Biomolecules — Subtopic Breakdown — Summary

2.1 Carbohydrates

Classification: Monosaccharides (glucose, fructose, galactose, ribose), disaccharides (sucrose, lactose, maltose), polysaccharides (starch, cellulose, glycogen).

Glucose structure: Aldohexose. Open-chain form: OHC–(CHOH) $_4$ –CH $_2$ OH. Formula $C_6H_{12}O_6$ . In solution exists mainly as cyclic pyranose. $\alpha$ -anomer: C1-OH down; $\beta$ -anomer: C1-OH up. Mutarotation = interconversion through open chain.

Reducing sugars test: Free anomeric carbon → can open to aldehyde/ketone → reduces Tollens' (silver mirror) and Fehling's (brick-red precipitate) reagents. Glucose, fructose, lactose, maltose = reducing. Sucrose = non-reducing.

Polysaccharide linkages:

Amylose: $\alpha$ -1,4 only (linear)
Amylopectin: $\alpha$ -1,4 (main chain) + $\alpha$ -1,6 (branches every ~24–30 units)
Glycogen: $\alpha$ -1,4 + $\alpha$ -1,6 (branches every ~8–12 units; more frequent than amylopectin)
Cellulose: $\beta$ -1,4 (linear; structural; indigestible by humans)

2.2 Proteins and Amino Acids

Amino acid general formula: $H_2N$ –CHR–COOH. Essential amino acids (PVT TIM HALL) cannot be synthesised by the body. Glycine = only achiral amino acid (R = H).

Peptide bond formation: –COOH + H $_2$ N– → –CO–NH– + H $_2$ O (condensation). The bond has partial double-bond character; restricted rotation.

Protein structure hierarchy:

Level	Stabilising Force	Example
Primary	Peptide bonds (covalent)	Insulin
Secondary	Hydrogen bonds	Keratin ( $\alpha$ -helix), Silk ( $\beta$ -sheet)
Tertiary	Disulfide, hydrophobic, ionic, H-bonds	Myoglobin
Quaternary	Same as tertiary (between subunits)	Haemoglobin

Denaturation: Heat, extreme pH, or chemicals disrupt 2°/3°/4° structure. Primary structure (peptide bonds) intact. Enzyme loses activity upon denaturation.

2.3 Vitamins

Fat-soluble: A, D, E, K (stored in liver/adipose; overdose possible). Water-soluble: all B vitamins + C (not stored; daily dietary requirement; excreted in urine).

2.4 Nucleic Acids

DNA double helix: B-form; antiparallel strands; base pairing: A–T (2 H-bonds), G–C (3 H-bonds). Phosphodiester backbone links 3' of one nucleotide to 5' of the next.

Sugar comparison: Deoxyribose (DNA) lacks the 2'-OH that ribose (RNA) carries. This makes DNA more stable than RNA.

RNA types: mRNA (linear, carries codon), tRNA (cloverleaf, anticodon loop, carries amino acid), rRNA (most abundant RNA, ribosome component).

Central dogma: DNA → (transcription) → mRNA → (translation) → Protein.