These cellular organelles  are essential to almost all of the principal metabolic pathways through the Krebs cycle . Believed to be a primitive prokarytic bacteria captured by a eukaryotic (with a nucleus) cell in the dim mists of time before the Cambrian geological era, mitochondria have a comparatively small loop of MtDNA (mitochondrial DNA) characteristic of prokaryotes that communicates chemically with the rest of the cell for which it provides energy. MtDNA is inherited from the mother since sperm mitochondria seldom if ever penetrate the ovum . Since mitochondria have been found to be critical in apoptosis (cell death)   one may inherit longevity from their mother.
The number and size of mitochondria vary considerably between cell types according to how much energy the cell needs. White adipose or fat cells have less than half the amount of muscle cells with liver cells having the most (~1200/cell). Brown adipose cells have considerably more mitochondria than white adipose cells because they play a critical role in infants' temperature regulation and thermogenesis in adults. Along with a few other cell types red blood cells have no mitochondria and are entirely dependent upon serum (blood plasma) glucose levels for their energy. Besides the changes exercise causes in the hormonal and immune systems mitochondria may be the key to understanding how fitness improves the basal metabolic rate.