Which process directly fixes carbon dioxide into organic molecules?

The process that directly fixes carbon dioxide into organic molecules is the Calvin Cycle. This cycle occurs in the stroma of chloroplasts in plants and is a crucial part of photosynthesis. During the Calvin Cycle, carbon dioxide is incorporated into five-carbon ribulose bisphosphate (RuBP) through the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO). This reaction produces a six-carbon intermediate that quickly splits into two three-carbon molecules called 3-phosphoglycerate (3-PGA).

The Calvin Cycle involves a series of enzymatic reactions that ultimately convert these three-carbon molecules into glucose and other carbohydrates, which are vital for the plant's energy storage and structural components. This process does not require light directly, as it uses the ATP and NADPH generated during the light-dependent reactions of photosynthesis.

In contrast, the light reactions of photosynthesis capture solar energy to produce ATP and NADPH but do not fix carbon dioxide themselves. Cellular respiration and fermentation are processes that occur in the breakdown of glucose to release energy, but they are not involved in the direct fixation of carbon dioxide into organic molecules.