Photosynthesis and respiration
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.
Which factor is limiting, from the graph
A rate graph for photosynthesis rises and then flattens, and the whole question is what the flat part means. While the line is rising, the factor on the x-axis is the limiting one — increase it and the rate increases. Where it levels off, that factor is no longer limiting and something else has become the ceiling: usually temperature, carbon dioxide concentration or the amount of chlorophyll. If a second line is drawn higher on the same axes, whatever was changed between the two was the factor limiting the first one. Name the specific factor rather than saying the rate plateaus.
Where the reactions happen, and why it matters
Photosynthesis takes place in chloroplasts, so it happens only in cells that have them — which is why root cells do not photosynthesise and why a variegated leaf tests negative for starch in its white regions. Aerobic respiration takes place in mitochondria, and cells that need a lot of energy have a lot of them: muscle, and sperm cells around the tail. Anaerobic respiration happens in the cytoplasm. Questions often supply an electron micrograph or a count of organelles and expect you to link structure to function, which is a mark you cannot get by knowing the equations alone.