Which statement best contrasts C3 and C4 photosynthesis regarding CO2 concentration and typical habitats?

Which statement best contrasts C3 and C4 photosynthesis regarding CO2 concentration and typical habitats?

• A. C3 uses NADPH as electron donor; C4 uses ferredoxin.
• B. C3 stores CO2 in vacuoles at night; C4 fixes CO2 only during the day.
• C. C3 fixes CO2 directly in mesophyll, whereas C4 concentrates CO2 in bundle-sheath cells via PEP carboxylase; C3 is common in cool climates.
• D. C3 occurs in desert shrubs; C4 in aquatic plants.

Answer: (C)

The key idea here is how C3 and C4 plants manage CO2 and why that matters in different environments. In C3 photosynthesis, CO2 is fixed directly in the mesophyll cells by RuBisCO as part of the Calvin cycle. In C4 photosynthesis, CO2 is first fixed in the mesophyll by PEP carboxylase to form a four-carbon compound, which is then shuttled to bundle-sheath cells where CO2 is released for fixation by RuBisCO. This separation creates a higher local CO2 concentration around RuBisCO in the bundle-sheath cells, which minimizes oxygen inhibition and reduces photorespiration, a problem that becomes significant in hot, sunny, dry conditions when stomata close to conserve water. Because of this, C3 plants—fixing CO2 directly in the mesophyll—tend to dominate in cool, moist climates where the risk of photorespiration is lower. C4 plants, with their CO2 concentrating mechanism in bundle-sheath cells, are better suited to hot, arid, or high-light environments, where minimizing photorespiration gives a real advantage. So the statement that best contrasts them notes the direct mesophyll CO2 fixation in C3 versus the CO2 concentrating mechanism in C4, and it also aligns with C3’s greater prevalence in cool climates.

The key idea here is how C3 and C4 plants manage CO2 and why that matters in different environments. In C3 photosynthesis, CO2 is fixed directly in the mesophyll cells by RuBisCO as part of the Calvin cycle. In C4 photosynthesis, CO2 is first fixed in the mesophyll by PEP carboxylase to form a four-carbon compound, which is then shuttled to bundle-sheath cells where CO2 is released for fixation by RuBisCO. This separation creates a higher local CO2 concentration around RuBisCO in the bundle-sheath cells, which minimizes oxygen inhibition and reduces photorespiration, a problem that becomes significant in hot, sunny, dry conditions when stomata close to conserve water.

Because of this, C3 plants—fixing CO2 directly in the mesophyll—tend to dominate in cool, moist climates where the risk of photorespiration is lower. C4 plants, with their CO2 concentrating mechanism in bundle-sheath cells, are better suited to hot, arid, or high-light environments, where minimizing photorespiration gives a real advantage.

So the statement that best contrasts them notes the direct mesophyll CO2 fixation in C3 versus the CO2 concentrating mechanism in C4, and it also aligns with C3’s greater prevalence in cool climates.