Written by Merel Dagher (PEACE Advisor)
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The correct answer is d. Rubisco is responsible for carbon fixation in the Calvin Cycle during photosynthesis. However, rubisco can also bind oxygen instead of carbon dioxide, because it hasn’t evolved to distinguish between the two. Evolutionarily, this is explained because early on in life, oxygen wasn’t as prevalent in the environment, so rubisco didn’t need a mechanism to distinguish the two. When oxygen enters the Calvin Cycle, it consumes ATP without producing any products, which is the phenomenon of photorespiration.