noun, plural: mitochondria
A spherical or rod-shaped organelle found within the cytoplasm of eukaryotic cells. It acts as the “powerhouse of the cell” as it generates most of the cell's supply of adenosine triphosphate (ATP) through the process of cellular respiration.
A mitochondrion consists of outer and inner membranes, an intermembrane space (space in between the membranes), the cristae (infoldings of the inner membrane), and the matrix (space within the inner membrane). The outer membrane contains several porins that form channels where certain molecules can freely diffuse. Unlike the outer membrane, the inner membrane does not contain porins and is highly impermeable to all molecules. Most ions and molecules would need special membrane transporters to enter or exit the matrix. The cristae, which are the foldings of the inner membrane, increase the surface area thereby increasing ATP production. The matrix contains enzymes, mitoribosomes, tRNA, and DNA. The mitochondrial DNA is genetically distinct from that in the nucleus. Since a mitochondrion has its own genetic material, and is capable of manufacturing its own RNAs and proteins, it is said to be a semi-autonomous, self-reproducing structure.
It is thought that the mitochondria might have originated from early bacteria that became so symbiotic with their hosts, the eukaryotic cells, that they evolved and become indispensable energy-yielding structures within the eukaryotic cells (endosymbiotic theory).