Which of the following statements is true?

Question

(a) The NADH dehydrogenase complex can pump more protons than can the cytochrome b-c1 complex.
(b) The pH in the mitochondrial matrix is higher than the pH in the intermembrane space.
(c) The proton concentration gradient and the membrane potential across the inner mitochondrial membrane tend to work against each other in driving protons from the intermembrane space into the matrix.
(d) The difference in proton concentration across the inner mitochondrial membrane has a much larger effect than the membrane potential on the total proton-motive force.

Answer 1

Choice (b) is the correct answer. The pumping of protons out of the matrix into the intermembrane space creates a difference in proton concentration between the two sides of the membrane, with the matrix at a higher pH (i.e., more alkaline) than the intermembrane space, which tends to equilibrate with the cytosol (which has a neutral pH). The electrons in NADH are at a higher energy than the electrons in reduced ubiquinone, but proton pumping is not determined simply by the energy of the electron donors [choice (a)]. Instead, the number of protons that can be pumped by each complex is determined by the difference in energy between the electrons in each substrate–product pair (i.e., the difference between the electrons in NADH and reduced ubiquinone, compared with that between reduced ubiquinone and reduced cytochrome c). The proton concentration gradient and the membrane potential generated by the electron-transport chain work in the same direction [choice (c)], creating a steep electrochemical gradient for protons across the membrane. For choice (d), the difference in proton concentration has a smaller effect than the membrane potential on the total proton-motive force.