Why is ATP synthesis reversible on the F1 subunit??? What is the point of this? Why is ATP synthesis reversible on the F1 subunit??? What is the point of this? They did some experiment a long time ago where they took F1 subunit and mixed it with ATP and water, with the Oxygen radioisotope marked. Then they watched the reaction and noticed that after ADP with the Pi was made, all 4 oxygens on the Pi had the marker on them. So they surmised that reaction must be reversible because how else could all the oxygens be the same? Ok, i understand that, i think. But heres what confuses me... The whole point of ATP synthase is to MAKE ATP. So why did they do the experiment starting with with ATP and water, unless they already suspected rxn was reversible and they were just tryng to prove it? I mean, they were studying how ATP is made right? So wouldnt the normal thing to do would be to start with reactants ADP + Pi + F1 subunit and then see what happens? Instead, they started with exact opposite reactants (ATP and water and F1).. My other question is: What is the advatage of the reaction being reversible, --for the molecule i mean? thank you for any help..