Give a stream of consciousness and then the final answer. In biological processes, the direction an ion moves by diffusion or active transport across a membrane is determined by the electrochemical gradient. In mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force. This potential energy is used for the synthesis of ATP by oxidative phosphorylation or photophosphorylation, respectively.

Answer this question: what uses the energy found within a proton gradient to drive the synthesis of atp?
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The important part for answering this question is: This potential energy is used for the synthesis of ATP by oxidative phosphorylation or photophosphorylation, respectively.
So, the final answer is oxidative phosphorylation.
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