They reproduce by fission in a manner similar to that of bacterial cells. In fact, the seeming independence of mitochondria from the eukaryotic cell's genetic code, as well as the resemblance of mitochondria-associated macromolecules to those of bacteria, are strong evidence for an endosymbiotic origin for the organelles. Mitochondrial membranes contain numerous transport systems for the import of metabolites and high energy intermediates, export of ATP which is utilized in the cytosol, and inorganic phosphate, which is returned to the matrix via a phosphate-proton symport that is driven by the chemiosmotic gradient.
Mitochondria are exceedingly important as storage tanks for calcium ions.
Calcium ion concentration is an important second messenger in cells. It must be precisely controlled in various intracellular compartments, or cellular function is compromised. Indeed, calcium itself is a mediator of many toxins. Mitochondria may act as 'sinks' to buffer the effects of calcium overload.