Which of the following molecules directly enters the Calvin Cycle?

The correct answer is carbon dioxide because it plays a crucial role in the Calvin Cycle, which is the phase of photosynthesis where glucose is synthesized. During the Calvin Cycle, carbon dioxide is fixed into a stable intermediate by the enzyme RuBisCO. This process occurs in the chloroplasts of plant cells, where carbon dioxide combines with ribulose bisphosphate (RuBP) to eventually produce glucose and other carbohydrates.

While ATP and NADPH are essential for providing the energy and reducing power needed for the reactions in the Calvin Cycle, they do not directly enter it as substrates. Instead, they are generated in the light-dependent reactions of photosynthesis and used to drive the reactions within the Calvin Cycle. Glucose, on the other hand, is a product of the Calvin Cycle rather than a starting material. Therefore, carbon dioxide is the key molecule that directly enters the Calvin Cycle to initiate the process of carbohydrate synthesis.