The inner membrane of mitochondria contains many proteins, has no pores and is very selective; It contains many enzyme complexes and transmembrane transport systems, which are involved in translocation of molecules. This membrane forms invaginations or folds called mitochondrial ridges, which considerably enlarge the surface for affixing these enzymes. In most eukaryotes, the folds form flattened partitions perpendicular to the mitochondrial axis, but some protists have a tubular or discoid shape. In the composition of the inner membrane there is a great abundance of proteins (80%), which are, moreover, exclusive of this organ, namely:
1. The electron transport chain, consisting of four fixed enzyme complexes and two mobile electron transporters:
- Complex I or NADH dehydrogenase containing flavon mononucleotide (FMN).
- Complex II or succinate dehydrogenase. Complexes I and II give electrons to coenzyme Q or ubiquinone.
- Complex III or cytochrome bc1 that yields electrons to cytochrome c.
- Complex IV or cytochrome c oxidase that gives off electrons to O2 to produce two water molecules.
2- An enzymatic complex, the H + ATP synthetase channel that catalyzes the synthesis of ATP (oxidative phosphorylation).
3- Carrier proteins that allow ions and other molecules to pass through the membrane, such as fatty acids, pyruvic acid, ADP, ATP, O2 and water. The following mitochondrial transporters may be highlighted:
- Adenine translocase nucleotide. It is responsible for transporting to the mitochondrial matrix the cytosolic ADP formed during the energy consuming reactions and, in parallel, translocates to the cytosol the newly synthesized ATP during oxidative phosphorylation.
- Phosphate translocase. Cytosolic phosphate translocation together with the proton to the matrix; Phosphate is essential for phosphorizing ADP during oxidative phosphorization.