okay, so here's another question: the reaction for G-3-P oxidizing to 1,3-BPG is: G-3-P + NAD+ + Pi --> 1,3-BPG + NADH + H+. I understand that the hydride ion leaves G-3-P and donates two electrons and one proton, which neutralizes the charge on NAD+ and gives NADH. But it is only giving one 'hydrogen', while the product side of the reaction shows another one (the hydrogen proton). Where does this hydronium ion come from? Why is it in the reaction? I read somewhere that it is pulled out of 'solution', but what does that mean?