What supplies energy for carbohydrate synthesis in the dark stage?

• A. NADPH
• B. Water
• C. NADP+
• D. ATP

Answer: (D)

The energy for carbohydrate synthesis in the dark stage comes from ATP. In the Calvin cycle, ATP supplies the energy needed to push through the endergonic steps, such as phosphorylating the carbon intermediates and regenerating the CO2 acceptor RuBP, which keeps the cycle looping. NADPH provides the reducing power to convert those intermediates into the carbohydrate, but it doesn’t supply the energy for the reactions in the same way ATP does. Water and NADP+ are involved in the light reactions, not as the energy source for the dark stage.