What is a limiting factor?
Reveal answer
A factor in short supply that restricts the rate of a process such as photosynthesis.
Key words: limiting factor
2(e) NUTRITION · RECALL
2.20 Factors affecting the rate of photosynthesis
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Name the three factors that can limit photosynthesis.
Reveal answer
Light intensity, carbon dioxide concentration and temperature.
Key words: light · carbon dioxide · temperature
Why does the rate stop rising as light intensity increases?
Reveal answer
Another factor (carbon dioxide concentration or temperature) becomes limiting.
Key words: another factor
Why does photosynthesis slow at very high temperatures?
Reveal answer
Enzymes that control photosynthesis are denatured.
Key words: denatured
Which factor is usually limiting at dawn, and on a bright summer afternoon?
Reveal answer
Dawn: light intensity. Bright afternoon: carbon dioxide concentration (or temperature).
Key words: light · carbon dioxide
How do glasshouse growers increase photosynthesis?
Reveal answer
Add carbon dioxide (paraffin heaters), heat, and artificial lighting, when the extra yield is worth the cost.
Key words: carbon dioxide · heating · lighting
Why does adding CO₂ make no difference to a plant in dim light?
Reveal answer
Light intensity is the limiting factor.
Key words: light
Why do plants release CO₂ at night?
Reveal answer
There is no photosynthesis but respiration continues.
Key words: respiration
What is meant by the term limiting factor?
Reveal answer
A factor whose shortage currently restricts the rate of a process.
Key words: limiting factor
What is meant by the term light intensity?
Reveal answer
Light power reaching a unit area.
Key words: light intensity
What is meant by the term saturation?
Reveal answer
A region where further increase in one factor gives little or no rate increase.
Key words: saturation
What is meant by the term optimum?
Reveal answer
The condition producing the highest measured rate.
Key words: optimum
Expected graph patterns, Light intensity. Typical pattern? Reason for plateau or fall?
Reveal answer
Typical pattern: Rate rises then plateaus. Reason for plateau or fall: Another factor becomes limiting.
Key words: Light intensity
Expected graph patterns, Carbon dioxide concentration. Typical pattern? Reason for plateau or fall?
Reveal answer
Typical pattern: Rate rises then plateaus. Reason for plateau or fall: Another factor or pathway capacity becomes limiting.
Key words: Carbon dioxide concentration
Expected graph patterns, Temperature. Typical pattern? Reason for plateau or fall?
Reveal answer
Typical pattern: Rate rises to an optimum then falls. Reason for plateau or fall: Enzymes denature and other stress responses occur.
Key words: Temperature
Expected graph patterns, Two factors together. Typical pattern? Reason for plateau or fall?
Reveal answer
Typical pattern: One curve may plateau higher than another. Reason for plateau or fall: Relieving a second limitation permits a higher rate.
Key words: Two factors together
True or false, and why? “Water can never limit photosynthesis.”
Reveal answer
False. Water stress can restrict photosynthesis, although the named specification factors are light, carbon dioxide and temperature.
True or false, and why? “The plateau proves light is no longer needed.”
Reveal answer
False. Light remains necessary; another factor now limits the rate.
True or false, and why? “Higher temperature always increases photosynthesis.”
Reveal answer
False. Rate falls above the optimum as enzymes and cells are disrupted.
True or false, and why? “Only one factor can limit a plant forever.”
Reveal answer
False. The limiting factor changes as conditions change.