Limiting factors
Change light, carbon dioxide and temperature, and see which one is actually holding the rate back at any moment.
- Limiting factors
- Rate graphs and plateaus
- Photosynthesis
Relative rate
40%
Limiting factor
light intensity
On the graph
rising part
Predict
The curve has flattened. You increase the light intensity further. What happens to the rate?
Experiment
- 1Set carbon dioxide to 30% and drag the light intensity from 0 to 100. Note where the curve flattens.
- 2With the light high, raise the carbon dioxide and watch the plateau lift.
- 3Set the temperature to 50 °C and explain why the rate falls even with plenty of light and carbon dioxide.
- 4Find a combination where temperature is the limiting factor and check the readout agrees with you.
Explain
A limiting factor is whichever requirement is in shortest supply. It alone sets the rate, so increasing anything else has no effect at all until the limiting factor is raised.
On the rising part of the graph, light is limiting — more light, more photosynthesis. On the plateau, light is in excess and something else has taken over, usually carbon dioxide concentration or temperature. That is why an examiner asking “what is limiting here?” never accepts “light” as the answer for the flat part.
Temperature behaves differently from the other two at the top end: past about 40 °C the enzymes controlling photosynthesis denature, so the rate falls rather than levelling off.