Where do the four ADPs come from in the second stage of glycolysis? In the first stage of glycolysis, the two molecules of ATP are broken down into 2 ADPs + 2 Pi through hydrolysis, then in the second stage of glycolysis they are phosphorilazed to obtain 2 ATPs. How can this process happen twice to yield 4 molecules of ATP if we only have 2 ADPs to begin with? Note: I'm not asking why the process happens twice, since it's easy to see that the Fructose-1,6-biphosphate is being broken down into two Glyceraldehyde phosphate molecules. I'm asking how can the process of producing two ATPs happen twice if we only broke down two ATP molecules into ADP.

There are many other sources of ADP in the cell: various proteins that use ATP as an energy source hydrolyze it to form ADP + phosphate, thereby extracting energy. This ADP can then be re-phosphorylated by glycolysis to form ATP again. If glycolysis did not produce a net yield of ATP it would be pointless, so it really _should_ be unbalanced in this sense.