What happens to the hydrogen ions when water is split in photosynthesis?

When water is split during the process of photosynthesis, specifically in the light-dependent reactions occurring in the thylakoid membranes, hydrogen ions are released into the thylakoid lumen. This process leads to an accumulation of hydrogen ions (protons) in that space, creating a concentration gradient across the thylakoid membrane.

The concentration gradient plays a crucial role in driving the synthesis of ATP through a process called chemiosmosis. As hydrogen ions flow back into the stroma (the fluid-filled space surrounding the thylakoids) through ATP synthase, they provide the energy necessary to convert ADP and inorganic phosphate into ATP. Thus, the concentration gradient that builds up due to the splitting of water is essential for the energy production in the light reactions of photosynthesis.

While the other options touch on aspects related to photosynthesis, they do not accurately depict what happens to hydrogen ions when water is split. For example, hydrogen ions are not released into the stroma directly, nor do they immediately transform into glucose or directly join with carbon dioxide at that moment of water splitting. These processes occur later in the Calvin cycle or in subsequent metabolic pathways.