Which statement is true about C3 vs C4 leaf anatomy and water-use efficiency?

• A. C4 reduces photorespiration and improves water-use efficiency.
• B. C3 uses Kranz anatomy with bundle-sheath cells.
• C. C4 uses mesophyll photosynthesis.
• D. There is no difference.

Answer: (A)

The main idea is that C4 leaf anatomy creates a CO2 concentration mechanism that lowers photorespiration and boosts water-use efficiency, especially under hot, bright, or dry conditions. In C4 plants, CO2 is first fixed in mesophyll cells by the enzyme PEP carboxylase into a four-carbon compound, which is then transported to bundle-sheath cells where CO2 is released for the Calvin cycle. This spatial separation, often described as Kranz anatomy, keeps CO2 levels high around the photosynthetic machinery and minimizes the oxygenation reaction of RuBisCO. With CO2 kept plentiful, the plant can tolerate less stomatal opening while still fixing substantial carbon, which translates into higher water-use efficiency. The other statements don’t fit: Kranz anatomy is characteristic of C4, not C3; C4 photosynthesis involves both mesophyll fixation and a bundle-sheath Calvin cycle (not just mesophyll photosynthesis); and there are real differences in physiology between C3 and C4, so there isn’t no difference.