The by the cell.The outer membrane functions

The mitochondrion is an organelle found in our cells that is sometimes referred to as a ‘cellular power plant.

‘ Each mitochondrion is enclosed in a membrane. Learn the structure and function of this membrane.

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Mitochondrial Outer Membrane

Mitochondria carry out aerobic respiration in our cells. They have their own DNA and are likely descended from bacteria that entered larger cells a very long time ago. Each mitochondrion has an outer membrane that separates it from the rest of the cell.The mitochondrial outer membrane is a double phospholipid membrane that separates the inside of the organelle from the rest of the cell.

It also helps define the inter-membrane space between itself and the mitochondrial inner membrane.The outer membrane is made of the same phospholipid bilayer that the cell’s own membrane is made of, and it functions in much the same way. Like the cell membrane, it regulates what goes into and out of the mitochondrion.


Before we get into specifics on the mitochondrial outer membrane, let’s take a closer look at mitochondria as a whole. The mitochondrion is the organelle where aerobic cellular respiration takes place. This organelle receives the products of glycolysis and breaks them down further to release more energy.

While much of the activity inside the mitochondrion takes place near or on the inner membrane, it is the outer membrane that determines which materials enter and leave.

The mitochondrion, showing the membranes, the inter-membrane space and other parts. The cristae are folds of the inner membrane.

Structure of the Outer Membrane

The basic structure of the membrane is similar to that of a eukaryotic cell membrane.

Both have proteins integrated into the phospholipid bilayer at a ratio of about 1:1. These proteins are responsible for allowing materials into the mitochondrion.Porins are channels formed by these proteins in the phospholipid bilayer. These channels allow smaller proteins to enter freely by diffusion. For larger proteins looking to enter the organelle, the membrane has a complex of proteins called the translocase of the outer membrane or TOM.

The TOM responds to signals from certain large proteins, opening a gateway to allow them in. These tend to be proteins made in the cytosol by the cell’s nuclear DNA. Many proteins needed by the mitochondrion are made outside the mitochondrion by the cell.The outer membrane functions in concert with the inner membrane, which also has protein complexes in it. Together, they allow proteins to enter the matrix, which is contained within the inner membrane.The outer membrane is also associated with the smooth endoplasmic reticulum.

Sometimes, lipids need to enter the mitochondrion. These come from the ER.

The outer and inner membranes of the mitochondrion. Note the small lollipop structures, which are phospholipids. Both membranes have protein complexes that allow proteins to enter the mitochondrion.

From the Journal of Cell Science.

mitochondrial membranes

The Inter-membrane Space

The outer membrane creates the inter-membrane space between the outer and inner membranes.

This space is vital to the electron transport chain, a step in aerobic respiration. During this step, hydrogen ions pass through the inner membrane and into the inter-membrane space. Once they build up in that space, they create an ion potential that powers the production of ATP.

The inner mitochondrial membrane. Note the H+ ions moving into the inter-membrane space.
inner mitochondrial membrane

Programmed Cell Death

In certain cases, the mitochondria may play a role in programmed cell death.

Programmed cell death is when a cell receives a signal either from within itself or from elsewhere, telling it to self-destruct. It is important in fetal development, where the skin between developing fingers and toes needs to separate. Several diseases can be caused by a failure of cell death, or by too much of it.When a cell needs to be killed off by the organism, one way of accomplishing this is to increase the permeability of the outer membrane of the mitochondrion. This causes certain proteins to leak out into the cytosol, causing cellular death.

Lesson Summary

The outer membrane is the gateway to the mitochondrion, just like the cell membrane is for the cell.

It has porins to allow smaller proteins in and protein complexes to allow bigger proteins in. It works in concert with the inner membrane to allow these proteins into the matrix.The two membranes also work together to create the inter-membrane space. This is necessary for aerobic respiration; specifically, the electron transport chain.

If the outer membrane opens up too much, it can let proteins into the cytosol that can cause cellular death.


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