pH of cytosol, intermembrane space, and matrix

[laczlacylaci]

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The correct answer to this question is A.
I understand that the matrix is ranked the highest pH here (less H+) is due to the proton gradient of pumping the H+ to the intermembrane space (less H+ before) from the matrix (more H+ now)

1) Why do we judge this ranking based on when the H+ are pumped out (M to I), why do we not care when the H+ ions are pumped back through the ATP synthase (I to M)?

2) How do you logic that cytosol has less H+ (higher pH) than the intermembrane space?

 

[theonlytycrane]

1) H+ isn't pumped back into the matrix, rather it goes down a gradient. This implies that the intermembrane space has the higher H+ concentration. Assuming the gradient dissipates and we won't pump anymore H+, the pH of both could be equal. But go with the normal ETC model of H+ driving ATP synthesis.

2) When we learn about oxidative phosphorylation, we learn about all this effort to pump H+ into the intermembrane space to drive ATP synthesis. If the cytosol had even more H+ than that, we would probably have a pathway to funnel those H+ down a gradient instead of pumping H+ at the complexes. I'm not sure what the cytosol pH is, but just stick to the model of ETC as in 1).

 

[laczlacylaci]

Thanks @theonlytycrane, that helped a lot!

 

[alpha-GTP]

The cytosol is usually maintained at a neutral pH (~7.4). The intermembrane space has the lowest pH because, as mentioned above, a proton gradient is maintained. It does not dissipate because energy from the ETC is used to continuously power proton pumps. Due to the gradient, the pH of the matrix is the lowest of the three.