Mitochondria (singular mitochondrion) are the 'electric generators' of cells. They take oxygen and produce carbon dioxide together with ATP, the fuel of most of the activities of the cell. Because they take oxygen and release CO2, it is easy to think they 'breathe'. In fact, this process has its own name very similar to breathing, cellular respiration - the respiration of cell.
A mitochondrion is covered by two layers (membranes). The outer is smooth and round. The inner is folded many times to allow maximum area possible for cellular respiration. In this inner membrane, many proteins essential for cellular respiration are present.
How scientists know its characteristics
A mitochondrion is about few micrometres in diameter. Although it is possible to see them under a light microscope, there is no way to tell what they do while they are in the cell.
So the scientists break up the cell, put its parts into a centrifuge (a spinning device in which different substances are separated, i.e. heavier objects lie further from the centre). When they have isolated our mitochondrion, they perform many chemical reactions on it to see what it can do. And they find it takes oxygen, produces carbon dioxide and ATPs! Some eukaryotes lacking mitochondria cannot consume oxygen. They even find this substance deadly.
Secrets discovered recently
Mitochondria are a power plants in our cell because they programme cell death. Mitochondria are also involved in signal transduction network; respond to signals from outside and emit signals themselves. Several facts have been discovered recently. First, DNA of mitochondrial code is unusual.Their DNA can be replicated, transcribed, and goes to form nucleoids. Second, Mitochondria and nuclei can communicate with each other. Nucleus communicates with mitochondria by imported proteins but still it isn't discovered how mitochondria responds back to the nucleus. Third, mitochondrion relocates protein. Inter membrane space of mitochondrial can oxidize disulphide bridges from sulfhydryl groups even though surrounded environment is highly reduced. The free energy of ATP shocks protein from mitochondrial 70-kDa heat, and it uses hydrolysis to move proteins across the mitochondrial inner membrane. Fourth, mitochondria can be divided and fused by machines, some proteins. Every two mitochondrial membranes have their own shape-changing machine to divide and fuse by itself. Last, our body still has maternally inherited genes from mitochondria DNA. Mitochondria specializes its energy and DNA of mitochondria are maternal. That uniparental inheritance of mitochondrial DNA helps to ensure avoidance of harmful mutation to encoded subunits; oxidative phosphorylation system letting co-optimized by adaptive mutation from nuclear encoded partner subunits. There are much more scientific insights to be made about Mitochondria.
The journey of evolution
If you still remember, we have said that mitochondrion(ancestry) used to be cells. At some time ago, there might have been a bacterium 'mitochondrion' migrating to a eukaryote. Mitochondria today in fact have their own DNAs. They can duplicate themselves. This relationship in biology, two species helping each other is called symbiosis.
Gottfried Schatz. The Magic Garden.annu.Rev.Biochem.2007. 76:673-78