USA Biology Olympiad (USABO) Practice Exam

What is true about the dark reaction (Calvin cycle) of photosynthesis?

1. It requires light

2. It uses carbon dioxide

3. It occurs in the thylakoids

4. It produces glucose directly

The correct answer is: It uses carbon dioxide

The dark reaction, also known as the Calvin cycle, is a crucial part of photosynthesis that takes place in the stroma of chloroplasts. One of its primary functions is to utilize carbon dioxide from the atmosphere to synthesize organic molecules, specifically glucose, through a series of enzymatic reactions. Carbon dioxide is fixed into a five-carbon sugar molecule known as ribulose bisphosphate (RuBP), leading to the production of three-carbon molecules that can eventually be converted into glucose and other carbohydrates. This process does not require light directly to occur, which distinguishes it from the light-dependent reactions that occur in the thylakoid membranes, where sunlight is captured and converted into chemical energy. In contrast to the Calvin cycle, the light reactions of photosynthesis are critical for capturing light energy and producing ATP and NADPH, which are then used in the Calvin cycle. The cycle can proceed both during the light and in the dark as long as there is sufficient ATP and NADPH produced from the light-dependent reactions. Overall, the true statement about the dark reaction is its reliance on carbon dioxide, emphasizing the cycle's role in carbon fixation as an essential process in converting inorganic carbon into organic forms suitable for plant metabolism.