Respiration: Releasing the Energy

Making food is only half the story. Food is useless until its energy is released — and that happens inside the cells of every living thing, all day and all night. The process is called (aerobic) respiration.

Compare that with photosynthesis and you will see something beautiful: respiration is photosynthesis running backwards. What one process builds, the other takes apart.

The released energy is used for growth, movement, repair, keeping warm, and every other job a living thing does. Plants respire too — they must, because they also need energy. A plant photosynthesises only in the light, but it respires all the time.

Worked example — testing for the products. Two flasks are set up overnight in a dark cupboard.

• Flask A: soaked, germinating bean seeds. Flask B: beans that were boiled first (so they are dead).
• In the morning the air from each flask is bubbled through limewater.

Result: the limewater from Flask A turns milky; the limewater from Flask B stays clear.

Milky limewater is the standard test for carbon dioxide. So the living beans released carbon dioxide in the dark — they were respiring. The dead beans released none. The darkness rules out photosynthesis, which makes the result impossible to misread.