Permeability of the Inner Membrane (With Diagram)

The most metabolites can readily pass through the outer mitochondrial membrane. In con­trast, the inner membrane is not freely permeable.

Except in special cases, sugars cannot pass through the membrane. Ions such as Na⁺, K⁺, and CI^– are impermeable as are NAD⁺, NADH, NADP⁺, NADPH, AMP, CDP, GDP, CTP, GTP, CoA, and acetyl-CoA.

These compounds collect in the matrix and are unable to mix or exchange with pools of these molecules in the cytosol.

There are, however, special transport systems in the inner membrane that are specific for select metabolites, and via these systems, ADP, ATP, P_i, pyruvate, and a number of TCA cycle intermediates are transported. The transport systems are species-specific and in most cases are associated with membrane proteins called carriers, translocate, or porters. Table 16-5 lists a number of these systems and indicates their functions.

These systems can function passively, facilitating the exchange of metabolites when there is a favorable concentration gradient, or they can function in an ac­tive manner if coupled to the energy-producing elec­tron transport system. During electron transport, there is an accumulation of hydroxyl ions in the ma­trix, but these hydroxyl ions can be transported exter­nally by a phosphate carrier, which simultaneously carries inorganic phosphate into the matrix on an ex­change basis (Fig. 16-31).

The phosphate accumu­lated can be partly consumed in ATP synthesis, with the ADP-ATP carrier functioning to bring ADP into the matrix. The phosphate can also be exchanged indi­rectly with external dicarboxylate or tricarboxylate required in TCA cycle reactions using the appropriate carboxylate carrier.

One of the clearest connections between active transport across the inner mitochondrial membrane and the electron transport system involves the move­ment of calcium ions into the matrix. The movement of Ca²⁺ into the matrix may be accompanied by an equivalent uptake of phosphate. It has been shown that for each pair of electrons transported through the electron transport system from NADH to oxygen, sixCa²⁺ are transported across the inner membrane into the matrix. However, when the Ca²⁺ is trans­ported, there is no phosphorylation of ADP.

Studies using specific inhibitors indicate that the portions of the respiratory chain that generates the energy for phosphorylation along the electron transport chain are also the energy-providing stages for the active transport of Ca²⁺.