Under abundant light and constant temperature, photosynthesis rate is limited by the availability of which gas?

Under abundant light and constant temperature, photosynthesis rate is limited by the availability of which gas?

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

Answer: (C)

When light is abundant and temperature is steady, the speed of photosynthesis is determined by how much carbon dioxide is available for fixation in the Calvin cycle. The plant’s enzyme Rubisco fixes CO2 to start assembling sugars, so if CO2 is scarce, the cycle slows and overall photosynthesis drops even though energy from light reactions is readily available. Once CO2 is plentiful, the process can run at its maximum capacity limited mainly by the enzymatic machinery and the supply of ATP and NADPH from the light reactions. Oxygen isn’t a substrate in the Calvin cycle; it’s a product of the process, so its level doesn’t limit the rate under these conditions. Water is essential, but with ample water it doesn’t constrain the rate. Nitrogen supports building chlorophyll and enzymes, but the immediate rate in this scenario is controlled by CO2 availability rather than nitrogen.

When light is abundant and temperature is steady, the speed of photosynthesis is determined by how much carbon dioxide is available for fixation in the Calvin cycle. The plant’s enzyme Rubisco fixes CO2 to start assembling sugars, so if CO2 is scarce, the cycle slows and overall photosynthesis drops even though energy from light reactions is readily available. Once CO2 is plentiful, the process can run at its maximum capacity limited mainly by the enzymatic machinery and the supply of ATP and NADPH from the light reactions.

Oxygen isn’t a substrate in the Calvin cycle; it’s a product of the process, so its level doesn’t limit the rate under these conditions. Water is essential, but with ample water it doesn’t constrain the rate. Nitrogen supports building chlorophyll and enzymes, but the immediate rate in this scenario is controlled by CO2 availability rather than nitrogen.