The inner mitochondrial membrane (IMM) is the
mitochondrial membrane which separates the
mitochondrial matrix
In the mitochondrion, the matrix is the space within the inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contains the mitochondrial DNA, ri ...
from the
intermembrane space.
Structure
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 increase the total membrane surface area compared to a smooth inner membrane and thereby the available working space for
oxidative phosphorylation
Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing chemical energy in order to produce adenosine t ...
.
The inner membrane creates two compartments. The region between the inner and outer membrane, called the intermembrane space, is largely continuous with the cytosol, while the more sequestered space inside the inner membrane is called the matrix.
Cristae
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 junctions
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
IMMT
Mitochondrial inner membrane protein is a protein that in humans is encoded by the ''IMMT'' gene.)
''IMMT'' encodes an inner mitochondrial membrane (IMM) protein in the nucleus. It is posttranslational transported to the IMM. Mic60/Mitofilin (e ...
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.
Composition
The inner membrane of mitochondria is similar in lipid composition to the membrane of
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
. 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
Phosphatidylcholines (PC) are a class of phospholipids that incorporate choline as a headgroup.
They are a major component of biological membranes and can be easily obtained from a variety of readily available sources, such as egg yolk or so ...
makes up about 26.5%,
cardiolipin
Cardiolipin (IUPAC name 1,3-bis(''sn''-3’-phosphatidyl)-''sn''-glycerol) is an important component of the inner mitochondrial membrane, where it constitutes about 20% of the total lipid composition. It can also be found in the membranes of most ...
25.4%, and
phosphatidylinositol 4.5%.
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 1.5%.
In the inner mitochondrial membrane, the protein-to-lipid ratio is 80:20, in contrast to the
outer membrane, which is 50:50.
Permeability
The inner membrane is freely permeable to
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
, 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
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
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 transporters 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.
IMM-associated proteins
See also
*
Citric acid cycle
The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and prote ...
*
Proton gradient
*
Mitochondrial trifunctional protein
Mitochondrial trifunctional protein (MTP) is a protein attached to the inner mitochondrial membrane which catalyzes three out of the four steps in beta oxidation. MTP is a hetero-octamer composed of four alpha and four beta subunits:
* HADHA
* HAD ...
*
Mitochondrial shuttle
*
Transport proteins
References
External links
Mitochondrial inner membrane (97 proteins)Orientations of Proteins in Membranes (OPM) database
{{Mitochondrial enzymes
Membrane biology
Mitochondria