The H+ that's vital in making ATP synthase work comes from the electron transport chain.
The electron transport chain can be found in the mitochondria. It is a series of protein complexes that are able to transfer electrons from the electron donors to the electron acceptors.
It should be noted that the transfer of the electrons from the electron donors to the electron acceptors is done through the redox reactions. This is illustrated in the H+ that made the ATP synthase work.
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