Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that yield them on hydrolysis) and act as the main source of energy. They are classified as: (1) monosaccharides - simple single-unit sugars such as glucose and fructose; (2) oligosaccharides (mainly disaccharides) - such as sucrose, maltose and lactose, which yield 2 to 10 monosaccharides on hydrolysis; and (3) polysaccharides - large polymers such as starch, glycogen and cellulose.

Proteins are polymers of amino acids joined by peptide bonds. Their four levels of structure are: (1) Primary structure - the linear sequence of amino acids in a polypeptide chain; (2) Secondary structure - local folding into an alpha-helix or beta-pleated sheet held by hydrogen bonds; (3) Tertiary structure - the overall three-dimensional folding of the whole chain, giving the functional shape; and (4) Quaternary structure - the association of two or more polypeptide chains, as in haemoglobin.

Lipids are organic compounds that are insoluble in water but soluble in organic solvents; they include fats, oils, phospholipids and steroids and are generally esters of fatty acids and glycerol. Functions: (1) they are a concentrated store of energy (fats give about twice the energy of carbohydrates); (2) phospholipids form the structural basis of cell membranes; and they also act as thermal insulation and protect internal organs.