General Science • BiologyHard
What is the primary evolutionary and physiological significance of C4 photosynthetic pathway adaptations found in tropical grasses and maize compared to standard C3 plants?
A. Spatial separation of carbon fixation and the Calvin cycle minimizes photorespiration under high temperatures and drought✓ Correct Answer
B. Complete elimination of dark reactions allows continuous carbohydrate production throughout day and night cycles
C. Direct fixation of nitrogen gas from the atmosphere replaces the need for soil nitrate absorption by roots
D. Replacement of chlorophyll pigments with carotenoids increases absorption efficiency across ultraviolet spectrum
💡 Solution & Explanation
C4 plants utilize PEP carboxylase to initially fix carbon dioxide into a four-carbon compound in mesophyll cells, which is then transported to bundle-sheath cells where the Calvin cycle operates at high carbon dioxide concentrations, effectively suppressing wasteful photorespiration. Dark reactions cannot proceed indefinitely without light-generated ATP and NADPH. Nitrogen fixation remains restricted to specialized rhizobia symbioses rather than C4 pathways. Chlorophyll remains the primary photosynthetic pigment in both C3 and C4 plants.
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