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Outer Mitochondrial Membrane – Function & Role

The outer mitochondrial membrane is the external lipid bilayer surrounding mitochondria. It controls molecular transport and plays a key role in cell metabolism and apoptosis.

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Things worth knowing about "Outer mitochondrial membrane"

The outer mitochondrial membrane is the external lipid bilayer surrounding mitochondria. It controls molecular transport and plays a key role in cell metabolism and apoptosis.

What is the outer mitochondrial membrane?

The outer mitochondrial membrane (OMM) is the external lipid bilayer that encloses every mitochondrion. It forms the outermost boundary of the mitochondrion and separates the organelle from the surrounding cytoplasm of the cell. Together with the inner mitochondrial membrane, it creates a double-membrane system that encloses the intermembrane space between the two layers.

Structure and Composition

The outer mitochondrial membrane is composed of a phospholipid bilayer embedded with various proteins. Its most characteristic components are porins, also known as VDAC proteins (Voltage-Dependent Anion Channels). These channel-forming proteins span the membrane and create pores through which small molecules and ions can freely diffuse.

Pore diameter: up to approximately 2-3 nanometres
Permeable to molecules up to approximately 5,000 Daltons
Impermeable to large proteins and macromolecules

Functions

The outer mitochondrial membrane fulfils several important biological roles:

Protective function: It shields the internal structures of the mitochondrion from the cytoplasmic environment.
Transport regulation: Through its porins, it controls which substances can enter or exit the mitochondrion.
Signal transduction: Proteins embedded in the outer membrane participate in cellular signalling pathways, particularly in the regulation of apoptosis (programmed cell death).
Protein import: It is involved in importing proteins into the mitochondrion that are encoded in the nucleus and synthesised in the cytoplasm.

Role in Apoptosis

One of the most significant functions of the outer mitochondrial membrane is its role in regulating programmed cell death. In response to apoptotic signals, pores open in the outer membrane through a process called mitochondrial outer membrane permeabilisation (MOMP). This releases cytochrome c from the intermembrane space into the cytoplasm, triggering the activation of caspases and ultimately leading to cell death. Proteins of the Bcl-2 family regulate this process and determine whether a cell survives or undergoes apoptosis.

Clinical Relevance

Disruptions in the function of the outer mitochondrial membrane are associated with several diseases:

Cancer: Alterations in apoptosis regulation via the outer membrane contribute to uncontrolled cell proliferation.
Neurodegenerative diseases: Mitochondrial dysfunction plays an important role in conditions such as Parkinson disease and Alzheimer disease.
Heart disease: Mitochondrial membrane function is relevant to cardiac muscle metabolism and the cellular response to ischaemia (oxygen deprivation).

Difference from the Inner Mitochondrial Membrane

The outer mitochondrial membrane is relatively smooth and less selective regarding the passage of small molecules compared to the inner membrane. The inner mitochondrial membrane is highly folded (these folds are called cristae) and is far more selective. It is the site of oxidative phosphorylation and ATP synthesis, the primary process by which cells generate energy.

References

Alberts, B. et al. - Molecular Biology of the Cell. 6th Edition. Garland Science, New York (2014).
Lodish, H. et al. - Molecular Cell Biology. 8th Edition. W. H. Freeman and Company, New York (2016).
Tait, S. W. & Green, D. R. - Mitochondria and cell death: outer membrane permeabilization and beyond. Nature Reviews Molecular Cell Biology, 11(9), 621-632 (2010). PubMed PMID: 20683470.

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