Outer mitochondrial membrane

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The outer mitochondrial membrane is the outer membrane of the mitochondrion, an organelle found in most eukaryotic cells. It is an example of a biological membrane. It comprises a lipid bilayer along with various integral membrane proteins embedded in that bilayer. It helps control the entry and exit of materials between the mitochondrion on the inside and the cytoplasm that surrounds it on the outside. The immediate inside side of the outer mitochondrial membrane is the intermembrane space.

The mitochondrion also has an inner mitochondrial membrane which is also a lipid bilayer; the intermembrane space separates the outer and inner mitochondrial membranes.


Item Value
Type of organisms whose cells contain the outer mitochondrial membrane Same as the organisms whose cells contain mitochondria: eukaryotic cells only, including plant cells, animal cells, and the cells of protists and fungi
Type of cells within the organisms that contain the outer mitochondrial membrane Same as the cells that contain mitochondria: all cells except red blood cells in mammals (other vertebrates do have mitochondria in their red blood cells).
Number of outer mitochrondrial membranes per cell Same as the number of mitochondria: 1 to 1000s, depending on the energy needs of the cell
Size angstrom thickness, compared with mitochondrial diameter of , so the thickness is about 1% of the diameter of the mitochondrion.
Location within mitochondrion The outer mitochondrial membrane fully encloses the mitochondrion.
What's on both sides of it The outside is the cytoplasm, i.e., the rest of the cell. The inside is the intermembrane space, that separates the outer and inner mitochondrial membranes.
Structural components Similar to any biological membrane, it has a lipid bilayer (comprising phospholipids) as well as large numbers of integral membrane proteins called porins.
Chemical constituents Phosopholipids and integral membrane proteins called porins; the ratio is about 1:1 by weight, similar to the cell membrane.
Evolutionary origin According to the endosymbiotic theory of mitochondrial origin, the mitochondrion descends from endosymbiotic prokaryotes inside the eukaryotic cell. The outer mitochondrial membrane correspondingly descends from a membrane created by the host cell to firewall the endosymbiont's access to the rest of the cell.
Control of the entry and exit of materials The outer mitochondrial membrane pretty freely allows small molecules to pass through, so the intermembrane space has a similar chemical composition as the cytosol. Large molecules are not allowed.