proteins and for outer membrane protein biogenesis Mitochondrial inner membrane The inner mitochondrial membrane is a structure that surrounds the mitochondrial matrix, characterized by many folds and compartments that form crista and is the site of oxidative phosphorylation and ATP synthesis. The high concentration of cardiolipin, a type of lipid and about 20% of the inner membrane composition, makes it impermeable to most molecules. Specialized transporters arranged in specific configurations are required to regulate the diffusion of molecules across the membrane. The inner membrane's structure causes a membrane potential of approximately 180 mV. Respiratory chain supercomplex The respiratory chain supercomplex is located in the cristae of the inner membrane. It is composed of multiple complexes that work together to drive oxidative phosphorylation and ATP synthesis. The complexes cannot function without the other parts of the respiratory supercomplex being present. The supercomplex is the site of the mitochondrial electron transport chain. NADH/ubiquinone oxidoreductase NADH/ubiquinone oxidoreductase, also known as complex I, is the first and largest protein in the mitochondrial respiratory chain. It consists of a membrane arm, embedded inside the inner mitochondrial membrane, and a matrix arm, extending out of the membrane. There are 78 transmembrane helices and three proton pumps. The junction of the two arms is the site of conduction of NADH to ubiquinol. Complex I is a scaffold needed for complex III and IV, and it will not function without these other complexes being present. Cytochrome c reductase, succinate dehydrogenase, and cytochrome c oxidase Cytochrome c reductase, also known as complex III, is the second protein in the respiratory chain. It pumps electrons from complex I, through succinate dehydrogenase (complex II) to cytochrome c (complex IV). Complex III and IV are proton pumps, pumping H+ protons out of the mitochondrial matrix, and work in conjunction with complex I to create the proton gradient found at the inner membrane. Cytochrome c is and electron carrier protein that travels between complex III and IV, and triggers apoptosis if it leaves the cristae. Complex IV passes electrons to oxygen, the final acceptor in the mitochondrial electron transport chain. Inner membrane translocases TIM complex The TIM complex is a protein translocase located on the inner membrane. It is part of the TOM/TIM supercomplex, which spans the intermembrane space. The TIM complex is responsible for sorting proteins into the mitochondrial matrix or into the membrane. TIM22 and TIM23 are the main subunits. TIM