What is the main source of carbon dioxide used by plants in the dark stage of photosynthesis?

What is the main source of carbon dioxide used by plants in the dark stage of photosynthesis?

• A. Air
• B. Water vapor
• C. Soil carbon dioxide
• D. Respiration

Answer: (A)

Carbon dioxide from the air is what plants use in the dark stage of photosynthesis. During the Calvin cycle, CO2 enters through the stomata in the leaves and diffuses into the chloroplasts, where the enzyme Rubisco fixes it to RuBP to form 3-phosphoglycerate. This reaction uses the ATP and NADPH produced by the light reactions, so once those energy carriers are available, the cycle can proceed even without light. Water vapor isn’t a source of carbon, soil CO2 around the roots isn’t typically used directly by the photosynthetic machinery, and the CO2 produced by respiration is released, not fixed. So the air is the main source of carbon dioxide for the dark stage.

Carbon dioxide from the air is what plants use in the dark stage of photosynthesis. During the Calvin cycle, CO2 enters through the stomata in the leaves and diffuses into the chloroplasts, where the enzyme Rubisco fixes it to RuBP to form 3-phosphoglycerate. This reaction uses the ATP and NADPH produced by the light reactions, so once those energy carriers are available, the cycle can proceed even without light. Water vapor isn’t a source of carbon, soil CO2 around the roots isn’t typically used directly by the photosynthetic machinery, and the CO2 produced by respiration is released, not fixed. So the air is the main source of carbon dioxide for the dark stage.