Why is it difficult to determine the optimal concentration of CO\(_2\) for photosynthesis when light intensity is very low?

• A. Because CO\(_2\) is not required for photosynthesis at low light intensities.
• B. Because low light intensity increases the rate of photorespiration instead of photosynthesis.
• C. Because low light intensity limits the rate of photosynthesis regardless of CO\(_2\) concentration.
• D. Because high CO\(_2\) levels inhibit chlorophyll production at low light.

Answer: (C)

Verified step by step guidance

1. Understand the relationship between light intensity and photosynthesis: Photosynthesis is a light-dependent process, meaning that light energy is required to drive the reactions that convert CO\(_2\) and water into glucose and oxygen. At very low light intensities, the energy available for photosynthesis is limited.
2. Recognize the role of CO\(_2\) in photosynthesis: CO\(_2\) is a substrate for the Calvin cycle, which occurs during the light-independent reactions of photosynthesis. However, the Calvin cycle relies on ATP and NADPH produced during the light-dependent reactions, which are constrained by low light intensity.
3. Analyze why CO\(_2\) concentration has limited impact at low light: Even if CO\(_2\) levels are increased, the rate of photosynthesis cannot significantly improve because the light-dependent reactions are the bottleneck. Without sufficient light, ATP and NADPH production is insufficient to support the Calvin cycle.
4. Clarify why other options are incorrect: CO\(_2\) is always required for photosynthesis, even at low light intensities. Low light intensity does not increase photorespiration; instead, it limits photosynthesis overall. High CO\(_2\) levels do not inhibit chlorophyll production; chlorophyll production is regulated by other factors.
5. Conclude that the correct answer is based on the limiting factor principle: At low light intensities, light availability is the primary limiting factor for photosynthesis, regardless of CO\(_2\) concentration.