Which of the following types of ion movement might be expected to require co-transport of protons from the mitochondrial intermembrane space to the matrix, inasmuch as it could not be driven by the membrane potential across the inner membrane? (
Assume that each ion being moved is moving against its concentration gradient.)
(a) import of Ca2+ into the matrix from the intermembrane space
(b) import of acetate ions into the matrix from the intermembrane space
(c) exchange of Fe2+ in the matrix for Fe3+ in the intermembrane space
(d) exchange of ATP from the matrix for ADP in the intermembrane space
Choice (b) is the correct answer. Because the inside of the membrane (the mitochondrial matrix) is more negative than the outside, in principle any traffic resulting in an increase in the positive charge in the matrix can be driven by the membrane potential. Hence, exchange of Fe2+ (or ATP) in the matrix for Fe3+ (or ADP) in the intermembrane space can be driven by the membrane potential and need not require the co-transport of protons down the pH gradient. The same applies to the import of Ca2+. But import of acetate ions into the matrix and exchange of Ca2+ in the matrix for Na+ in the intermembrane space result in an increase in the amount of negative charge in the matrix and they therefore cannot be driven by the charge difference between the two mitochondrial compartments.
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