A balanced diet
- Carbohydrates (starch and sugars) give energy. Lipids (fats and oils) store energy and insulate the body. Proteins are needed for growth and repair.
- Vitamins and minerals keep us healthy: vitamin C prevents scurvy, vitamin D and calcium keep bones and teeth strong, and iron is needed for red blood cells (a lack causes anaemia).
- Fibre keeps food moving through the gut and prevents constipation. Water is needed for all the body's reactions.
- Eating too much can lead to obesity; not enough can lead to starvation. Both are malnutrition.
Food tests
- Starch: add iodine solution. Orange-brown turns blue-black.
- Sugar (glucose): add Benedict's solution and heat in a water bath. Blue turns green, yellow, orange or brick-red.
- Protein: add biuret solution. Blue turns purple.
- Fats and oils: shake with ethanol, then add water: a cloudy white layer forms. (Or rub on paper: a translucent spot.)
The digestive system
- Mouth (teeth chew food; saliva contains amylase) → oesophagus → stomach (hydrochloric acid kills bacteria; protease starts digesting protein) → small intestine (enzymes finish digestion, and nutrients are absorbed into the blood) → large intestine (water is absorbed) → rectum → anus.
- The liver makes bile, which is stored in the gall bladder and breaks fats into small droplets. The pancreas makes enzymes.
- The small intestine is long and lined with villi: tiny finger-like folds that give a huge surface area. They have thin walls and a good blood supply for absorption.
Enzymes and plant nutrition
- Enzymes are biological catalysts: they speed up reactions without being used up. Digestive enzymes break large, insoluble molecules into small, soluble ones that can be absorbed.
- Amylase (a carbohydrase) breaks down starch into sugars. Protease breaks down proteins into amino acids. Lipase breaks down fats into fatty acids and glycerol.
- Enzymes work best at an optimum temperature (about 37 °C in the body) and pH. High temperatures denature them, so they stop working.
- To investigate amylase, mix it with starch and test a drop with iodine every 30 seconds; when the iodine stays orange-brown, the starch has gone.
- Plants make their own food by photosynthesis, but they need mineral ions from the soil: nitrates for proteins, phosphates for roots, potassium for flowers and fruit, and magnesium for chlorophyll.
Key terms
- Balanced diet
- Eating the right amounts of all the nutrients, fibre and water.
- Carbohydrate
- A nutrient that gives energy, such as starch or sugar.
- Lipid
- A fat or oil, used to store energy and insulate the body.
- Protein
- A nutrient needed for growth and repair.
- Vitamin
- A nutrient needed in small amounts to stay healthy, such as vitamin C.
- Mineral
- A nutrient such as calcium or iron, needed in small amounts.
- Fibre
- Keeps food moving through the gut.
- Deficiency disease
- An illness caused by a lack of a nutrient, such as scurvy.
- Enzyme
- A biological catalyst that speeds up reactions.
- Amylase
- An enzyme that breaks down starch into sugars.
- Protease
- An enzyme that breaks down proteins into amino acids.
- Lipase
- An enzyme that breaks down fats into fatty acids and glycerol.
- Villi
- Tiny folds in the small intestine that increase the surface area.
- Bile
- Made by the liver; breaks fats into small droplets.