Mitochondrion

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Mitochondria are visible as thread-like structures in the light microscope. Mitochondria are selectively stained with a fluorescent dye. The nucleus and cell membrane are not visible.

In cell biology, a mitochondrion (from Greek mitos thread + khondrion granule) is an organelle found in most eukaryotic cells, including those of plants, animals, fungi, and protists. A few cells, such as the trypanosome protozoan, have a single large mitochondrion, but usually a cell has hundreds or thousands of mitochondria. The exact number of mitochondria depends on the cell's level of metabolic activity: more activity means more mitochondria. Mitochondria can occupy up to 25% of the cell's cytosol.

Mitochondria are sometimes described as "cellular power plants", because their primary function is to convert organic materials into energy in the form of ATP via the process of oxidative phosphorylation.

Mitochondrion structure

The outer and inner membranes of the mitochondria are composed of phospholipid bilayers studded with proteins, much like a typical cell membrane. The two membranes, however, have very different properties. The outer mitochondrial membrane, which encloses the entire organelle, is composed of about 50% phospholipids by weight and contains a variety of enzymes involved in such diverse activities such as the oxidation of epinephrine (adrenaline), the degradation of tryptophan, and the elongation of fatty acids. The outer mitochondrial membrane contains numerous integral proteins called porins, which contain a relatively large internal channel (about 2-3 nm) that allow ions and small molecules to move in and out of the mitochondrion. Large molecules, however, cannot traverse the outer membrane without being actively transported. Mostly every cell contains mitochondrion but each of them have different amounts.

The inner mitochondrial membrane, in contrast, contains more than 100 different polypeptides, and has a very high protein to phospholipid ratio (more than 3:1 by weight, which is about 1 protein for 15 phospholipids). Additionally, the inner membrane is rich in an unusual phospholipid, cardiolipin, which is usually characteristic of bacterial plasma membranes. The inner membrane does not contain porins, however, and is highly impermeable; almost all ions and molecules require special membrane transporters to enter or exit the matrix. The inner membrane also has foldings called the cristae which allow the inner membrane to have a greater surface area for the electron transport chains.

The mitochondrial matrix

In addition to various enzymes, the mitochondrial matrix also contains ribosomes and several molecules of DNA. Thus, mitochondria possess their own genetic material, and the machinery to manufacture their own RNAs and proteins. (See: protein synthesis). This nonchromosomal DNA encodes a small number of mitochondrial peptides (13 in humans) that are integrated into the inner mitochondrial membrane, along with polypeptides encoded by genes that reside in the host cell's nucleus.

Mitochondrial functions

Although the primary function of mitochondria is to convert organic materials into cellular energy in the form of ATP, mitochondria play an important role in many metabolic tasks, such as:

• 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)

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. A mutation in the genes regulating any of these functions can result in a variety of mitochondrial diseases.

Energy conversion

As stated above, the primary function of the mitochondria is the production of ATP. This is done by metabolizing the major products of glycolysis: pyruvate and NADH (glycolysis is performed outside the mitochondria, in the host cell's cytosol). This metabolism can be performed in two very different ways, depending on the type of cell and the presence or absence of oxygen.

Pyruvate: the citric acid cycle

Each pyruvate molecule produced by glycolysis is actively transported across the inner mitochondrial membrane, and into the matrix where it is combined with coenzyme A to form acetyl CoA. Once formed, acetyl CoA is fed into the citric acid cycle , also known as the tricarboxylic acid (TCA) cycle or Krebs cycle. This process creates 3 molecules of NADH and 1 molecule of FADH₂, which go on to participate in the electron transport chain.

With the exception of succinate dehydrogenase, which is bound to the inner mitochondrial membrane, all of the enzymes of the citric acid cycle are dissolved in the mitochondrial matrix.

NADH and FADH₂: the electron transport chain

This energy from NADH and FADH₂ is transferred to oxygen (O₂) in several steps via the electron transfer chain. 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).

As the proton concentration increases in the intermembrane space, a strong concentration gradient is built up. The main 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. Peter Mitchell was awarded the 1978 Nobel Prize in Chemistry for his work on chemiosmosis. Later, 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.

Under certain conditions, protons may be allowed to re-enter the mitochondial matrix without contributing to ATP synthesis. This process, known as proton leak or mitochondrial uncoupling, results in the unharnessed energy being released as heat. This mechanism for the metabolic generation of heat is employed primarily in specialized tissues, such as the "brown fat" of newborn or hibernating mammals.

Use in population genetic studies

Main article: mitochondrial genetics

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 mitochondrial DNA sequences to draw conclusions about genealogy and evolution. See: mitochondrial Eve, genetic genealogy, genealogical DNA test.

Recent studies have, however, cast doubt on this hypothesis. Kraytsberg et al. showed that mitochondrial recombination is possible in humans (Science 304:981, May 2004, pubmed #15143273).

Origin

As mitochondria contain ribosomes and DNA, and are only formed by the division of other mitochondria, it is generally accepted that they were originally derived from endosymbiotic prokaryotes. Studies of mitochondrial DNA, which is circular and employs a variant genetic code, show their ancestor was a member of the Proteobacteria [Futuyma 2005]. In particular, the pre-mitochondrion was probably related to the rickettsias.

A few groups of unicellular eukaryotes lack mitochondria: the symbiotic microsporidians, metamonads, and entamoebids, and the free-living pelobionts. On rRNA trees these groups appeared as the most primitive eukaryotes, suggesting they appeared before the origin of mitochondrion, but this is now known to be an artifact of long branch attraction - they are apparently derived groups and retain genes or organelles derived from mitochondria. Thus it appears that there are no primitively amitochondriate eukaryotes, and so the origin of mitochondria may have played a critical part in the development of eukaryotic cells.

Reference

• Douglas J. Futuyma (2005). On Darwin's Shoulders. Natural History 114 (9): 64–68.

External links

• Mitochondrion Reconstructed by Electron Tomography
• Review of evidence addressing whether mitochondria form cellular networks or exist as discrete organelles

See also

• Chemiosmotic hypothesis
• Chloroplast
• Electrochemical potential
• Endosymbiotic theory
• Glycolysis
• Lynn Margulis
• Mitochondrial disease
• Mitochondrial DNA
• Mitochondrial genetics
• Submitochondrial particle

Mitochondrial structure

• Outer membrane
• Inner membrane
• Intermembrane space
• Crista
• Matrix

Fiction

• The midi-clorians of the Star Wars universe are fictional life-forms inside cells that provide the Force. George Lucas took inspiration from the endosymbiotic theory.
• A Wind in the Door posits fictional "farandolae" which are to mitochondria what mitochondria are to cells.
• In the acclaimed novel and video game Parasite Eve, mitochondria are shown to be their own independent organisms, using animals and plants as a form of "transportation," causing a major biological disaster when they decide to set themselves free.

This article contains material from the Science Primer published by the NCBI, which, as a US government publication, is in the public domain at http://www.ncbi.nlm.nih.gov/About/disclaimer.html.

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