Name a gas essential for the dark stage of photosynthesis.

Name a gas essential for the dark stage of photosynthesis.

• A. Oxygen
• B. Nitrogen
• C. Methane
• D. Carbon dioxide

Answer: (D)

The dark stage relies on carbon dioxide as the carbon source to build sugars. In the Calvin cycle, CO2 is fixed onto ribulose-1,5-bisphosphate by the enzyme RuBisCO, creating a six-carbon intermediate that splits into two molecules of 3-phosphoglycerate. Using ATP and NADPH from the light reactions, these are converted into sugar phosphates and ultimately into glucose, starch, or sucrose. Without CO2, the cycle cannot synthesize carbohydrate. Oxygen is produced during the light reactions and isn’t used in the dark stage, while nitrogen and methane aren’t substrates for carbohydrate synthesis.

The dark stage relies on carbon dioxide as the carbon source to build sugars. In the Calvin cycle, CO2 is fixed onto ribulose-1,5-bisphosphate by the enzyme RuBisCO, creating a six-carbon intermediate that splits into two molecules of 3-phosphoglycerate. Using ATP and NADPH from the light reactions, these are converted into sugar phosphates and ultimately into glucose, starch, or sucrose. Without CO2, the cycle cannot synthesize carbohydrate. Oxygen is produced during the light reactions and isn’t used in the dark stage, while nitrogen and methane aren’t substrates for carbohydrate synthesis.