KA Ox phos question

[basophilic]

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What would occur if all available hydrogen ions were used by ATP synthase?
Please choose from one of the following options.

• It would decrease the levels of inorganic phosphate in the intermembrane space
• It would increase the levels of inorganic phosphate in the mitochondrial matrix
• It would decrease the the ph of the mitochondrial matrix
• It would increase the ph of the mitochondrial intermembrane space

D is the correct answer and I get why. But why is C incorrect?

 

[RH8448]

I have to look this up. Right now I'm thinking that maybe it forms water with the oxygen?

 

[desertrat12]

I have to look this up. Right now I'm thinking that maybe it forms water with the oxygen?

I think you are correct. I think it is easy to think of the proton gradient only being formed from increasing the protons in the inner membrane space via the pumping out of protons from complex's 1,3,4, however the gradient is also helped out by decreasing the protons in the matrix. The decrease is done by, of course, pumping the protons out, but also by the formation of water when molecular oxygen is reduced to form water. So, in your question, I am thinking that C is incorrect because the extra hydrogens are combined to form water.

However, in order to form water the molecular oxygen must be reduced by the electrons that have been transferred along in the ETC. If all the available hydrogens are used up by ATP synthase and yet there is no decrease in pH of the mitochondrial matrix this would mean many of them would need to be used to make water. However, the process of making water also involves the transfer of electrons from a functioning ETC, and if the ETC were functioning then hydrogens would still be pumped out and thus there would still be available hydrogens outside of the matrix. I may be misunderstanding this a little and there may be other ways for the oxygen to be reduced to water, but from what I just presented in order for C to be true then the ETC would have to also cease functioning (so no water was being made). The question stem never states that the electron transport chain has ceased functioning. Even though we could potentially assume this given the fact that ATP synthase has used up all the hydrogens and for this to happen we could probably assume that no other hydrogens were being pumped into the inner membrane space, it is much less of a jump to conclude that D is the correct answer. It seems like C could potentially be correct but requires making assumptions that probably go beyond what we are presented with in the question stem.

 

[amy_k]

http://oregonstate.edu/instruction/bi314/fall12/figure_14_11.jpg

I'm imagining that, since the ETC is driven by a proton gradient, the very last H+ from the ETC is pumped through into the matrix by ATP synthase, and then everything stops. All previous H+ pumped into the mitochondrial matrix has been used to make H2O, but we need two H+ for every O, so that very last H+ that is pumped through combines with the waiting OH to form the last H2O, and the mitochondrial matrix pH is neutralized in terms of H+, as usual, whereas the intermembrane space is less acidic than usual because of the lack of H+ gradient.