Which gas is fixed during the light-independent reactions?

The light-independent reactions of photosynthesis, commonly referred to as the Calvin cycle, primarily involve the fixation of carbon dioxide. During these reactions, carbon dioxide is taken from the atmosphere and combined with a five-carbon sugar, ribulose bisphosphate (RuBP). This process is facilitated by the enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO), which catalyzes the reaction that incorporates carbon dioxide into organic molecules.

The fixation of carbon dioxide is crucial because it provides the organic carbon needed to produce glucose and other carbohydrates, which serve as energy sources for the plant. The products of the Calvin cycle can ultimately be used in various metabolic processes to support the plant's growth and energy needs.

In contrast, oxygen is a byproduct of the light-dependent reactions, hydrogen is not fixed during photosynthesis in a gaseous state, and nitrogen fixation occurs primarily in certain bacteria and not directly during the light-independent processes of photosynthesis. Thus, carbon dioxide is the correct answer as it is the gas that is actually fixed during the light-independent reactions.