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The conversion from Glycerate 3-Phosphate to Triose Phosphate requires:

$ATP \rightarrow ADP + P_i$

$NADPH \rightarrow NADP + H^+$

So what happens to the $H^+$?

Is it in-effect "recycled" back to the Light Dependent reaction.

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  • $\begingroup$ The H+ is used for the photophosphorylation part, in which the H+ is pumped.. so yes, you are correct... $\endgroup$ – TanMath Oct 10 '15 at 17:54
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First off, I believe you have the production/consumption of the proton off, at least for the reaction with NADPH. Most reactions with NAD(P)H go like NADPH + H+ + (oxidized compound) --> NAD(P)+ + (reduced compound). In fact, in most reaction mechanisms the hydrogen that's actually on the NADPH is ends up covalently bonded to the reduced compound.

That said, acid-base reactions are some of the fastest reactions that can occur in the cell. The cellular environment is pH buffered, so any excess H+ ions would be rapidly taken up by the excess of buffering agents which are floating around in the cell. Likewise, when a hydrogen ion is needed in a reaction, it's pulled from the general pool of free hydrogen ions, and it is rapidly replaced from that same pool of buffering agents. As a consequence of this, it normally doesn't make much sense to talk about where hydrogen ions flow in the cell - there's just a general hydrogen ion pool which supplies and takes up any hydrogen ions that are needed/produced by the reactions in the cell.

The big exception to this is where hydrogen ion movement is restricted by things like membranes. The TCA cycle and the light-dependent reactions of photosynthesis being major examples of this. In those situations hydrogen ions are pumped across the membrane during the reaction, and there is a distinction between hydrogen ions "inside" and "outside" the membrane. But there isn't any distinction between any of the hydrogens on one particular side of the membrane.

The light independent reactions of the Calvin cycle, though, happen within the chloroplast stroma and don't pump hydrogen ions across the membrane. So the hydrogen ions generated/consumed by these reactions go into/come from the general stromal pool of hydrogen ions. Here they can be used by any reaction that requires/generates hydrogen ions - a major one of these would be the light-dependent reactions, which take hydrogen ions from the stroma and pump them to the lumen (and then release them back to capture the energy for phosphorylation). But the hydrogen atoms from the Calvin cycle enzymes aren't in any way "earmarked" for the light dependent reactions.

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