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, ribo ...

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 A crista (; plural cristae) is a fold in the inner membrane of a mitochondrion. The name is from the Latin for ''crest'' or ''plume'', and it gives the inner membrane its characteristic wrinkled shape, providing a large amount of surface area fo ...

, 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. 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 Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membra ...

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 (enc ...

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 among ...

. This phenomenon can be explained by the

endosymbiont hypothesis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possi ...

of the origin of mitochondria as prokaryotes internalized by a eukaryotic host cell. In pig heart mitochondria,

phosphatidylethanolamine Phosphatidylethanolamine (PE) is a class of phospholipids found in biological membranes. They are synthesized by the addition of cytidine diphosphate- ethanolamine to diglycerides, releasing cytidine monophosphate. ''S''-Adenosyl methionine can ...

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 soybea ...

makes up about 26.5%, cardiolipin 25.4%, and

phosphatidylinositol Phosphatidylinositol (or Inositol Phospholipid) consists of a family of lipids as illustrated on the right, where red is x, blue is y, and black is z, in the context of independent variation, a class of the phosphatidylglycerides. In such molecul ...

4.5%. In ''

S. cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have bee ...

'' mitochondria, phosphatidylcholine makes up 38.4% of the IMM, phosphatidylethanolamine makes up 24.0%, phosphatidylinositol 16.2%, cardiolipin 16.1%,

phosphatidylserine Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid and is a component of the cell membrane. It plays a key role in cell cycle signaling, specifically in relation to apoptosis. It is a key pathway for viruses to enter cells via ap ...

3.8%, and

phosphatidic acid Phosphatidic acids are anionic phospholipids important to cell signaling and direct activation of lipid-gated ion channels. Hydrolysis of phosphatidic acid gives rise to one molecule each of glycerol and phosphoric acid and two molecules of fatty ac ...

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 wel ...

, 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'' (franchis ...

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 In biology, a transporter is a transmembrane protein that moves ions (or other small molecules) across a biological membrane to accomplish many different biological functions including, cellular communication, maintaining homeostasis, energy produ ...

s exist to allow specific molecules to cross this barrier. There are several

antiport An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma mem ...

systems embedded in the inner membrane, allowing exchange of

anions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

between the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

and the mitochondrial matrix.

IMM-associated proteins

References

External links

Mitochondrial inner membrane (97 proteins)
Orientations of Proteins in Membranes (OPM) database {{Mitochondrial enzymes Membrane biology Mitochondria