Why is the mitochondrial inner membrane impermeable to protons?

The mitochondrial inner membrane's impermeability to protons is crucial for maintaining the proton gradient necessary for ATP synthesis. This gradient is created during the process of oxidative phosphorylation, where electrons are transferred through a series of protein complexes in the electron transport chain. As electrons move, protons are pumped from the mitochondrial matrix into the intermembrane space, creating a higher concentration of protons outside the matrix.

This differential distribution of protons across the membrane establishes a potential energy difference, known as the proton motive force. When protons flow back into the mitochondrial matrix through ATP synthase, this energy is harnessed to synthesize ATP from ADP and inorganic phosphate, which is essential for various cellular functions.

Other choices do not directly relate to the primary role of the impermeability of the inner membrane in ATP synthesis. Protecting mitochondrial DNA, facilitating electron transport, and regulating nutrient transport are important, but they are not the main reasons for the membrane's selective permeability to protons. The preservation of the proton gradient is crucial for the effective production of ATP, which is ultimately why this choice is the correct answer.