During photosynthesis, the energy from sunlight is used to split water molecules. What happens to the hydrogen ions that are split from the water molecules?

Splitting water releases hydrogen ions into the thylakoid space, where they build up and create a concentration gradient. As the ions flow out through ATP synthase, their movement provides energy to add a phosphate group to ADP and form ATP. Therefore, the correct choice is “They flow out of the thylakoid, providing energy to add a phosphate group to ADP.”

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