Mitochondria

Mitochondria (singular mitochondrion) are the source of energy production within a cell. They are semiautonomous and self producing organelles, residing in the cytoplasm of eukaryotic cells. Converting cellular energy metabolites in the Kreb’s Cycle, through the process of oxidative phosphorylation they produce adenosine triphosphate (ATP) which is used to power other processes in the cell.

Their function is essential to efficient energy production. Without them eukaryotic cells would be dependent on anaerobic glycolysis for their ATP. Glycolysis releases very little free energy but in the mitochondria the metabolism of sugars is much more efficient and provides 15 times more ATP than is produced through glycolysis.

Mitochondria take up a large portion of the cytoplasmic volume of eukaryotic cells. They are rod shaped organelles  with an inner and an outer membrane. The outer membrane limits the organelle. The inner membrane folds in on itself forming the cristae mitochondriales, giving the appearance of partitions and chambers within the organelle in cross section. The cristae number and shape vary according to the type of tissue and organism. Cristae serve to increase the surface area of the inner membrane..

Mitochondria contain their own genome which is separate and distinct from the genome of the cell. Theoretically, mitochondria may have been separate unicellular organisms at one time and were subsumed in a symbiotic relationship into eukaryotic cells at some point in the evolutionary process.

from en.citizendium.org