Vitamin A (retinol, retinal, retinoic acid): essential for vision (retinal is chromophore in rhodopsin), epithelial cell differentiation, immune function. Deficiency: night blindness, xerophthalmia, keratomalacia. Sources: liver, egg yolk, dairy, carotenoids (carrots, sweet potato). Excess: hypervitaminosis A (liver damage, teratogenic). Vitamin D (cholecalciferol D3, ergocalciferol D2): synthesised in skin from 7-dehydrocholesterol by UV-B. Activated in liver (25-OH) then kidney (1,25-(OH)2 = calcitriol). Promotes intestinal Ca2+ and PO43- absorption. Deficiency: rickets (children — soft bones, bow legs), osteomalacia (adults — bone pain). Excess: hypercalcaemia. Vitamin E (tocopherol): fat-soluble antioxidant. Deficiency: haemolytic anaemia in newborns, ataxia. Vitamin K (phylloquinone K1, menaquinone K2): required for blood clotting factors II, VII, IX, X (carboxylation of glutamate residues). Deficiency: excessive bleeding, haemorrhagic disease of newborn.

Vitamin B1 (thiamine): coenzyme in pyruvate dehydrogenase and α-ketoglutarate dehydrogenase. Deficiency: Beriberi (peripheral neuropathy, heart failure) — wet beriberi (oedema) and dry beriberi (neurological). Vitamin B2 (riboflavin): FMN and FAD (electron carriers in respiratory chain). Deficiency: cheilosis, angular stomatitis, glossitis. Vitamin B3 (niacin, nicotinamide): NAD+ and NADP+ (coenzymes in many redox reactions). Deficiency: Pellagra (3Ds: Dermatitis, Diarrhoea, Dementia). Vitamin B5 (pantothenic acid): component of CoA. Deficiency: rare, "burning feet" syndrome. Vitamin B6 (pyridoxine): coenzyme for aminotransferases, decarboxylases. Deficiency: peripheral neuropathy, seborrheic dermatitis, microcytic anaemia. Vitamin B7 (biotin): carboxylation reactions. Deficiency: dermatitis, alopecia (rare — avidin in raw eggs binds biotin). Vitamin B9 (folic acid): one-carbon transfers, essential for DNA synthesis and neural tube development. Deficiency: megaloblastic anaemia, neural tube defects (spina bifida). Vitamin B12 (cobalamin): contains cobalt. Coenzyme for methylmalonyl-CoA mutase and methionine synthase. Deficiency: pernicious anaemia, subacute combined degeneration of spinal cord.

Vitamin C is water-soluble, most thermolabile vitamin (destroyed by heat and oxidation). Chemical structure: enediol of lactone of L-gulonic acid. Functions: (1) Coenzyme for hydroxylation of proline and lysine in collagen synthesis (collagen is most abundant protein — 30% of total protein). (2) Antioxidant: reduces oxidative damage. (3) Enhances iron absorption (reduces Fe3+ to Fe2+, which is better absorbed). (4) Immune function: promotes WBC activity. Deficiency — Scurvy: (historically devastated sailors before citrus was discovered). Symptoms: bleeding gums (impaired collagen in blood vessel walls), loose teeth, poor wound healing, perifollicular haemorrhages, corkscrew hairs, anaemia. Source: fresh fruits and vegetables (citrus, kiwi, capsicum, broccoli). RDA: 65-90 mg/day. Linus Pauling controversially advocated megadoses (1-10g/day) for preventing colds — not proven by clinical trials.

Vitamin D3 (cholecalciferol) synthesised in skin: 7-dehydrocholesterol + UV-B (290-315 nm) → pre-vitamin D3 → vitamin D3 (thermal isomerisation). Absorbed from diet (D2 or D3 in fortified foods, fatty fish, egg yolk). Liver: 25-hydroxylase → 25-OH-D3 (calcidiol, storage form, measured in blood to assess Vit D status). Kidney: 1α-hydroxylase → 1,25-(OH)2-D3 (calcitriol, active form). Calcitriol binds VDR (Vitamin D Receptor) → nuclear receptor → transcription factor → upregulates calcium transport proteins in intestine, kidney, bone. Rickets prevention: fortified milk, cod liver oil. Vitamin D deficiency is now recognised as epidemic in many countries (indoor lifestyle, sunscreen use, dark skin). Associated with: osteoporosis, increased cancer risk, autoimmune diseases, cardiovascular disease. Daily sunlight exposure (15-20 min, arms and legs) generates 10,000-25,000 IU vitamin D3.

Proteins: polymers of amino acids linked by peptide bonds (-CO-NH-). 20 standard amino acids. Primary structure: sequence of amino acids. Secondary structure: α-helix (right-handed, 3.6 residues/turn, H-bonds within chain), β-pleated sheet (H-bonds between chains), β-turns. Tertiary structure: 3D fold stabilised by: disulphide bonds (Cys-Cys), hydrophobic interactions, ionic bonds (salt bridges), H-bonds. Quaternary structure: association of multiple subunits. Haemoglobin: 4 subunits (2α+2β), cooperative O2 binding. Denaturation: loss of 3D structure by heat, acid, base, heavy metals, detergents. Primary structure maintained (peptide bonds intact). Biological activity lost. Sometimes reversible (renaturation). Enzymes: biological catalysts, protein in nature (except ribozymes). Active site is critical for function. Allosteric enzymes: regulated by molecules binding at sites other than active site.

Monosaccharides: simplest carbohydrates. Aldoses (CHO group) and ketoses (C=O group). D-glucose (aldohexose): most important fuel. D-fructose (ketohexose): fruits. Galactose: in lactose. Ribose, deoxyribose: in nucleic acids. Disaccharides: maltose (glucose-glucose, α-1,4), sucrose (glucose-fructose, α-1,β-2), lactose (galactose-glucose, β-1,4). Reducing sugars: free aldehyde/ketone group, react with Fehlings, Benedicts, Tollens. Sucrose: non-reducing (glycosidic bond between anomeric carbons of both sugars). Polysaccharides: starch (amylose α-1,4 + amylopectin α-1,4 and α-1,6). Glycogen (α-1,4 and α-1,6, more branched than amylopectin). Cellulose (β-1,4, structural, humans cannot digest). Chitin (β-1,4-N-acetylglucosamine, fungal cell walls, insect exoskeleton).

DNA: deoxyribonucleic acid. Double helix (Watson-Crick, 1953). Antiparallel strands. Base pairs: A=T (2 H-bonds), G≡C (3 H-bonds). Sugar: deoxyribose. Backbone: phosphodiester bonds (5→3 direction). B-form DNA: 10 bp/turn, pitch 34 Å. A-form (less common): 11 bp/turn. Z-form (left-handed): in GC-rich sequences. RNA: ribonucleic acid. Single-stranded (mostly). Sugar: ribose (2-OH group present). Uracil replaces thymine. Types: mRNA (messenger), tRNA (transfer, cloverleaf secondary structure, anticodon loop, CCA-3 for amino acid attachment), rRNA (ribosomal, structural). Nucleotides: base + sugar + phosphate. Nucleosides: base + sugar. ATP: adenosine triphosphate, universal energy currency. High-energy phosphoanhydride bonds. ΔG° for hydrolysis = -30.5 kJ/mol.

Lipids: diverse group of biological molecules soluble in organic solvents (hydrophobic). Fats and oils: triacylglycerols (triglycerides). Fatty acids + glycerol. Saturated (no double bonds): palmitic (C16), stearic (C18). Solid at room temperature. Animal fats. Unsaturated (one or more C=C): oleic (C18:1, cis), linoleic (C18:2, ω-6), linolenic (C18:3, ω-3). Liquid at room temperature. Plant/fish oils. Saponification: triglyceride + NaOH → soap (sodium salt of fatty acid) + glycerol. Soap: hydrophilic head (-COO-Na+) and hydrophobic tail (long carbon chain). Micelles in water. Phospholipids: membrane components. Glycerophospholipids: glycerol backbone. Lecithin (phosphatidylcholine): major membrane component, emulsifier. Steroids: four-ring structure. Cholesterol: membrane component, precursor to hormones. Bile salts, sex hormones (testosterone, oestrogen), adrenocortical hormones (cortisol, aldosterone), vitamin D. Waxes: long chain fatty acid esterified with long chain alcohol. Beeswax (myricyl palmitate). Protective function.