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The splitting of water is an endergonic (non-spontaneous) reaction, and thus would require energy (chemical work to be done) in order to happen.

In Photosystem II, an enzyme catalyzes this splitting, but where does it get the energy from? Does it use ATP?

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2 Answers 2

The electrones which are generated from splitting water are later used to split CO2.

The general formula is:

General formula of photosynthesis

The Photosystem II does the first part of the reaction by splitting up water and transferring electrons to plastoquinone and also by generating H+ ions. Water gets oxidized (spends electrons) in this reaction, CO2 in the end is reduced (receives electrons). 4 photons are needed for splitting 1 water molecule and 8 photons to liberate one molecule of oxygen. For green plants the energy for this reaction comes completely from light. In the process the energy of the electrons is also used to generate ATP, not to use it. A more detailed view can be found in the schematic diagram "Z-scheme" in the Wikipedia page on photosynthesis:

enter image description here

The figure shows the flow of the electrons and the points when they are brought to higher energy levels by light. The energy of the light is then converted in a proton gradient which is then used to generate ATP.

Its also possible to exchange the role of the oxygen with sulphur, the energy source is then usually heat. This is done by sulphur reducing bacteria in the deep sea in the vicinity of black smokers.

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Catalysis is about reducing the free energy barrier (aka. activation energy) of a reaction, so it does not require any energy. In photolysis (e.g. splitting water) you get the energy from the absorbed photons.

The exact process is called the Joliot-Kok cycle:

Kok cycle

  • Figure 1 - Joliot-Kok cycle - source

So the photon separates the charges on the P680, after that the activated P680 activates the Yz intermedier, which forces the enzyme to the next step (Sx) in the reaction.

The overall process comprises three types of reaction sequences: (a) light-induced charge separation leading to formation of the radical ion pair P680+QA(-) ; (b) reduction of plastoquinone to plastoquinol at the QB site via a two-step reaction sequence with QA(-) as reductant and (c) oxidative water splitting into O2 and four protons at a manganese-containing catalytic site via a four-step sequence driven by P680+ as oxidant and a redox active tyrosine YZ acting as mediator.

So the process does not involve ATP or NAD or something like that just the "redox active tyrosine YZ". ATP and NADPH are created after the PS2 part of photosynthesis.

light-dependent reactions of photosynthesis

  • Figure 2 - Light-dependent reactions of photosynthesis - electron transport chain - source

The photolysis of water is about storing the energy coming from the photons (light) in ATP and NADPH. In the Calvin cycle the cells use the stored energy to reduce CO2 into carbohydrates. For instance glucose can be delivered to the cells in the root, which can use it as food.

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