A number of sites (including this one) that provide descriptions of photosynthesis state that high energy electrons 'pump' ions across a membrane. What is the actual 'pumping' mechanism? I've looked at Wikipedia and at a number of YouTube lectures/tutorials but so far have only found statements as to the where and when but not the how of this important process.
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$\begingroup$ This is a good question, but the answer is likely to be very technical, and I think these mechanisms are still bein debated; see for example ncbi.nlm.nih.gov/pubmed/21545285 calling this "one of the most challenging research subjects in life science" ... $\endgroup$– RolandCommented Mar 23, 2016 at 20:22
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$\begingroup$ Thanks for your interest. I've had a further hunt round and found an article in Wikipedia on active transport . A little way down there's a section about secondary active transport that I think could be related to the pumping mechanism in photosynthesis - but I'm not qualified to say! $\endgroup$– JohnPCommented Mar 24, 2016 at 9:47
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Short answer: Electrons flow through membranes by floating through kind of channels made out of iron-sulfur clusters.
Long answer: Let's take a look at the electron transport chain in the inner mitochodrial membrane. There is a proton gradient across the membrane building up a potential difference by pumping protons across the membrane as electeons flow through the respiratory chain. They (electrons) like to flow throught the respiratory chain because they can go from enzyme to enzyme each with a lower standart free energy. These enymes together form one big complex within the inner membrane with Fe-S clusters enabeling electrons to flow through the membrane by giving them a kind of a power stroke (see here). This as an simplyfied answer on a example.
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$\begingroup$ Thanks for your answer. I think I'm starting to understand this a little. I think what I was missing was that: "Whenever a molecule is reduced by acquiring an electron, the electron (e -) brings with it a negative charge. In many cases, this charge is rapidly neutralized by the addition of a proton (H+) from water, so that the net effect of the reduction is to transfer an entire hydrogen atom, H+ + e -". The quote is from this site that I found really helpful - though I'll need to study it a good deal more. As a layman I found 'pump' misleading. $\endgroup$– JohnPCommented Mar 24, 2016 at 17:40
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$\begingroup$ Nice! Still, I think it should be kept in mind that these are very recent findings; the structure of complex I was only recently resolved, and the precise mechanisms are likely to be debated for some time to come. $\endgroup$– RolandCommented Mar 24, 2016 at 21:35
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$\begingroup$ Moreover there are many other kinds of proton pumps that work differently. $\endgroup$– WYSIWYGCommented Mar 25, 2016 at 8:55
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$\begingroup$ Thanks for the additional comments @WYSIWYG: I trust that the principle remains the same i.e. a set of redox reactions with the H+ transported across the membrane in association with the electron as it 'neutralises' the negative charge of the electron? I'm new to biology so want to make sure I've understood the basic ideas/processes correctly. $\endgroup$– JohnPCommented Mar 28, 2016 at 12:08