USA Biology Olympiad (USABO) Practice Exam

What is the likely result if a chemical inhibits proton diffusion out of the thylakoid through ATP synthase?

1. The proton motive force decreases

2. The light reaction rate increases

3. The proton motive force increases

4. No effect on ATP production

The correct answer is: The proton motive force increases

Inhibiting proton diffusion out of the thylakoid through ATP synthase would prevent protons from moving back into the stroma, which is crucial for ATP synthesis during the light reactions of photosynthesis. Normally, as protons flow down their concentration gradient via ATP synthase, this process drives the conversion of ADP and inorganic phosphate into ATP, fueled by the proton motive force. When the diffusion of protons is inhibited, the buildup of protons inside the thylakoid lumen would increase, leading to a greater concentration gradient across the thylakoid membrane. The increased concentration of protons would enhance the proton motive force, as it is dependent on both the difference in proton concentration and electrical charge across the membrane. Although ATP synthase would be inhibited and unable to utilize this gradient effectively for ATP production, the actual force driving protons across the membrane would be heightened due to the increased proton concentration. This scenario highlights that while ATP production may be impaired, the fundamental components and driving forces behind proton transport are affected in a way that increases the proton motive force. Understanding the relationship between proton concentration, the functionality of ATP synthase, and the implications on the proton motive force is essential in grasping how inhibiting proton diffusion impacts photos