The digestive system is an example of an organ system in which several organs work together to digest and absorb food. Students should be able to describe the nature of enzyme molecules and relate their activity to temperature and pH changes. • Enzymes catalyse specific reactions in living organisms due to the shape of their active site. • Students should be able to use the 'lock and key theory' as a simplified model to explain enzyme action. • Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream. • Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch. • Proteases break down proteins to amino acids. • Lipases break down lipids (fats) to glycerol and fatty acids. • The products of digestion are used to build new carbohydrates, lipids and proteins. Some glucose is used in respiration. • Bile is made in the liver and stored in the gall bladder. It is alkaline to neutralise hydrochloric acid from the stomach. It also emulsifies fat to form small droplets which increases the surface area. The alkaline conditions and large surface area increase the rate of fat breakdown by lipase.