Final answer:
The light-dependent reactions of photosynthesis take place during the day as they require sunlight to proceed. These reactions generate essential molecules like ATP and NADPH, which are then used in the Calvin cycle, occurring independent of light.
Step-by-step explanation:
The light-dependent reactions of photosynthesis occur exclusively during the day. This is because these reactions depend directly on sunlight to drive the process. During the light-dependent reactions, chlorophyll in the thylakoid membranes of chloroplasts absorbs sunlight, which is then converted into chemical energy stored in ATP and NADPH. These molecules are critical for the subsequent light-independent reactions, also known as the Calvin cycle, which can function during both day and night as they don't require light but depend on the products of the light-dependent reactions.
Environmental factors like day length and temperature may influence the efficiency of these processes, but the fundamental requirement for the light-dependent reactions remains sunlight. The so-called "dark reactions" (Calvin cycle or light-independent reactions) are often misconstrued to occur at night, which is incorrect since they take place in the stroma of chloroplasts and can occur any time the products of the light-dependent reactions are available.