Biomolecules
Living systems are built from complex biomolecules - carbohydrates, proteins, nucleic acids, lipids - together with simple molecules like vitamins and hormones.
Carbohydrates
Optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis. Classified by behaviour on hydrolysis:
- Monosaccharides: cannot be hydrolysed further (glucose, fructose, ribose). An aldose has a -CHO group, a ketose a >C=O group; named also by carbon number (triose to heptose).
- Oligosaccharides (2-10 units), chiefly disaccharides (sucrose, maltose, lactose).
- Polysaccharides (many units): starch, cellulose, glycogen - the non-sugars.
Reducing sugars reduce Fehling's solution and Tollens' reagent; all monosaccharides are reducing.Glucose (D-(+)-glucose) is an aldohexose; its structure was deduced from HI (n-hexane, straight chain), HCN/NH2OH (carbonyl), Br2 water (gluconic acid, aldehydic CO), pentaacetate (5 -OH) and HNO3 (saccharic acid, primary -OH). It exists as alpha and beta anomers of a six-membered pyranose hemiacetal. Fructose is a ketohexose, D-(-)-fructose, forming a five-membered furanose ring.
Disaccharides are joined by a glycosidic (oxide) linkage: sucrose (glucose + fructose, non-reducing, gives invert sugar), maltose (two glucose, reducing), lactose (galactose + glucose, reducing). Polysaccharides: starch (amylose + amylopectin, alpha-glucose), cellulose (beta-glucose, C1-C4), glycogen (animal starch, highly branched).
Proteins
Polymers of about twenty alpha-amino acids linked by peptide bonds (amide -CO-NH-). Amino acids have -NH2 and -COOH groups, exist as zwitter ions (amphoteric); essential amino acids must come from diet. Structure: primary (sequence), secondary (alpha-helix, beta-pleated sheet via H-bonds), tertiary (overall folding, fibrous/globular), quaternary (sub-unit arrangement). Denaturation by heat/pH destroys 2 and 3 structures (H-bonds) but keeps the primary structure - e.g. boiling egg white.Enzymes
Biocatalysts, almost all globular proteins, highly specific; they lower the activation energy. Named with the suffix -ase (maltase, sucrase).Vitamins
Accessory food factors. Fat soluble: A, D, E, K (stored in liver/fat). Water soluble: B group and C (excreted, not stored except B12). Deficiencies: A - night blindness/xerophthalmia; B1 - beri beri; B2 - cheilosis; B6 - convulsions; B12 - pernicious anaemia; C - scurvy; D - rickets/osteomalacia; E - fragile RBCs; K - increased clotting time.Nucleic Acids
Polynucleotides (DNA, RNA). Nucleoside = base + sugar; nucleotide = base + sugar + phosphate. DNA: 2-deoxyribose, double helix, bases A, G, C, T (A-T, C-G pairing). RNA: ribose, single strand, bases A, G, C, U; types mRNA, rRNA, tRNA. DNA is the basis of heredity; RNA carries out protein synthesis.Hormones
Intercellular messengers from endocrine glands: steroids (estrogens, androgens), polypeptides (insulin, glucagon, endorphins), amino acid derivatives (epinephrine, thyroxine from tyrosine). Insulin lowers and glucagon raises blood glucose; testosterone and estradiol drive secondary sex characters.