Mitochondrion

A mitochondrion is an organelle found in the cells of most eukaryotes. Mitochondria are sometimes described as cellular "power plants" because their primary function is to manufacture adenosine triphosphate (ATP), which is used as a source of energy.

The number of mitochondria found in different types of cells varies widely. At one end of the spectrum, the Trypanosome protozoan has one large mitochondrion; by contrast, human liver cells normally have between one and two thousand each.

Table of contents

1 Structure
2 Energy conversion
3 Other functions
4 Use in population genetic studies
5 The endosymbiotic hypothesis
6 See also

Structure

Mitochondria are composed of folds called cristae, which give a much increased surface area on which chemical reactions can occur.

The outer membrane encloses the entire organelle and contains channels made of protein complexes through which molecules and ions can move in and out of the mitochondrion. Large molecules are excluded from traversing this membrane.
The inner membrane, folded into cristae, encloses the matrix (the internal fluid of the mitochondrion). It contains several protein complexes. Stalked particles are found on the cristae: these are the ATP synthase enzyme molecules, which produce ATP.
The intermembrane space between the two membranes contains enzymes that use ATP to phosphorylate other nucleotides and that catalyze other reactions.

Figure 1 : Mitochondrion. 1. Inner membrane. 2. Outer membrane. 3. Crista. 4. Matrix.

"Mitochondrion" literally means 'thread granule', which is what they look like under a light microscope: tiny rod-like structures present in the cytoplasm of all cells. The matrix contains soluble enzymes that catalyze the oxidation of pyruvate and other small organic molecules. Parts of the Krebs cycle occur within mitochondria. The matrix also contains several copies of the mitochondrial DNA (usually 5-10 circular DNA molecules per mitochondrion), as well as special mitochondrial ribosomes, tRNAs, and proteins needed for DNA replication.

When the cell divides, mitochondria replicate by fission. They also replicate if the long-term energy demands of a cell increase. For example, fat storage cells, which require little energy, have very few mitochondria, but energy-demanding muscle cells tend to have many. Mitochondria are generally theorised to be highly adapted symbiotic bacteria, probably belonging to the alpha-proteo bacteria (with the closest known candidate being Rickettsia, the causative agent of typhus), and are believed to have been incorporated only once (compare chloroplast).

Energy conversion

Mitochondria convert the potential energy of food molecules into ATP. The production of ATP is achieved by the Krebs cycle (see citric acid cycle), electron transport and oxidative phosphorylation. Without oxygen, these processes cannot occur.

The energy from food molecules (e.g., glucose) is used to produce NADH and FADH₂ molecules, via glycolysis and the Krebs cycle. This energy is transferred to oxygen (O₂) in several steps. The protein complexes in the inner membrane (NADH dehydrogenase, cytochrome c reductase, cytochrome c oxidase) that perform the transfer use the released energy to pump protons (H⁺) against a gradient (the concentration of protons in the intermembrane space is higher than that in the matrix). An active transport system (energy requiring) pumps the protons against their physical tendency (in the "wrong" direction) from the matrix into the intermembrane space.

As the proton concentration increases in the intermembrane space, a strong diffusion gradient is built up. The only exit for these protons is through the ATP synthase complex. By transporting protons from the intermembrane space back into the matrix, the ATP synthase complex can make ATP from ADP and inorganic phosphate (P_i). This process is called chemiosmosis and is an example of facilitated diffusion. Part of the 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E. Walker for their clarification of the working mechanism of ATP synthase.

Other functions

Mitochondria have several important functions besides the production of ATP. This variety of functions corresponds to the variety of mitochondrial diseases.

Some mitochondrial functions are performed only in specific types of cells. For example, mitochondria in liver cells contain enzymes that allow them to detoxify ammonia, a waste product of protein metabolism. These enzymes are not made in the mitochondria of cardiac cells.

Mitochondria also play a role in the following:

apoptosis
glutamate-mediated excitotoxic neuronal injury
cellular proliferation
regulation of the cellular redox state
heme synthesis
steroid synthesis
heat production (enabling the organism to stay warm)

Use in population genetic studies

Because eggs destroy the mitochondria of the sperm that fertilize them, the mitochondrial DNA of an individual derives exclusively from the mother. Individuals inherit the other kinds of genes and DNA from both parents jointly. Because of the unique matrilineal transmission of mitochondrial DNA, scientists in population genetics and evolutionary biology often use data from mitochondiral DNA sequences to draw conclusions about genealogy and evolution. See: mitochondrial Eve.

The endosymbiotic hypothesis

Mitochondria are unusual among organelles in that they contain ribosomes and their own genetic material. Mitochondrial DNA is circular and employs characteristic variants of the standard eukaryotic genetic code.

These and similar pieces of evidence motivate the endosymbiotic hypothesis — that mitochondria originated as prokaryotic endosymbionts. Essentially this widely accepted hypothesis postulates that the ancestors of modern mitochondria were independent bacteria that colonized the interior of the ancient precursor of all eukaryotic life.