In the light-dependent reactions of photosynthesis, H+ ions pass to the thylakoid space. Where do these protons come from?

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    $\begingroup$ Photolysis of water. It is the first step of the light reactions. BTW, H⁺ is ubiquitous. $\endgroup$ – WYSIWYG Jun 29 '15 at 13:49
  • $\begingroup$ And the electrons go to PhotoSystem II? $\endgroup$ – Benjli Jun 29 '15 at 14:04
  • $\begingroup$ Is this a homework question? It really looks much like it. Additionally: Please have a look at the reaction chart on the Wikipedia page on photosynthesis which explains both questions. $\endgroup$ – Chris Jun 29 '15 at 14:43
  • $\begingroup$ No, I have a biology course in Univeristy, and I just want to be sure I know it before the exam. $\endgroup$ – Benjli Jun 29 '15 at 14:44
  • $\begingroup$ @Benjli Read our homework policy. If you have not put an effort in answering a basic question then we consider it as homework. Can you add some points on how you analysed this problem? $\endgroup$ – WYSIWYG Jun 29 '15 at 14:53

The photolysis of water is coupled to the reduction of plastaquinone (Q) in photosystem II (PSII) as summarized in this diagram, adapted from Berg et al.:

Establishment of hydrogen ion gradient in photosynthesis

The overall reaction (which balances) is:

2 H2O + 2 Q + 4 H+ = O2 + 2 QH2 + 4 H+

But the 4 H+ on both sides of the equation are not the same. The generation of the hydrogen ions in the thylakoid lumen is coupled to the uptake of hydrogen ions from the stroma as the plastoquinone is reduced. This is made possible because of the membrane disposition of the components of photosystem II. Hence an electochemical gradient is established that can be used for generating the ATP needed for photosynthesis.

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