Biological molecules

Carbohydrates

• Monosaccharides (glucose, fructose, galactose); α-glucose and β-glucose differ in the position of the OH group on carbon 1.
• Disaccharides form by condensation, making a glycosidic bond: maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose). Hydrolysis breaks the bond.
• Polysaccharides: starch (amylose – unbranched helix, 1,4 links; amylopectin – branched, 1,4 and 1,6 links) and glycogen (highly branched) are compact energy stores; cellulose (β-glucose, 1,4 links, alternate units inverted) forms hydrogen-bonded microfibrils giving strength to cell walls.

Lipids

Triglycerides: glycerol + three fatty acids joined by ester bonds. Saturated fatty acids have no C=C. Phospholipids have a hydrophilic phosphate head and hydrophobic tails, forming bilayers. Functions: energy store, insulation, membranes, waterproofing.

Proteins

• Amino acids join by peptide bonds (condensation).
• Primary structure: sequence of amino acids. Secondary: α-helix and β-pleated sheet held by hydrogen bonds. Tertiary: 3D folding held by hydrogen, ionic and disulfide bonds and hydrophobic interactions. Quaternary: more than one polypeptide (haemoglobin).
• Globular proteins (enzymes, haemoglobin) are soluble; fibrous proteins (collagen, keratin) are structural.

Water

Polar, forms hydrogen bonds: a good solvent, high specific heat capacity, high latent heat of vaporisation (cooling by sweating), cohesion (transpiration stream).

Check yourself

1. Name the bond between two monosaccharides.
2. Why is glycogen well suited as an energy store?
3. Which bonds hold the secondary structure of a protein?