البناء الضوئي والتنفّس
Biology · GCSE · The School
Two equations, opposite directions
Photosynthesis: carbon dioxide + water → (light energy, chlorophyll) glucose + oxygen. Respiration: glucose + oxygen → carbon dioxide + water + energy. One stores energy in a chemical; the other releases it. Plants do BOTH — the common error is thinking plants only photosynthesise. They respire day and night, like every living thing.
Limiting factors
Photosynthesis speeds up with more light, more carbon dioxide, or higher temperature — until one of them runs short, and that one becomes the LIMITING factor. On a graph the rate rises then plateaus: the plateau shows something else has become the limit. Commercial greenhouses raise CO₂ and light precisely to push that plateau higher.
When oxygen runs out
Aerobic respiration uses oxygen and releases a lot of energy. Anaerobic respiration works without it but releases far less: in muscles it produces lactic acid (the burn, and the oxygen debt you repay by panting); in yeast it produces ethanol and carbon dioxide, which is bread and brewing. Same purpose, different chemistry, much less energy.
Seal a pondweed in water with a carbon dioxide indicator. In bright light photosynthesis outruns respiration, carbon dioxide is used up, and the indicator shows less of it. In darkness only respiration runs, carbon dioxide rises, and the indicator shows more. At dim light there is a point where the two exactly balance and the indicator does not move at all — the compensation point. Respiration never stopped at any stage.