Photosynthesis and Plant Nutrition: Question 8

Worked solution

Step 1: Rule out light intensity and carbon dioxide

The stem states clearly that light intensity and carbon dioxide concentration are both kept high and constant throughout the whole investigation. This means neither of them can be responsible for the fall in rate above 35°C, so options A and C must be wrong, they contradict information already given in the question.

Step 2: Identify what is changing

The only variable being deliberately changed is temperature. As temperature rises from 10°C to 35°C, the rate of photosynthesis climbs steadily (4 → 9 → 15 → 18), showing that a rising temperature is helping the rate up to this point. But between 35°C and 45°C, the rate falls sharply (18 → 10 → 2), even though temperature keeps rising.

Step 3: Explain the fall using enzymes

Photosynthesis depends on enzyme-controlled reactions. Enzymes work fastest at an optimum temperature; above this optimum, the heat starts to disrupt the enzymes’ structure, denaturing them so they can no longer catalyse the reactions of photosynthesis as effectively. As more and more enzyme molecules are denatured with rising temperature, the rate of photosynthesis falls further, which matches the sharp drop seen between 35°C and 45°C. This makes option B correct.

Option D wrongly attributes the fall to a permanent, complete closure of the stomata; this is not the accepted explanation, and a rate of 2 units at 45°C shows the reaction has slowed greatly, not stopped completely.

Final answer

The rate falls because the enzymes controlling photosynthesis become increasingly denatured above the plant’s optimum temperature, option B.