Mitochondrial membranes — identify the incorrect statement about the outer membrane Which statement about the mitochondrial outer membrane is not true?

Introduction / Context:
Mitochondria are double-membraned organelles with distinct permeability properties for inner and outer membranes. Understanding which membrane forms the crucial diffusion barrier clarifies how the proton motive force is generated and maintained during oxidative phosphorylation.

Given Data / Assumptions:

• Outer membrane contains porins that allow free diffusion of small solutes and ions up to a size limit.
• Inner membrane is the primary barrier that maintains the proton gradient.
• Protons and many small ions can equilibrate across the outer membrane channels.

Concept / Approach:

The statement claiming impermeability of the outer membrane to hydrogen ions is incorrect. The outer membrane, via porins such as VDAC, is permeable to many ions and small molecules. The proton gradient that drives ATP synthesis is established across the inner membrane, which is highly selective and comparatively impermeable to protons and most solutes.

Step-by-Step Solution:

Verification / Alternative check:

Measurements show that intermembrane space pH is close to cytosolic pH, whereas matrix pH is higher due to proton pumping across the inner membrane. This observation aligns with outer membrane permeability to small ions, including protons.

Why Other Options Are Wrong:

Options B, C, D, and E accurately describe the outer membrane and its physiological consequences.

Common Pitfalls:

Attributing inner membrane selectivity to the outer membrane, or misremembering where the proton gradient is maintained.

Final Answer:

The outer membrane is impermeable to hydrogen ions