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Nutrition

Biology · WAEC and JAMB · SS2 and SS3

Covers how plants make food and how animals process it. The examiner almost always wants a word equation, a named enzyme, or an adaptation explained.

What you need to know

  • Photosynthesis needs light, chlorophyll, carbon dioxide and water, and produces glucose and oxygen.
  • The leaf is adapted for it: broad and flat for surface area, thin for short diffusion distance, stomata for gas exchange, veins for transport.
  • Limiting factors: light intensity, carbon dioxide concentration and temperature. Whichever is in shortest supply caps the rate.
  • Classes of food: carbohydrates, proteins, fats and oils, vitamins, mineral salts, water, roughage.
  • Digestion in the mouth: salivary amylase converts starch to maltose.
  • In the stomach: pepsin converts protein to peptides, in acidic conditions.
  • In the duodenum: bile emulsifies fat; lipase converts fat to fatty acids and glycerol.
  • Absorption happens in the ileum, adapted by villi, microvilli, a rich blood supply and a thin wall.

Key terms

Autotroph
An organism that makes its own food, as green plants do.
Enzyme
A biological catalyst, made of protein, that speeds up a reaction without being used up.
Emulsification
Breaking fat into small droplets to increase surface area for lipase. Bile does this; it is not an enzyme.

Formulae

  • Photosynthesis: 6CO2 + 6H2O --light, chlorophyll--> C6H12O6 + 6O2
  • Aerobic respiration: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy

Worked example

Explain why a destarched leaf partly covered with foil shows starch only in the uncovered part.

  1. Destarching removes existing starch so any found afterwards is newly made.
  2. The covered part received no light, so photosynthesis could not occur there.
  3. The uncovered part had light, chlorophyll, CO2 and water, so it photosynthesised and stored starch.

The mistake to avoid

Bile is NOT an enzyme. It emulsifies fat and neutralises stomach acid. Calling it an enzyme is one of the most common errors in the paper.

In the exam

For any food test, state the reagent, the procedure and the colour change. "Benedict's solution, heat, brick-red precipitate" is a full answer; "Benedict's" alone is not.