Figure 1.

Mitochondria are composed of two membranes: an outer mitochondrial membrane and an inner mitochondrial membrane (IMM). The IMM is unique in that it has a very high concentration of proteins and is rich in cardiolipin, a very anionic phospholipid that plays a crucial role in the formation of cristae curvature and is important for efficient function of the electron transport chain (ETC). The mitochondrial matrix enclosed by the IMM contains not only mitochondrial DNA but also the major enzymes of the tricarboxylic acid cycle, which provide reducing equivalents in the form of NADH and FADH₂ to the ETC. The IMM is compartmentalized into numerous cristae that greatly increase the surface area of the IMM. The four power-generating protein complexes of the ETC (complexes I–IV) reside on these cristae in the IMM, and the proton gradient generated across the IMM as a result of electron transfer from complex I to complex IV drives the production of ATP by the F0F1–ATPase (complex V). All these complexes must be assembled properly for efficient electron transfer to take place. ADP, adenosine diphosphate; ATP, adenosine triphosphate; ATPase, adenosine triphosphatase; FADH₂, reduced flavin adenine dinucleotide; NADH, reduced nicotinamide adenine dinucleotide.