Mitochondria

Mitochondria are best known for their role in cellular respiration. In many eukaryotic cells, they make much of the ATP—the molecule cells use to transfer usable chemical energy—through oxidative phosphorylation. Glycolysis begins in the cytoplasm; later stages, including the citric acid cycle and the electron-transport processes that power ATP synthesis, take place in mitochondria. The inner membrane's chemical gradient helps drive this synthesis. This is why mitochondria are often called the cell's powerhouses, though the phrase captures only part of their biology.

A mitochondrion has an outer membrane and a more selective inner membrane. The inner membrane folds into cristae, increasing the area where much of the energy-generating machinery is located; the inner compartment, called the matrix, contains enzymes used in metabolism as well as mitochondrial DNA. Mitochondria vary in number and form among cell types. Cells with high energy requirements may contain many, while mature human red blood cells have none.

Mitochondria contain a small genome separate from the DNA in the cell nucleus. In humans, mitochondrial DNA has 37 genes; 13 encode proteins used in oxidative phosphorylation. Most proteins that make up mitochondria are instead encoded by nuclear genes and imported into the organelle. Human mitochondrial DNA is generally inherited from the mother, because the egg supplies most of the embryo's cytoplasm; this describes the usual pattern, not an absolute rule for every species or circumstance.

The endosymbiotic theory explains mitochondria as descendants of bacteria that came to live inside an ancestral cell and became integrated into it over evolutionary time. Evidence includes mitochondria's own circular DNA, their two membranes, and their division by a process resembling bacterial fission. Modern mitochondria are not independent organisms: most of their proteins are encoded in the nucleus and delivered to them. This combination of inherited bacterial-like features and dependence on the host cell is central to the theory.

Mitochondria also take part in processes such as calcium handling, heat generation, cell signaling, and the regulation of programmed cell death. Their activity is therefore connected to many aspects of cell physiology, not just energy supply. Mitochondrial disorders are diverse: some involve changes in mitochondrial DNA, while many result from mutations in nuclear genes that encode proteins needed by mitochondria. A mitochondrial association does not by itself show that a condition is inherited through mitochondrial DNA.