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.
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 roundprotein 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 MIC60/Mic60 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. 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 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, 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 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.
