AQA · Biology · 8461
GCSE Biology: Rate of photosynthesis
Bioenergetics · AQA GCSE Biology (8461), specification point 4.4.1.2. Cards are generated from the exam board's own specification for "Rate of photosynthesis", and every card links back to the page it came from. Specification © AQA — used with attribution.
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What is a limiting factor in photosynthesis?
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A factor that is in shortest supply and so restricts the rate of photosynthesis, even if other factors are increased.
The rate of photosynthesis is affected by temperature, light intensity, carbon dioxide concentration, and the amount of ?.
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chlorophyll
On a graph of photosynthesis rate against light intensity, what does the plateau show?
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Light intensity is no longer limiting; another factor (e.g. CO₂ concentration or temperature) has become the limiting factor.
Why does the rate of photosynthesis fall at high temperatures?
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The enzymes that control photosynthesis become denatured, so the reaction slows and stops.
Why does increasing carbon dioxide concentration raise the rate of photosynthesis?
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Carbon dioxide is a reactant in photosynthesis, so more of it lets the reaction happen faster (until another factor becomes limiting).
What is the relationship between light intensity and distance from a light source?
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Inverse square law: light intensity is proportional to 1/distance² — it falls with the square of the distance.
If the distance between a lamp and pondweed doubles, what happens to the light intensity?
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It falls to a quarter of its original value (inverse square law: 1/2² = 1/4).
Why do greenhouse growers not simply maximise heat, light and carbon dioxide?
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Adding heat, light or CO₂ costs money, so growers balance the higher rate of photosynthesis against the cost to keep the greenhouse profitable.
Which aquatic organism is used to investigate the effect of light intensity on photosynthesis?
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Pondweed (e.g. Elodea or Cabomba).
How is the rate of photosynthesis measured in the pondweed practical?
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By counting the bubbles of oxygen released per minute (or measuring the volume of gas collected) at different distances from a lamp.
Why does a lack of chlorophyll reduce the rate of photosynthesis?
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Chlorophyll absorbs the light energy needed for photosynthesis, so less chlorophyll (e.g. in diseased or magnesium-deficient leaves) means less light is captured.
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