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In glycolysis, 2 ATP molecules are produced from each triose phosphate molecule (to give a total of 4 ATP produced; 2 net produced as 2 were initially used up in the reaction).

I don't understand how this would be the case, as each triose phosphate only has one phosphate group available to form ATP from ADP, meaning that one ATP is produced per triose phosphate. I know this cannot be the case as this would mean a 0 net production of ATP defeating the purpose of glycolysis, but i can't get my head around why.

out of curiosity, would the other remaining two phosphates that are required come from other sources, i.e. 2 phosphates + the 2 phosphates already from the two trios phosphate molecules to give us the 4 ATP molecules (2 net ATP)

Many thanks in advance.

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    $\begingroup$ Possible duplicate of Where do the four ADPs come from in the second stage of glycolysis? $\endgroup$ – Vance L Albaugh Aug 11 '16 at 0:50
  • $\begingroup$ @VanceLAlbaugh — I agree that the two questions are related and would be better as one, but the other question wants to know where the ADPs that accept the phosphate come from, whereas this asks about where the phosphates that are not from ATP come from. That is not covered in the other question or its accepted answer. $\endgroup$ – David Aug 12 '16 at 12:39
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Though David has already answered as I wished to write one for this question here's my answer$-$

You say that

In glycolysis, 2 ATP molecules are produced from each triose phosphate molecule ... I don't understand how this would be the case, as each triose phosphate only has one phosphate group

but you are wrong here as each triose phosphate is destined to undergo phosphorylation before they are dephosphorylated to yield 2 ATPs each.

Here are the reactions:Glycolysis

You can see that out of the two trioses Glyceraldehyde-3-phosphate is phosphorylated into a compound (1,3-bisphosphoglycerate) that has two phosphoryl groups. Similarly Dihydroxy acetone phosphate the other triose is first converted into glyceraldehyde-3-phosphate by an isomerisation reaction catalysed by triosephosphate isomerase. This glyceraldehyde-3-PO$_4$ formed undergoes phosphorylation to form a two phosphoryl group containing compound.

would the other remaining two phosphates that are required come from other sources..?

Yes the Phosphate group added to Glyceraldehyde-3-PO$_4$ is derived from orthophosphate (HPO43−).

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    $\begingroup$ Probably just loose writing. I imagine what the OP meant was something like "In the second stage of glycolysis, after one hexose is converted to two triose molecules in the aldolase reaction, each of these produce 2 ATPs". And his concern seemed to be that the aldolase reaction produced triose monophosphates (from the previous di-phosphorylated hexose) so where did the other phosphate come from? He was looking for more ATP input and failed to notice the orthophosphate. $\endgroup$ – David Aug 12 '16 at 17:51
  • $\begingroup$ @David Probably. $\endgroup$ – Tyto alba Aug 12 '16 at 17:57
  • $\begingroup$ @David I have seen people using the term OP quite often here. What does it stand for? Is it Owner of Post? $\endgroup$ – Tyto alba Aug 12 '16 at 18:01
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    $\begingroup$ — I believe it stands for Original Poster, but don't quote me on that. Certainly it refers to the person who posted the question. I really shouldn't use it because Poster takes no time to write, and acronyms like this present a barrier to new users. They imply that there is an in-group who know these things. That is why I have always kicked students hard who write dissertations full of lab abbreviations. After that I guess I'll have to refer to the poster in future. $\endgroup$ – David Aug 12 '16 at 18:30
  • $\begingroup$ @A.P Does this answer clear your doubts? Let me know if I could help. $\endgroup$ – Tyto alba Sep 2 '16 at 17:50
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The other two phosphates come from inorganic phosphate* in solution (Pi — in my diagram below) at the glyceraldehyde phosphate dehydrogenase stage. (Two, of course, because there are two molecules of triose.)

G3P dehydrogenase

This is covered in Berg et al. Section 16.1.5.

*The term inorganic phosphate is used because there are various different anions in solution derived from phosphoric acid, depending on the pH. At physiological pH the predominant species is the double-negativly charged, monohydrogen phosphate.

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