Biological molecules

Biological molecules and food tests

Carbohydrates (C, H, O) are built from simple sugars: glucose joins to form starch (plant store), glycogen (animal store) and cellulose (cell walls). Proteins (C, H, O, N, sometimes S) are chains of amino acids. Fats and oils (C, H, O) are made from fatty acids and glycerol. Food tests: iodine turns blue-black with starch; Benedict's (heated) turns brick-red with reducing sugars; biuret turns purple with protein; ethanol emulsion test gives a cloudy white emulsion with fat; DCPIP decolourises with vitamin C. DNA is a double helix with base pairs A–T and C–G.