Biology · Plant Physiology - Photosynthesis
A researcher compares net CO2 exchange in three plants — a C3 grass, a C4 maize, and a CAM succulent — under identical conditions: high temperature (38°C), low humidity, and CO2 at 400 ppm. Which of the following correctly predicts the relative efficiency and the key enzymatic reason?
- 1C3 > CAM > C4 — C3 plants perform better under high light and high temperature conditions due to direct fixation
- 2C4 > CAM > C3 — C4 suppresses photorespiration via spatial CO2 concentration using PEP carboxylase, while CAM is limited by slow nocturnal CO2 storage; both outperform C3 whose RuBisCO oxygenase activity is exacerbated by heat✓
- 3CAM > C4 > C3 — PEP carboxylase in CAM fixes 10× more CO2 per unit time than RuBisCO because of its higher Vmax
- 4C3 > C4 > CAM — RuBisCO efficiency decreases from C3 to CAM due to increasing structural complexity of photosynthetic cells
Correct answer
C4 > CAM > C3 — C4 suppresses photorespiration via spatial CO2 concentration using PEP carboxylase, while CAM is limited by slow nocturnal CO2 storage; both outperform C3 whose RuBisCO oxygenase activity is exacerbated by heat
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