In the scenario presented above, the aerobic bacterium which was ingested by the anaerobic bacterium was proto mitochondrion, in other words, the organism which made possible the production of energy from oxygen. Symbiosis, the relationship of mutual benefit between two species such as the one described above, is believed by some to have been the process by which mitochondria became organelles in eukaryotes. Before delving into the evidence for the theory, however, the functions and structure of the present-day mitochondrion must be addressed. Mitochondria are the eukaryotic organelles which carry out oxidative respiration, the final step in cellular respiration. Oxygen is necessary for the eukaryotic cell because mitochondria use it as the terminal electron acceptor in the electron transport chain, ultimately resulting in a proton gradient which drives ATP synthesis.
Mitochondria are present to varying degrees in different eukaryotic cells.
Cells requiring lots of energy such as in muscle tissue and liver have proportionately more mitochondria than cells requiring less energy such as bone. The features examined here are size, membrane structure, protein status, and genetic information. It has two membranes, the innermost of which folds in at many points along its perimeter in a formation resembling a maze. The manner in which the cristae are arranged keeps these enzymes segregated according to their utility, an example of the high level of organization within the mitochondrion. Mitochondria are unique among all other organelles because they contain their own DNA, i.e., separate from the cell nucleus' DNA, and its configuration is circular.