Mitochondria of the lungs under an electron microscope, courtesy Louisa Howard, via Wikipedia
Mitochondria (singular Mitochondrion) from the Greek for "thread granule" are discrete organelles that appear in most cells that have a nucleus. In human physiology, they play the primary role in attaching phosphates (from combining oxygen with glucose) to adenosine diphosphate to form adenosine triphoshate, the source of energy most commonly used in cellular functions. In addition, they play a role in other cell functions such as differentiation, cell growth, and cell death.
The number of mitochondria in a cell largely depends on its function. Some cells may contain just one while others may contain thousands (like the cells of the liver).
Mitochondria has its own DNA, discrete from that found in the rest of the cells of the body. All mitochondria in the human body descend from the mitochondria in the female egg cell. Although sperm cells contain mitochondria to drive the flagellum, they are not involved in reproduction.
Several diseases are related to the function of mitochondria. Among the ones that are related to mitochondrial function are diabetes mellitus, MERRF syndrome, Wilson's disease, Parkinson's disease, schizophrenia, bipolar disorder, and Alzheimer's disease.