ZIMSEC O Level · Combined Science · Form 1, Form 2
Nutrition is how living things obtain food for energy, growth and repair.
Photosynthesis
Green plants make their own food in the chloroplasts of leaves:
carbon dioxide + water → (light energy, chlorophyll) → glucose + oxygen
Factors that affect the rate: light intensity, carbon dioxide concentration and temperature.
Test a leaf for starch: boil in water, then in ethanol (to remove chlorophyll), rinse, then add iodine solution. Blue-black shows starch is present.
A balanced diet
- Carbohydrates (sadza, bread): energy. Fats and oils: stored energy and insulation.
- Proteins (beans, meat, fish, matemba): growth and repair.
- Vitamins and minerals: health, e.g. vitamin C prevents scurvy, iron prevents anaemia.
- Fibre: helps food move through the gut. Water: needed for all reactions.
Food tests: iodine for starch (blue-black), Benedict's for reducing sugar (brick-red when heated), biuret for protein (purple), ethanol emulsion for fat (cloudy white).
Human digestion
Digestion breaks large insoluble food molecules into small soluble ones that can be absorbed.
- Mouth: teeth chew; amylase in saliva starts digesting starch.
- Stomach: acid and protease (pepsin) digest protein.
- Small intestine: enzymes complete digestion; villi absorb digested food into the blood.
- Large intestine: absorbs water; waste leaves as faeces.
Check yourself
- Write the word equation for photosynthesis.
- Which reagent is used to test for protein, and what colour shows a positive result?
- Why is the leaf boiled in ethanol during the starch test?
Show answers
- carbon dioxide + water → glucose + oxygen (with light and chlorophyll)
- Biuret reagent; purple/violet.
- To remove the chlorophyll so the colour change can be seen.
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