What supplies energy to the dark stage of photosynthesis?

• A. Light energy
• B. Water energy
• C. ATP
• D. Heat energy

Answer: (C)

The energy for the dark stage comes from ATP produced during the light reactions. In the Calvin cycle, ATP provides the energy to drive the endergonic steps that fix carbon and rebuild the molecule that accepts CO2, fueling processes like converting intermediates to later stages and regenerating the CO2 acceptor. NADPH, while essential, supplies reducing power (electrons) for the reduction steps rather than energy in the form of ATP. Light energy isn’t used directly in the dark stage, and heat energy isn’t a driver of these biochemical reactions.