Almost all eukaryote cells have mitochondria, and often in large numbers. The most important function of the mitochondria is to generate chemical energy, and thereby regulate the cell’s metabolism. This is a two-part process that consists of the cell converting certain chemical compounds into fuel and then eliminating the chemical compounds the cell cannot use. From the cell’s food and a special molecule known as adenosine diphosphate (ADP), mitochondria produce another special type of molecule called adenosine triphosphate (ATP). ATP is an energy carrier. Once it breaks back down into ADP, it releases energy that can be used by all parts of the cell for their various functions. ADP enters a mitochondrion from holes in the outer membrane called porins. The synthesis of ATP occurs in the inner membrane of the mitochondrion. ATP then exits the mitochondrion through the same porins through which the ADP entered.
During the essential metabolism process, sometimes protons re-enter the matrix, the space enclosed by the inner membrane of the mitochondrion. This is called a proton leak, and it results in the production of heat. While this heat may be negligible in most cells, in a certain type of tissue called the brown adipose tissue (brown fat), a special type of prominent protein called thermogenesis facilitates this heat production by the proton leak. Brown adipose tissue deteriorates over time in humans as they age. Since there is more brown fat in babies than in adults, and brown fat generates heat, it helps keep babies warm. Since babies are much smaller than adults, and therefore have less muscle tissue to generate body heat, this is an essential survival mechanism.
Mitochondria also play a key role in the homeostasis of calcium in the cell. Mitochondria store calcium in the matrix. The calcium interaction between the mitochondria and the ER – intake, storage, release – is essential for the proper operation of nerve cells and the release of hormones.
While most of a cell’s DNA (Deoxyribonucleic acid) is contained in its nucleus, mitochondria also have some DNA, called mtDNA. Although it is not known for sure, many theorize that mtDNA was actually part of the DNA of simpler organisms, like bacteria, that eventually became part of the cell of more complex living beings. The most unique feature of mtDNA is that in most multicellular organisms, mtDNA is inherited from the mother. This can help in tracing maternal lineage.
Mitochondria replicate their own DNA and divide to make copies of themselves in a process called binary fission, much like the way bacteria multiply. In multicellular organisms, this often occurs within a cell when the cell needs more energy.
Mitochondria act as the powerhouse of the cell and regulate its metabolism. They play a part in keeping infants warm, maintaining homeostasis of calcium in the cell, and even have their own special type of DNA. Mitochondria are one of the most important organelles of a eukaryotic cell.