[Campbell Biology P.201] According to Figure 8.10, glutamine synthesis from glutamic acid is... | Practice Question

According to Figure 8.10, glutamine synthesis from glutamic acid is an endergonic process that is not spontaneous. However, when coupled with ATP hydrolysis, the overall reaction becomes spontaneous. Which statement best explains this phenomenon?

  • A: ATP hydrolysis alone provides sufficient energy for the reaction.
  • B: The positive ΔG of glutamine synthesis is overcome by the even more negative ΔG of ATP hydrolysis, resulting in a net negative ΔG.
  • C: The phosphorylated intermediate increases the activation energy of the reaction.
  • D: Coupling reactions always makes them spontaneous, regardless of ΔG values.

Explanation

The overall reaction is spontaneous because the large negative free-energy change from ATP hydrolysis ΔG = -7.3 kcal/mol outweighs the positive free-energy change required for glutamine synthesis ΔG = +3.4 kcal/mol , resulting in a net negative ΔG -3.9 kcal/mol for the coupled reaction.