An energy difference across a membrane
Chemiosmosis couples ion movement to chemical work. A proton electrochemical gradient has two components: a difference in proton concentration and a difference in electrical potential. A membrane that restricts uncontrolled proton passage keeps that stored energy from immediately dissipating.
ATP synthase provides a controlled route for protons to move down the gradient. Their movement drives rotation and conformational changes that support ATP formation from ADP and inorganic phosphate. Merely having ATP synthase is insufficient; the gradient and usable substrates must also be present.
Example: a mitochondrion with a proton leak
In a respiring mitochondrion, electron transport pumps protons from the matrix toward the intermembrane space. Proton return through ATP synthase can support ATP production.
Now imagine an additional route that lets protons return without passing through ATP synthase. This leak weakens coupling: electron transport may continue while less of its energy is captured as ATP. Energy is instead dissipated, including as heat.
Apply the idea
When evaluating a membrane-energy system, identify the proton pump, the two compartments, and the route of proton return. Distinguish stopping electron transport from allowing protons to bypass ATP synthase; they need not have the same effect on respiration.
Related question
Apply this knowledge
Use the concept guide to understand the reasoning, then return to the complete question and worked answer.
Where Do Light-Dependent Reactions Occur?Sources
These references support the core concepts and interpretation boundaries explained above.