What atmospheric substance is essential for activating the Calvin cycle?

The Calvin cycle, also known as the light-independent reactions of photosynthesis, primarily relies on carbon dioxide (CO2) to produce glucose. During this process, CO2 is fixed into an organic molecule through a series of enzymatic reactions facilitated by the enzyme RuBisCO. This is the first major step in the cycle, where carbon fixation occurs, allowing the plant to convert inorganic carbon into organic forms. Without the presence of CO2, the cycle cannot initiate, and thus glucose production would be compromised.

In contrast, while oxygen is a byproduct of the light-dependent reactions and nitrogen plays a role in plant metabolism and growth, they do not directly activate the Calvin cycle. Hydrogen, although involved in the overall photosynthetic process, is not a factor in the activation of the Calvin cycle itself. Therefore, carbon dioxide is the critical atmospheric substance for initiating and sustaining the reactions of the Calvin cycle.