Apentix/AP Biology/Unit 3: Cellular Energetics/Question
AP Biologyhardmcq1 pt

Glutamine synthetase drives glutamate plus ammonia toward glutamine, a synthetic step that is unfavorable on its own, by coupling it to ATP hydrolysis. Which reasoning explains why the coupled sequence proceeds?

A.The enzyme lowers activation barriers enough to reverse the sign of the overall free energy change
B.Spending ATP makes glutamine permanently unstable so it must form regardless of conditions
C.Rapid ATP consumption forces product accumulation irrespective of thermodynamic direction
D.Coupling adds the large negative free energy of hydrolysis onto the positive cost of the synthetic step, yielding a net negative sum the enzyme channels along one pathway

Explanation

Core Concept

Free energy changes add when reactions share intermediates. Hydrolyzing ATP releases roughly 30 kJ per mole, far exceeding the modest positive price of joining glutamate and ammonia. By first transferring the phosphate onto glutamate itself, the enzyme merges both transformations into one sequence whose combined free energy change is clearly negative. The overall reaction therefore proceeds spontaneously, with catalysis merely lowering barriers along a route that is now favorable.

Correct Answer

DCoupling adds the large negative free energy of hydrolysis onto the positive cost of the synthetic step, yielding a net negative sum the enzyme channels along one pathway

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