A single mitochondrion is a bean-shaped organelle enclosed by a double-membrane system. The outer membrane of mitochondria is smooth and contains many porins – the integral membrane transporters. Porins enable free diffusion of ions and small uncharged molecules through the outer mitochondrial membrane but limit the transport of molecules larger than 5000 Daltons. Further, the outer mitochondrial membrane forms a unique structure called membrane contact sites with other subcellular organelles, such as endoplasmic reticulum, peroxisome, endosome, lysosome, and plasma membrane.
The outer mitochondrial membrane is structurally and functionally different from the inner mitochondrial membrane that has many invaginations called cristae. The cristae accommodate many protein complexes and enzymes for oxidative phosphorylation (OXPOS) or respiratory functions. The arrangement of the inner membrane into cristae is dependent on the membrane shaping proteins such as the mitochondrial contact sites and cristae organizing system or MICOS.
The characteristic shape of the mitochondrial membranes is crucial for its functions, including ATP synthesis, autophagy (or mitophagy), stem cell differentiation, and innate immune response. However, to facilitate specific functions, the mitochondrial membranes undergo structural modifications. For example, when respiratory reactions are triggered, the cristae density increases to accommodate more enzymes and metabolites, thus improving the rate of oxidative phosphorylation.
Also, the mitochondrial membranes can undergo fusion or fission to orchestrate the cellular needs. For instance, many mitochondria can fuse to combine their enzymes, cofactors, and other resources to increase the efficiency of ATP production when needed. In contrast, individual mitochondria can undergo fission (or fragmentation) to produce more reactive oxygen species that can mediate efficient degradation of internal organelles in a dying cell. Moreover, fragmentation of the mitochondrion is a critical mechanism necessary for equal distribution of mitochondria into daughter cells during cell division.
