What must happen for the Calvin cycle to proceed?

The Calvin cycle, which occurs in the stroma of chloroplasts, is fundamental for synthesizing glucose from carbon dioxide (CO2). For the Calvin cycle to proceed, the initial step is the fixation of carbon dioxide, which relies on the availability of substrates that are produced during the light-dependent reactions of photosynthesis. During these reactions, water is split, releasing oxygen and generating ATP and NADPH. These products are essential for providing the energy and reducing power required for the Calvin cycle to convert CO2 into glucose.

Water splitting plays a critical role in ensuring a continuous supply of electrons, which not only contribute to the formation of ATP and NADPH but also maintain the overall efficiency and sustainability of the photosynthetic process. The generation of ATP and NADPH during the light-dependent reactions significantly drives the Calvin cycle forward, allowing it to synthesize carbohydrates effectively.

Thus, the necessity of water splitting aligns with the biochemical needs of the Calvin cycle, making it the correct choice in explaining what is essential for the cycle to proceed. Other options either do not contribute directly to the process or are unrelated to the fundamental requirements of carbon fixation and sugar production.