MITOCHONDRIA
In cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 10 micrometers (μm) in diameter. Mitochondria 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 a range of other processes, such as signaling, cellular differentiation, death cell, as well as the control of the cell cycle and cell growth. Mitochondria have been implicated in several human diseases, including mitochondrial disorders and cardiac dysfunction, and may play a role in the aging process.
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:
Outer membrane
Intermembrane space
Inner membrane
Cristae
Matrix
A mitochondrion contains outer and inner membranes composed of phospholipid bilayers and proteins. The two membranes, however, have different properties. Because of this double-membraned organization, there are five distinct compartments within the mitochondrion.
1. Outer membrane
The outer mitochondrial membrane, which encloses the entire organelle, has a protein-to-phospholipid ratio similar to that of the eukaryotic plasma membrane. It contains large numbers of integral proteins called porins.
2. Intermembrane space
The intermembrane space is the space between the outer membrane and the inner membrane. Because the outer membrane is freely permeable to small molecules, the concentrations of small molecules such as ions and sugars in the intermembrane space is the same as the cytosol. However, as large proteins must have a specific signaling sequence to be transported across the outer membrane, the protein composition of this space is different from the protein composition of the cytosol. One protein that is localized to the intermembrane space in this way is cytochrome c.
3. Inner membrane
The inner membrane is rich in an unusual phospholipids, cardiolipin. Unlike the outer membrane, the inner membrane does not contain porins and is highly impermeable to all molecules. The inner mitochondrial membrane contains proteins with five types of functions:
Those that perform the redox reactions of oxidative phosphorylation
ATP synthase, which generates ATP in the matrix
Specific transport proteins that regulate metabolite passage into and out of the matrix
Protein import machinery.
Mitochondria fusion and fission protein
4. Cristae
The inner mitochondrial membrane is compartmentalized into numerous cristae, which expand the surface area of the inner mitochondrial membrane, enhancing its ability to produce ATP. For typical liver mitochondria the area of the inner membrane is about five times greater than the outer membrane. This ratio is variable and mitochondria from cells that have a greater demand for ATP, such as muscle cells, contain even more cristae.
5. Matrix
The matrix is the space enclosed by the inner membrane. It contains about 2/3 of the total protein in a mitochondrion.
The matrix is important in the production of ATP with the aid of the ATP synthase contained in the inner membrane. The matrix contains a highly-concentrated mixture of hundreds of enzymes, special mitochondrial ribosomes, tRNA, and several copies of the mitochondrial DNA genome. Of the enzymes, the major functions include oxidation of pyruvate and fatty acids, and the citric acid cycle.
