Nutrition and Digestion
Nutrients and their tests
Candidates must know the food groups and their roles: carbohydrates for energy, proteins for growth and repair, fats for insulation and energy storage, vitamins and minerals in small amounts, water and dietary fibre. Learn the food tests: iodine for starch (blue-black), Benedict's for reducing sugar (brick-red on heating), Biuret for protein (purple), ethanol emulsion for fat (milky white). A common trap is forgetting that Benedict's requires heating in a water bath.
The digestive system
Digestion is the breakdown of large insoluble molecules into small soluble ones that can be absorbed. Mechanical digestion (teeth, churning) increases surface area; chemical digestion uses enzymes. Know where each enzyme works: amylase in saliva and the small intestine (starch to maltose), protease in the stomach and small intestine (protein to amino acids), lipase in the small intestine (fat to fatty acids and glycerol). The stomach produces hydrochloric acid, which kills bacteria and provides acidic pH for protease.
Absorption and the villus
The small intestine lining is folded into villi, giving a large surface area, a thin wall, and a rich blood supply. These features speed absorption of glucose and amino acids into the blood, and fatty acids and glycerol into the lymph.
Plants make their own food
Photosynthesis uses carbon dioxide and water with light energy and chlorophyll to make glucose and oxygen. The limiting factor idea is examined often: increasing light, carbon dioxide or temperature raises the rate until another factor becomes limiting.
Worked example and trap
A leaf kept in the dark then tested with iodine stays brown, because no starch was made without light. Rule of thumb: a positive iodine test means starch, not sugar.