The mitochondria are the organelles in which cellular respiration (the process by which organic molecules are broken down to release energy which the cell can store for later use) takes place. A cell may have only one mitochondrion, but it is much more common to find cells with many, usually thousands, of mitochondria.
The mitochondria each have two membranes, one on the outside which separates it from the cell's cytoplasm, and a second membrane found on the inside which is folded upon itself many times. This second membrane, which is sometimes called the crystae, has a very large surface area on which the respiratory reactions can occur.
Interestingly, biologists have found that mitochondria have their own DNA, separate from the cell's DNA. In addition, mitochondria can create their own proteins and reproduce to form new mitochondria. This evidence suggests that perhaps mitochondria were once separate living organisms which somehow became incorporated into eukaryotic cells.
