The inner mitochondrial membrane (IMM) is the mitochondria
l membrane which separates the mitochondrial matrix from the intermembrane space.
The structure of the inner mitochondrial membrane is extensively folded and compartmentalized. The numerous invaginations of the membrane are called cristae
, separated by crista junctions from the inner boundary membrane juxtaposed to the outer membrane. Cristae significantly increases the total membrane surface area compared to a smooth inner membrane and thereby the available working space.
The inner membrane creates two compartments. The region between the inner and outer membrane, called the intermembrane space
which is largely continuous with the cytosol, and the more sequestered space inside the inner membrane, called matrix
For typical liver mitochondria, the area of the inner membrane is about 5 times as large as the outer membrane due to cristae. This ratio is variable and mitochondria from cells that have a greater demand for ATP, such as muscle cells, contain even more cristae. Cristae membranes are studded on the matrix side with small round protein complexes known as F1 particles
, the site of proton-gradient driven ATP synthesis. Cristae affect overall chemiosmotic
function of mitochondria.
Cristae and the inner boundary membranes are separated by junctions. The end of cristae are partially closed by transmembrane protein complexes that bind head to head and link opposing crista membranes in a bottleneck-like fashion. For example, deletion of the junction protein MIC60/Mic60 (formerly Mitofilin/Fcj1) leads to a reduced inner membrane potential and impaired growth and to dramatically aberrant inner membrane structures which form concentric stacks instead of the typical invaginations.
The inner membrane of mitochondria is similar in lipid composition to the membrane of bacteria
. This phenomenon can be explained by the endosymbiont hypothesis
of the origin of mitochondria as prokaryotes internalized by a eukaryotic host cell.
In pig heart mitochondria, phosphatidylethanolamine
makes up the majority of the inner mitochondrial membrane at 37.0% of the phospholipid composition. Phosphatidylcholine
makes up about 26.5%, cardiolipin
25.4%, and phosphatidylinositol
In ''S. cerevisiae
'' mitochondria, phosphatidylcholine makes up 38.4% of the IMM, phosphatidylethanolamine makes up 24.0%, phosphatidylinositol 16.2%, cardiolipin 16.1%, phosphatidylserine
3.8%, and phosphatidic acid
In the inner mitochondrial membrane, the protein-to-lipid ratio is 80:20, in contrast to the outer membrane
, which is 50:50.
The inner membrane is freely permeable to oxygen
, carbon dioxide, and water only.
It is much less permeable to ions and small molecules than the outer membrane, creating compartments by separating the matrix
from the cytosolic environment. This compartmentalization is a necessary feature for metabolism. The inner mitochondrial membrane is both an electrical insulator and chemical barrier. Sophisticated ion transporter
s exist to allow specific molecules to cross this barrier. There are several antiport
systems embedded in the inner membrane, allowing exchange of anions
between the cytosol
and the mitochondrial matrix.
* Citric acid cycle
* Proton gradient
* Mitochondrial trifunctional protein
* Mitochondrial shuttle
* Transport proteins
Mitochondrial inner membrane (97 proteins)
Orientations of Proteins in Membranes (OPM) database