The concentration of H⁺ ions inside the mitochondrial matrix is lower than it is in the cytosol or the mitochondrial intermembrane space. What would be the immediate effect of a membrane-permeable compound that carries and releases protons into the mitochondrial matrix?

(a) inhibition of the electron-transport chain (b) inhibition of ATP synthesis (c) increased import of ADP into the matrix (d) inhibition of the citric acid cycle

(b) inhibition of ATP synthesis

Explanation:

The inner mitochondrial membrane is impermeable to protons. Electron transfer from NADH and FADH2 to molecular oxygen via electron transport chain generates electrochemical gradient by pumping the protons from matrix to intermembrane space. This electrochemical gradient drives ATP synthesis by down the concentration gradient transport of protons back to the matrix via proton channels.

A compound able to carry protons back to the mitochondrial matrix would dissipate the electrochemical gradient. In the absence of proton concentration gradient across the inner mitochondrial membrane, ATP synthesis would stop.