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Mitochondrial Membranes

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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.

Overview

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A single mitochondrion is a bean-shaped organelle enclosed by a double-membrane system. The outer membrane of mitochondria is smooth and contains many porins - the integral membrane transporters. Porins enable free diffusion of ions and small uncharged molecules through the outer mitochondrial membrane but limit the transport of molecules larger than 5000 Daltons. Further, the outer mitochondrial membrane forms a unique structure called membrane contact sites with other subcellular organelles, such as endoplasmic reticulum, peroxisome, endosome, lysosome, and plasma membrane.

The outer mitochondrial membrane is structurally and functionally different from the inner mitochondrial membrane that has many invaginations called cristae. The cristae accommodate many protein complexes and enzymes for oxidative phosphorylation (OXPOS) or respiratory functions. The arrangement of the inner membrane into cristae is dependent on the membrane shaping proteins such as the mitochondrial contact sites and cristae organizing system or MICOS.

The characteristic shape of the mitochondrial membranes is crucial for its functions, including ATP synthesis, autophagy (or mitophagy), stem cell differentiation, and innate immune response. However, to facilitate specific functions, the mitochondrial membranes undergo structural modifications. For example, when respiratory reactions are triggered, the cristae density increases to accommodate more enzymes and metabolites, thus improving the rate of oxidative phosphorylation.

Also, the mitochondrial membranes can undergo fusion or fission to orchestrate the cellular needs. For instance, many mitochondria can fuse to combine their enzymes, cofactors, and other resources to increase the efficiency of ATP production when needed. In contrast, individual mitochondria can undergo fission (or fragmentation) to produce more reactive oxygen species that can mediate efficient degradation of internal organelles in a dying cell. Moreover, fragmentation of the mitochondrion is a critical mechanism necessary for equal distribution of mitochondria into daughter cells during cell division.

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