How would doubling the atmospheric CO\(_2\) concentration most likely affect C\(_3\) plants?

• A. It would increase the rate of photosynthesis due to reduced photorespiration.
• B. It would cause C\(_3\) plants to switch to the C\(_4\) pathway.
• C. It would decrease the rate of photosynthesis due to increased photorespiration.
• D. It would have no effect on the rate of photosynthesis.

Answer: (A)

Verified step by step guidance

1. Understand the difference between C\(_3\) and C\(_4\) plants: C\(_3\) plants use the Calvin cycle for carbon fixation, where the enzyme RuBisCO fixes CO\(_2\) into a 3-carbon compound. C\(_4\) plants, on the other hand, have an additional mechanism to concentrate CO\(_2\) around RuBisCO, reducing photorespiration.
2. Recognize the role of CO\(_2\) concentration in photosynthesis: In C\(_3\) plants, RuBisCO can bind to either CO\(_2\) or O\(_2\). When CO\(_2\) levels are low, RuBisCO is more likely to bind to O\(_2\), leading to photorespiration, which wastes energy and reduces photosynthetic efficiency.
3. Analyze the effect of doubling atmospheric CO\(_2\): Increasing CO\(_2\) concentration reduces the likelihood of RuBisCO binding to O\(_2\), thereby decreasing photorespiration and increasing the efficiency of the Calvin cycle in C\(_3\) plants.
4. Evaluate the options: Doubling CO\(_2\) would not cause C\(_3\) plants to switch to the C\(_4\) pathway, as this is a genetically determined trait. It would also not decrease photosynthesis or have no effect, as reduced photorespiration directly enhances photosynthetic rates in C\(_3\) plants.
5. Conclude that the most likely effect of doubling atmospheric CO\(_2\) on C\(_3\) plants is an increase in the rate of photosynthesis due to reduced photorespiration.