Describe the difference between C3 and C4 photosynthetic pathways and why C4 crops often outperform C3 crops under high temperature and low CO2 conditions.

• A. C3 plants fix CO2 directly via Rubisco; C4 concentrates CO2 in bundle-sheath cells, reducing photorespiration. C4 thus offers higher water and nitrogen use efficiency and better performance in hot, dry environments.
• B. C4 plants perform worse at high temperatures than C3.
• C. C3 plants concentrate CO2 and perform better in hot environments.
• D. C4 plants are identical to C3 with no advantage.

Answer: (A)

The main idea is how carbon fixation is organized in the two pathways and how that affects efficiency when it's hot and CO2 is scarce. In C3 photosynthesis, CO2 is fixed directly by Rubisco in the mesophyll cells to form a three-carbon compound. But Rubisco also reacts with O2, especially when CO2 is low or temperatures are high, leading to photorespiration, which wastes energy and carbon. Under hot, dry conditions, plants tend to close stomata to conserve water, which lowers internal CO2 and makes photorespiration more pronounced in C3 plants.

In C4 photosynthesis, the first CO2 fixation happens in the mesophyll not with Rubisco but with PEP carboxylase, which grabs CO2 and forms a four-carbon compound. This compound is then moved to bundle-sheath cells, where CO2 is released close to Rubisco for the Calvin cycle. This concentrates CO2 around Rubisco, so hydrogen ions are less likely to react with O2, dramatically reducing photorespiration. The result is higher photosynthetic efficiency at high temperatures and under low ambient CO2.

Because CO2 can be concentrated, C4 plants often keep their stomata more closed or less opened, which lowers water loss and improves water-use efficiency. They also tend to use nitrogen more efficiently since less Rubisco is needed to achieve the same carbon gain. All of this helps explain why C4 crops outperform C3 crops under high temperature and low CO2 conditions.