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In chemistry, I studied the decomposition of water as being $2H_2O_{(l)} \rightarrow 2H_{2(g)} + O_{2(g)}$. However, when water is split, the equation is $2H_2O_{(l)} \rightarrow 4H^+ + 4e^- +O_{2(g)}$. Why is that? How does $H_2$ just ionize?

Moreover, what is the mechanism for photolysis? I would have thought that splitting water molecules requires a ton of energy; yet it can be accomplished simply with light energy during photosynthesis?

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    $\begingroup$ Have you read about the photosynthesis in detail? I think this will explain the question. $\endgroup$
    – Chris
    Apr 19, 2015 at 10:36
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    $\begingroup$ I don't want to write the same again: biology.stackexchange.com/a/23829/3703 use this and read about redox reactions. $\endgroup$
    – inf3rno
    Apr 29, 2015 at 22:32
  • $\begingroup$ The part of photosynthesis occurs in the granum region of thylakoid region of a chloroplast when light use absorbed by chlorophyll. It is the break down of water molecule into hydrogen and oxygen under Influence of light when photons are absorbed, it causes the hydrogen to bind to an acceptor, subsequently releasing oxygen. $\endgroup$ Feb 1, 2018 at 20:34

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Water is split in chloroplasts in the light reaction of photosynthesis. Chlorophyll, acting as a photopigment, captures sunlight and transfers that energy to an electron pair of a water molecule. Under the influence of a water-splitting enzyme (George et al, 1989) it is separated into 2 protons, molecular oxygen and a free electron pair.

Reference
George et al. Science 1989; 243(4892): 789-91

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The splitting of water in chloroplasts is catalysed by the Oxygen Evolving Complex. It's all about redox reactions but in general hydrogen is the less electronegative of biological elements so it is easily ionised in many life reactions.

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