries specific amino acids to ribosome), rRNA (structural component of ribosome). Key NEET distinction: RNA has uracil instead of thymine, and ribose instead of deoxyribose.

Chemistry Biomolecules contributes 2–4 NEET questions per year from Class 12. The most frequently tested topics are: (1) Reducing vs non-reducing sugars — sucrose is non-reducing; all monosaccharides are reducing. (2) Levels of protein structure — know which bonds hold each level (H-bonds: secondary; disulfide, hydrophobic: tertiary; same forces: quaternary). (3) Enzyme characteristics — specificity, lock-and-key model, effect of temperature and pH, enzyme inhibition. (4) Chargaff rules and DNA/RNA differences — nucleotide percentage calculations. (5) Vitamin deficiency diseases — fat vs water soluble, specific disease pairs. Note: Biology (Zoology/Botany) Biomolecules chapter has more weightage in NEET than the Chemistry chapter, so coordinate study with the zoology biomolecules content.