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

• A. Because low light intensity limits the rate of photosynthesis, making changes in CO\(_2\) concentration have little effect.
• B. Because low light intensity increases the rate of CO\(_2\) uptake by plants.
• C. Because low light intensity causes plants to stop using CO\(_2\) entirely.
• D. Because CO\(_2\) becomes toxic to plants at low light intensities.

Answer: (A)

Verified step by step guidance

1. Understand the relationship between light intensity and photosynthesis: Photosynthesis is a process that depends on both light intensity and the availability of carbon dioxide (CO\(_2\)). Light provides the energy required for the photosynthetic reactions, while CO\(_2\) is a raw material used in the Calvin cycle to produce glucose.
2. Recognize the limiting factor concept: When one factor (e.g., light intensity) is in short supply, it becomes the limiting factor for the rate of photosynthesis. This means that even if other factors, such as CO\(_2\) concentration, are increased, the overall rate of photosynthesis will not significantly change because the process is constrained by the limiting factor.
3. Analyze the effect of low light intensity: At very low light intensities, the energy available for photosynthesis is minimal. This limits the light-dependent reactions, which produce ATP and NADPH needed for the Calvin cycle. As a result, the rate of photosynthesis is already very low, regardless of CO\(_2\) concentration.
4. Consider the role of CO\(_2\) under low light conditions: Since the Calvin cycle requires ATP and NADPH from the light-dependent reactions, increasing CO\(_2\) concentration will have little to no effect on the rate of photosynthesis when light intensity is very low. The process is already limited by the lack of energy from light.
5. Conclude why determining the optimal CO\(_2\) concentration is difficult: Because the rate of photosynthesis is primarily limited by light intensity under these conditions, changes in CO\(_2\) concentration do not significantly impact the process. This makes it challenging to determine the optimal CO\(_2\) concentration for photosynthesis when light intensity is very low.