Final answer:
The Calvin cycle can occur day or night because it uses ATP and NADPH from the light-dependent reactions, not sunlight directly, to fix carbon into carbohydrates.
Step-by-step explanation:
The carbon-fixing reactions, also known as the Calvin cycle or light-independent reactions, can go on day or night in plants because they do not require sunlight to proceed. Instead, these reactions use the ATP and NADPH produced in the light-dependent reactions that occur during the day.
The enzyme RuBisCO catalyzes the fixation of CO₂ into organic molecules, which can happen any time as long as ATP and NADPH are available. This cycle includes three stages: fixation, reduction, and regeneration, and allows the plant to fix carbon into carbohydrates regardless of the time of day.