Photosynthesis: Question 8

Worked solution

Step 1: Recall how temperature affects enzyme-catalysed reactions

Many steps of photosynthesis (particularly the reactions of the Calvin cycle, catalysed by enzymes such as rubisco) are enzyme-controlled. As with any enzyme-catalysed reaction:

• Below the optimum temperature, raising the temperature increases the kinetic energy of both enzyme and substrate molecules, so they move faster and collide more often and more energetically. This increases the frequency of successful collisions in which the substrate binds to the enzyme’s active site, so the rate of reaction (here, carbon fixation by rubisco) increases.
• Above the optimum temperature, the additional kinetic energy causes the atoms within the enzyme molecule to vibrate too much, breaking the hydrogen bonds and ionic bonds that hold the enzyme’s tertiary structure in shape. This distorts the shape of the active site (denaturation), so substrate molecules can no longer bind as effectively, and the rate of reaction falls sharply.

Since the question states that light intensity and carbon dioxide concentration are both kept non-limiting, the enzyme-controlled Calvin cycle reactions are what actually set the pace of the overall process as temperature changes, giving the classic rise-then-fall pattern with a peak at the optimum temperature.

Step 2: Evaluate each option

• A: correct, this describes the standard explanation for a temperature optimum in an enzyme-catalysed process: increasing kinetic energy raises the rate up to the optimum, then denaturation of enzymes such as rubisco (via breakage of the bonds maintaining tertiary structure) sharply reduces the rate above it.
• B: incorrect. Light absorption by chlorophyll is a photochemical process driven by light energy, not by temperature; temperature does not directly increase how much light is absorbed, and chlorophyll is a pigment, not an enzyme, so “bleaching” is not the mechanism responsible for the fall in rate described here.
• C: incorrect. The rate of photolysis is driven by light energy at photosystem II, not primarily by temperature, and reduced NADP does not become “unusable”; the fall in rate at high temperature is due to denaturation of enzymes, not oversupply of reduced NADP.
• D: incorrect. Temperature does have a direct effect on the rate of the enzyme-catalysed photosynthetic reactions themselves (as described in option A); while respiration rate does also increase with temperature, this is a separate process and does not account for the described pattern in the rate of photosynthesis itself.

Final answer

• The characteristic rise-then-fall pattern in the rate of photosynthesis with temperature, even when light and carbon dioxide are non-limiting, is best explained by the effect of temperature on the enzyme-catalysed reactions of the Calvin cycle: increasing kinetic energy raises the rate up to an optimum, then denaturation of enzymes such as rubisco sharply reduces it, option A.