Kaplan Bio FL #6

[wrathofgod64]

Based on the chemiosmotic model, which of the following will occur when isolated mitochondria that have been maintained in a solution at pH 8 until saturation are suddenly transferred into a second solution at pH 4 containing ADP and Pi (inorganic phosphate)?

A. No ATP will be produced.
B. There will be a burst of ATP production.
C. The rate of ATP production will be unchanged.
D. ATP will be converted to ADP and Pi.


I chose A, because I thought that if you take a mitochondria and stick it in a pH 4 solution, then both the intermembrane, and matrix will be pH 4, and so the gradient will cease to exist; hence, no ATP. However, Kaplan answered B, saying that when you put the mitochondria in a pH 4 solution, the matrix will remain at pH 8, but the intermembrane space will become pH 4, and so the gradient will exist and ATP produced. Why?

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[Adamska]

You're overthinking....if the equilibrium with a basic solution has been established, as soon as you transfer the mitochondria into an acidic environment the proton gradient will be apparent, forcing all the hydrogen ions to rush in through the ATP synthase (which uses the energy to phosphorylate ADP and Pi)

 

[wrathofgod64]

but for that to happen, you have to assume that the matrix remains at pH 8, and the intermembrane suddenly drops to pH 4. I don't see why one drops and the other doesn't.

 

[rls303]

I also think this answer is A, because I thought the proton gradient is towards the intermembrane space, thus making that region more acidic?

Or could it be D, because the rush of protons inside the intermembrane space would force the protons back into the matrix and then reverse the activity of Atpsynthase?

Can anyone clarify/explain?

Thanks!

 

[t1me]

The porin on the outer membrane are very permeable to protons and a bunch of other crap while the inner membrane is not. This is why protons will travel back inside the matrix via ATP synthase only, because there's not much other options (not counting uncouplers).