Mitochondria (singular mitochondrion) are membrane-bound organelles found in most eukaryotic cells (the cells that make up plants, animals, fungi, and many other forms of life). 

The word mitochondrion comes from the Greek μίτος, mitos, i.e. "thread", and χονδρίον, chondrion, i.e. "granule".

Diagram of parts of a mitochondrion 1 Outer membrane, 1.1 Porin, 2 Intermembrane space, 2.1 Intracristal space, 2.2 Peripheral space, 3 Lamella, 3.1 Inner membrane, 3.11 Inner boundary membrane, 3.12 Cristal membrane, 3.2 Matrix, 3.3 Cristæ, 4 Mitochondrial DNA, 5 Matrix granule, 6 Ribosome, 7 ATP synthase .image: wikipedia

Mitochondria range from 0.5 to 1.0 micrometer (μm) in diameter. These structures are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy. 

In addition to supplying cellular energy, mitochondria are involved in other tasks such as signaling, cellular differentiation, cell death, as well as the control of the cell cycle and cell growth.

Mitochondria in human health

Mitochondria have been implicated in several human diseases, including mitochondrial disorders and cardiac dysfunction, and may play a role in the aging process. 

More recent research indicates that autism, especially severe autism, is correlated with mitochondrial defects.

The amount of mitochondria vary in cells and tissues

Several characteristics make mitochondria unique. The number of mitochondria in a cell varies widely by organism and tissue type. Many cells have only a single mitochondrion, whereas others can contain several thousand mitochondria. 

The organelle is composed of compartments that carry out specialized functions. These compartments or regions include the outer membrane, the intermembrane space, the inner membrane, and the cristae and matrix. Mitochondrial proteins vary depending on the tissue and the species.

 In humans, 615 distinct types of proteins have been identified from cardiac mitochondria, whereas in rats, 940 proteins have been reported. 

Mitchondrion have their own DNA

The mitochondrial proteome is thought to be dynamically regulated.  Although most of a cell's DNA is contained in the cell nucleus, the mitochondrion has its own independent genome. Further, its DNA shows substantial similarity to bacterial genomes.