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Heat stress decreases the light-dependent reaction of photosynthesis - AQA - A-Level Biology - Question 1 - 2018 - Paper 2
Question 1
Heat stress decreases the light-dependent reaction of photosynthesis. Explain why this leads to a decrease in the light-independent reaction. Another effect of hea... show full transcript
Worked Solution & Example Answer:Heat stress decreases the light-dependent reaction of photosynthesis - AQA - A-Level Biology - Question 1 - 2018 - Paper 2
Step 1
Explain why this leads to a decrease in the light-independent reaction.
Answer
Heat stress affects the light-dependent reactions, which in turn reduces the production of ATP and NADPH. These compounds are crucial for the light-independent reactions, also known as the Calvin cycle. Without adequate ATP and NADPH, the Calvin cycle cannot proceed efficiently, thereby leading to a decrease in the overall rate of photosynthesis.
Step 2
Explain why.
Answer
Rubisco is the key enzyme that catalyzes the fixation of carbon dioxide in the Calvin cycle. If the activity of rubisco decreases, the rate of carbon dioxide assimilation slows down. This results in reduced production of glucose and other carbohydrates, which are essential for plant energy and growth. Thus, a reduced rubisco activity directly limits the rate of photosynthesis.
Step 3
Step 4
Use all the information to evaluate their conclusion.
Answer
The scientists' conclusion is supported by the observed relationship between temperature and enzyme activity shown in Figures 1 and 2. As temperature increases, rubisco activity rises to an optimum level before declining, indicating potential denaturation at high temperatures. This aligns with the claim that heat stress reduces rubisco activity. Additionally, rubisco activase also shows temperature sensitivity, as its effectiveness is crucial for maintaining rubisco function. Thus, the evaluation is valid as both the enzyme and its activator are affected by high temperatures, ultimately limiting photosynthesis under heat stress conditions.