Inner mitochondrial membrane

This article describes a component of the organelle mitochondrion in cells.
Unless otherwise specified, information about this component is about its in situ occurrence in vivo, i.e., its occurrence in its usual location in living cells.

Definition

The inner mitochondrial membrane (IMM) is the inner membrane of the mitochondrion, an organelle found in most eukaryotic cells. It is an example of a biological membrane. It comprises a lipid bilayer. It is highly folded in order to increase its area, with the folds called cristae. It controls the entry and exit of materials between the intermembrane space of mitochondrion on its outside, and the mitochondrial matrix on the inside.

The movement of materials as well as the voltage maintained across the inner mitochondrial membrane are critical to the mitochondrion's role in energy production.

Summary

Item	Value
Type of organisms whose cells contain the inner 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 inner 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 inner mitochondrial membranes per cell	Same as the number of mitochondria: 1 to 1000s, depending on the energy needs of the cell
Size	$~70$ angstrom thickness (very approximate), similar to the outer mitochondrial membrane, compared with mitochondrial diameter of $0.5-1.0\mu m$, so the thickness is about 1% of the diameter of the mitochondrion. The area is about five times that of the outer mitochondrial membrane, due to the folds called cristae. High surface area is helpful for its goal of facilitating more energy generation through the transfer of materials (mostly protons) via the membrane.
Location within the mitochondrion	It is close to but not at the boundary of the mitochondrion.
What's on both sides of it	It fully encloses the mitochondrial matrix. Outside it is the intermembrane space of mitochondrion, and further out is the outer mitochondrial membrane.
Electrochemical gradient across the membrane	The outside (the intermembrane space) is more positively charged than the inside (the mitochondrial matrix), and has an excess of protons. Moving protons from inside to outside, against the gradient, requires and absorbs energy. Moving protons back inside releases energy that can be captured through either ATP production or heat generation. The electrochemical potential difference maintained across the membrane is about 180 mV.
Structural components	Similar to any biological membrane, it has a lipid bilayer (comprising phospholipids) as well as integral membrane proteins.
Chemical constituents	Integral membrane proteins and phospholipids, in a 80:20 ratio (unlike 50:50 for the outer mitochondrial membrane). The chemical composition is similar to that of the cell membrane of bacteria, consistent with the endosymbiotic theory of mitochondrial origin.
Evolutionary origin	According to the endosymbiotic theory of mitochondrial origin, the mitochondrion descends from endosymbiotic prokaryotes inside the eukaryotic cell. The inner mitochondrial membrane, correspondingly descends from the cell membrane of the endosymbiont.
Control of the entry and exit of materials	The inner mitochondrial membrane allows free passage for only: oxygen, carbon dioxide, and water. It is much more selective about allowing ions and small molecules, with carriers needed to facilitate the movement of such molecules. This is necessary for it to allow for the buildup of an electrochemical gradient (if it were too permeable, diffusion through it would drain any gradient). This is needed for its role in energy storage.
Function	It helps with energy storage and transfer to ATP by maintaining an electrochemical gradient and controlling the entry and exit of materials to increase or decrease the gradient.