Mitochondria have a semi-autonomous life of their own. Their framework, if not their contents, is independent of information in the cell nucleus. During cell division, daughter-cell mitochondria are produced by division of the mitochondria of the parent. During sexual reproduction, the mitochondria come with the egg from the mother, and later divide and increase in number. Mitochondria usually are located in the cell at places where energy is needed (along myofibrils in muscle cells, or in regions of secretory activity requiring ATP), or where stored energy is available (near fat globules in the cytoplasm). In liver cells mitochondria are capable of free motion within the cytoplasm. They are not fixed, static organelles.

There is an old but recently resurrected suggestion that mitochondria in cells are the highly specialized remains of respiring bacteria, which at one time established a symbiotic relationship with larger, nucleated cells that were incapable of respiration. The host cell supplied its own waste product, pyruvate as food for the guest, which in turn made better use of it and donated some of its excess ATP back to the host.

Functionally, the host cell and the guest bacterium would stand in a relationship similar to that of a cow and the cellulose-digesting bacteria in its rumen. In time, host and guest gradually became increasingly interdependent, and many of the genetic functions once possessed by the guest were transferred to the nucleus of the host. With its own bacterialike inner membrane, and wrapped completely in a hostlike outer membrane, a mitochondrion is really outside the eucaryotic cell even though it is physically surrounded by it.

This theory was first proposed many years ago on the basis of a general resemblance between mitochondria and bacteria, but was neglected for lack of evidence. Recent new evidence, involving bacterial and mitochondrial membrane structure, DNA, polymerases, ribosomes, and inhibition by antibiotics, has made this old theory not only respectable but probably correct. The same lines of research suggest that chloroplasts in photosynthesis probably are the relics of once-symbiotic blue-green algae.