Food and nutrients
Nutrition is how organisms obtain and use food. The food we eat contains seven classes of nutrients, each with a job in the body.
| Nutrient | Main job | Sources |
|---|---|---|
| Carbohydrates | energy | maize, rice, bread, cassava, bananas |
| Proteins | growth and repair | meat, fish, eggs, beans, milk |
| Fats and oils | stored energy, insulation | groundnuts, cooking oil, butter |
| Vitamins | protect against disease and keep tissues healthy | fruits, vegetables |
| Minerals | build bones and teeth, make blood | milk, leafy vegetables, fish |
| Water | reactions, transport, temperature control | drinks, fruits |
| Fibre (roughage) | keeps food moving through the gut | whole grains, vegetables |
Energy in food
Try itHow much energy is in a meal?
Choose the grams of each nutrient in a meal. See the energy from each and how the nutrients share the total.
How plants make food
Green plants are autotrophs: they make their own food by photosynthesis. Animals are heterotrophs: they must eat other organisms.
Digestion
Digestion breaks large insoluble food molecules into small soluble ones that can be absorbed into the blood. Enzymes speed it up: amylase digests starch, protease digests protein and lipase digests fat.
| Part of the gut | What happens there |
|---|---|
| Mouth | Teeth chew the food. Saliva contains amylase, which starts to digest starch. |
| Oesophagus | Muscles push the food down to the stomach (peristalsis). |
| Stomach | Acid kills germs. Protease starts to digest protein. |
| Small intestine | Digestion is finished. Digested food is absorbed into the blood through the villi. |
| Liver and pancreas | The liver makes bile, which helps to digest fat. The pancreas makes digestive enzymes. |
| Large intestine | Water is absorbed from the leftovers. |
| Rectum and anus | Faeces are stored, then egested. |
Key terms
- Nutrient
- A substance in food that the body needs.
- Balanced diet
- A diet with all nutrients in the right amounts.
- Photosynthesis
- How green plants make glucose from carbon dioxide and water using light.
- Enzyme
- A protein that speeds up a chemical reaction.
Summary
- Seven classes of nutrient: carbohydrates, proteins, fats, vitamins, minerals, water and fibre.
- Energy: about 17 kJ/g for carbohydrate and protein, 37 kJ/g for fat.
- Plants make food by photosynthesis; animals eat food.
- Digestion uses enzymes to break food into small soluble molecules.