Mitochondrial Membranes

A mitochondrion is enclosed by two concentric membranes—a smooth outer membrane and a densely folded inner membrane, each having distinct structure and properties.

The outer membrane is lipid-rich and forms a unique structure with the endoplasmic reticulum membrane that aids lipid transport between these organelles.

Unlike the outer membrane, the inner membrane is protein-rich and has many invaginations called cristae.

These folds increase the inner membrane's surface area, allowing many electron transport chain enzymes to be packed in the mitochondria.

The mitochondrial membranes define two submitochondrial compartments—the intermembrane space between the outer and inner membranes and the matrix enclosed by the inner mitochondrial membrane.

The composition of these compartments depends on the permeability of the two mitochondrial membranes.

The outer mitochondrial membrane contains transporter proteins called porins that permit the free diffusion of ions and small, uncharged molecules into the intermembrane space. Consequently, the intermembrane space has the same pH and ionic composition as the cytoplasm.

In contrast, the inner membrane acts as a tight diffusion barrier. It contains specific membrane transport proteins that allow selective ions and metabolites in and out of the matrix.

Therefore, the mitochondrial matrix contains only selected ions, molecules, and enzymes responsible for energy metabolism within the mitochondrion.