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In my book, under the topic chemiosmotic hypothesis, it is given that proton gradient is created by accumulation of protons in the lumen of thylakoids. One of the ways by which the gradient is created is mentioned as below....

' As electrons move through the photosystems protons are transported across the membrane. This happens because the the primary acceptor of electrons which is located towards the outer side of the membrane transfers its electron NOT TO AN ELECTRON CARRIER BUT TO AN H CARRIER. Hence this molecule removes proton from the stroma while transporting an electron.

I suppose they are calling plastoquinone as the H carrier, but then, why is it not an electron carrier? It is involved in electron transport isn't?

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By 'H carrier', I think the author means 'a proton plus an electron carrier' or a H atom carrier.

When PQ accepts an electron, is may also pick up a proton from the outside (it now has accepted a H, that is a proton plus an electron).

If the electron is donated to another acceptor, a proton may be lost on the inside. PQ is now back in its original state, but the net result is that a proton has been 'transported' across the membrane (but it is almost certainly not the same proton) and an electron has been 'carried' from donor to acceptor.

(Oxidation is the loss of an electron or electrons. Reduction is the gain of an electron or electrons. But accepting or losing a proton is neither an oxidation nor a reduction, but an ionization)

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