If photosynthesis needs light, CO₂ and working enzymes, then the rate depends on all three. The useful question is: which one is holding the rate back right now?
Key idea
The rate of photosynthesis rises with light, carbon dioxide and temperature, until something else becomes the limiting factor. At high temperature the rate falls, as the enzymes of the Calvin cycle denature.
Why it works
Think of a factory line that needs two parts. Doubling one part does nothing if you are out of the other. In dim light, the light reactions cannot make ATP and NADPH fast enough, so adding light speeds things up. In bright light, the Calvin cycle cannot use them any faster, because it is waiting for CO₂ (or because the enzymes are working at their limit at that temperature). Then extra light does nothing: the curve is flat.
Worked example: Which factor is limiting?
On the low-CO₂ curve above, what limits the rate at 10% light? At 70% light?
- At 10%, adding light raises the rate steeply: light is limiting.
- At 70%, the curve is flat, so more light does nothing. The high-CO₂ curve is higher there, so CO₂ is limiting.
Answer: 10%: light. 70%: carbon dioxide.
Worked example: A lamp and a water plant
A student counts oxygen bubbles from a water plant with a lamp 40, 30, 20 and 10 cm away: 8, 14, 30 and 32 bubbles per minute. What happens between 20 and 10 cm, and why?
- Moving the lamp closer gives more light.
- From 40 to 20 cm the rate climbs, so light was limiting.
- From 20 to 10 cm it barely changes (30 to 32): another factor, such as CO₂ or temperature, is now limiting.
Answer: The rate levels off; light is no longer the limiting factor.
Greenhouse growers use this: on bright days they may add CO₂ to the air to raise the plateau, and they keep the temperature in the range where the enzymes work best.
Common mistake
Thinking more light always means more photosynthesis. Past a point the curve levels off because carbon dioxide or temperature is limiting. To find the limiting factor, ask: "If I raise this one, does the rate go up?" Only raising the limiting factor helps.