MYP Biology sample question: Photosynthesis and limiting factors

A student investigates how light intensity affects the rate of photosynthesis. The rate rises as light intensity increases, then levels off.

1. (a)Write the word equation for photosynthesis.[2]
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   • • carbon dioxide + water → glucose + oxygen (1)
   • • in the presence of light and chlorophyll (1)
2. (b)Explain why the rate levels off at high light intensity.[2]
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   • • Another factor is now limiting the rate (1)
   • • for example carbon dioxide concentration or temperature (1)
3. (c)Explain why some growers add carbon dioxide to greenhouses.[1]
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   • • It increases the rate of photosynthesis, so plants grow faster and give higher yields (1)

On a graph, a plateau means a different factor is now limiting. Name the factor and say why it matters.