Factors affecting the rate
Factor
Effect
Why
Light intensity rate rises with light intensity, then levels off
light provides the energy; once there is enough, something else limits
Carbon dioxide concentration rate rises, then levels off
carbon dioxide is a reactant
Temperature rate rises to an optimum, then falls sharply
enzymes work faster when warmer, then denature
Amount of chlorophyll more chlorophyll, faster
more light absorbed; reduced by disease or magnesium deficiency
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What limits the rate of photosynthesis light intensity rate of photosynthesis rate rises, then levels off — something else has become the limiting factor carbon dioxide concentration rate of photosynthesis the same shape — more CO₂ helps, until something else runs short temperature rate of photosynthesis rate rises, peaks at the optimum, then falls steeply as the enzymes DENATURE The limiting factor is whichever one is in shortest supply — it is the thing holding the rate back. On a bright warm day it is usually carbon dioxide; on a cold bright day it is temperature; at dawn and dusk it is light. Note the difference: the first two graphs LEVEL OFF, but the temperature graph PEAKS and then crashes, because enzymes are destroyed by heat. Three graphs of rate of photosynthesis: against light intensity (rising then flat), against carbon dioxide concentration (rising then flat), and against temperature (rising to a peak then dropping steeply)
Measuring the rate: count bubbles of oxygen per minute from
pondweed, or better, measure the volume of oxygen collected in a
fixed time. Rate = = = ÷ \div ÷
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In the full topic
• Factors affecting the rate• Limiting factors [Higher Tier]• The inverse square law [Higher Tier]• Greenhouses [Higher Tier]Read the free biology topics →