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Nature & Animals

Which outcome occurs when PEP carboxylase activity exceeds rubisco’s fixation rate in C4 plants' carbon fixation pathways?

A)Photorespiration rate is minimized
B)Stomata close to regulate water loss
C)ATP demand for pathway increases
D)Initial carbon fixation decreases

💡 Explanation

When PEP carboxylase activity exceeds rubisco’s rate, the C4 pathway efficiently captures CO2 into oxaloacetate, increasing the concentration around rubisco and preventing photorespiration, but the conversion processes increase ATP consumption. Therefore higher ATP usage results, rather than minimized photorespiration, stomatal closure which involve separate mechanisms related to water balance, or decreased carbon fixation.

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