Where does hydrogen for ATP Synthase come from?

Hydrogen ions (protons) for ATP synthase primarily come from the electron transport chain during cellular respiration. As electrons are transferred through the chain, energy is released, which pumps protons from the mitochondrial matrix into the intermembrane space, creating a proton gradient. This gradient is then utilized by ATP synthase to convert ADP and inorganic phosphate into ATP as protons flow back into the matrix. Additionally, in photosynthesis, protons come from the splitting of water molecules in the light-dependent reactions.