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Getting started: life processes, photosynthesis and respiration

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Two reactions keep almost all life going: photosynthesis makes food and respiration releases the energy in it. You met both at KS3; GCSE asks you to use them all the time.

What makes something alive?

At KS3 you learned seven life processes, often remembered as MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
At International GCSE you will meet eight characteristics: the same ideas plus controlling internal conditions.

Photosynthesis

Plants make their own food by photosynthesis, in the chloroplasts.
carbon dioxide + water → glucose + oxygen
Light energy is absorbed by the green pigment chlorophyll.
The glucose is used for respiration, stored as starch, or turned into substances such as cellulose and proteins.

Respiration is not breathing

Aerobic respiration happens in the mitochondria of all living cells, plants included, all the time.
glucose + oxygen → carbon dioxide + water (energy is transferred)
When muscles are short of oxygen they also respire anaerobically: glucose → lactic acid. This transfers much less energy.
Breathing is different: it is moving air into and out of the lungs.
Worked example

A plant is kept in the dark for two days. Its leaves are then tested and contain no starch. Explain why.

  1. With no light, the plant cannot photosynthesise, so no new glucose is made.
  2. The plant keeps respiring, day and night.
  3. It breaks down its stored starch to glucose for respiration until the starch is used up.

Answer: No photosynthesis in the dark, but respiration continues and uses up the stored starch.

Key idea

Photosynthesis: carbon dioxide + water → glucose + oxygen, in chloroplasts, using light. Aerobic respiration: glucose + oxygen → carbon dioxide + water, in mitochondria of all living cells, all the time. Breathing is not respiration.

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