Biomolecules — Top 20 Questions

Glucose gives a pentaacetate with acetic anhydride (one acetate per −OH group). This confirms glucose has 5 −OH groups. (The aldehyde group does not react with acetic anhydride under normal conditions.) The 5 OH groups are at C1, C2, C3, C4, and C6 in the open chain form.

Sucrose is a non-reducing sugar — it does not reduce Tollen's or Fehling's reagent. This is because in sucrose, the anomeric carbons of both glucose and fructose are involved in the glycosidic bond (C1 of glucose and C2 of fructose) — no free anomeric −OH group available for ring opening to aldehyde/ketone form.

Secondary structure: regular local folding patterns — α-helix (coiled, H-bonds between C=O of one amino acid and N−H of another 4 residues away) and β-pleated sheet (H-bonds between extended strands). Stabilised by intramolecular hydrogen bonds. Primary = amino acid sequence; Tertiary = 3D folding.

Essential amino acids cannot be synthesised by the human body and must be obtained from diet. Lysine is an essential amino acid. The 9 essential amino acids are: His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val (remember: 'HILLMFTTV'). Glycine, alanine, and glutamic acid are non-essential.

One complete turn of the B-form DNA double helix contains 10 base pairs and has a pitch of 3.4 nm (each base pair is 0.34 nm apart). The helix is right-handed. This was established by Watson and Crick (1953) based on Franklin's X-ray crystallography data.

RNA contains Adenine, Guanine, Cytosine, and Uracil. DNA contains Adenine, Guanine, Cytosine, and Thymine. Uracil replaces Thymine in RNA. Uracil pairs with Adenine (like Thymine). Uracil lacks the 5-methyl group present in Thymine.

Sucrose + H₂O (or invertase enzyme) → glucose + fructose. The equimolar mixture of glucose and fructose is called 'invert sugar' (because sucrose is dextrorotatory (+66.5°) but the product mixture is laevorotatory (−19.9°) — inversion of optical rotation). This is why bee honey (which contains invertase) is sweeter than sucrose.

Water-soluble vitamins: Vitamin C and all B vitamins (B1, B2, B3, B5, B6, B7, B9, B12). Fat-soluble vitamins: A, D, E, K. Water-soluble vitamins are not stored in large amounts in the body → need regular dietary intake. Vitamin C (ascorbic acid) deficiency causes scurvy.

Protease (peptidase) catalyses hydrolysis of peptide bonds (−CO−NH−) in proteins → amino acids. Examples: pepsin (stomach), trypsin, chymotrypsin (intestine). Lipase: hydrolysis of lipids/fats → fatty acids + glycerol. Amylase: starch hydrolysis → maltose/glucose. Nuclease: hydrolysis of nucleic acids.

Cellulose is the structural polysaccharide of plant cell walls — gives rigidity and strength. It consists of glucose units linked by β-1,4-glycosidic bonds. Humans cannot digest cellulose (lack cellulase enzyme) — it acts as dietary fibre. Starch (energy storage in plants), Glycogen (energy storage in animals), Sucrose (disaccharide).

At the isoelectric point (pI), the amino acid exists predominantly as a zwitterion (dipolar ion) with equal positive (−NH₃⁺) and negative (−COO⁻) charges → net charge = 0. At pH < pI: positively charged. At pH > pI: negatively charged. At pI, amino acid has minimum solubility and does not migrate in electric field.

Enzymes are biological catalysts that: (1) are highly specific (one enzyme, one substrate — lock and key model), (2) lower activation energy, (3) have optimum pH (usually 6-8, pepsin: pH 2) and temperature (37°C for most human enzymes), (4) are NOT consumed in reaction, (5) do NOT change equilibrium constant.

Fructose is a ketose but is a reducing sugar. In alkaline conditions of Tollen's test, fructose undergoes isomerisation via enediol intermediate to give glucose and mannose (aldoses) — which then reduce Ag⁺. This is why fructose, despite being a ketone, gives positive Tollen's test.

In nucleic acids, the sugar (deoxyribose in DNA, ribose in RNA) and phosphate groups are connected by phosphodiester bonds — forming the backbone of the polynucleotide chain. Glycosidic bond: between sugar and nitrogenous base. Peptide bond: in proteins. Hydrogen bond: holds the two strands of DNA together.

Denaturation: disruption of secondary (H-bonds in α-helix/β-sheet), tertiary (disulfide, hydrophobic, ionic bonds), and quaternary structures — but primary structure (peptide bonds, amino acid sequence) remains INTACT. Causes: heat, pH change, organic solvents, heavy metals. Example: egg white protein (albumin) denatures on heating (coagulation).

Natural rubber is cis-poly(isoprene): (−CH₂−C(CH₃)=CH−CH₂−)n. The monomer is isoprene (2-methylbuta-1,3-diene, CH₂=C(CH₃)−CH=CH₂). Rubber is produced in the latex of Hevea brasiliensis tree. Vulcanization (with sulphur) cross-links chains → improves elasticity and strength.

ATP: adenine (purine base) + ribose (pentose sugar) + three phosphate groups. The bond between second and third phosphate (β-γ bond) is a high-energy bond (~30.5 kJ/mol). Hydrolysis ATP → ADP + Pi releases energy used for cellular work. ATP is often called 'energy currency of the cell'.

Glucose forms a 6-membered ring (5 carbons + 1 oxygen) by intramolecular hemiacetal formation between C1 aldehyde and C5 hydroxyl group. This 6-membered ring is called the pyranose form (after pyran, the parent compound). Fructose primarily forms a 5-membered ring (furanose form).

DNA is the genetic material — it carries genetic information encoded in the sequence of bases and transmits it through generations (replication). The information in DNA is transcribed into mRNA, which is then translated into proteins. DNA is the blueprint of life, maintained in the nucleus.

Vitamin D (calciferol) is synthesised in the skin from 7-dehydrocholesterol upon UV light (sunlight) exposure. It is important for calcium absorption and bone health. Deficiency causes rickets (in children) and osteomalacia (in adults). It is both a vitamin (obtained from diet) and a hormone (synthesised in body).