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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.


Mitochondrion structure

Cross-section of a mitochondrion, showing: (1) inner membrane, (2) outer membrane, (3) cristae, (4) matrix

Depending on the cell type, mitochondria can have very different overall structures. At one end of the spectrum, the mitochondria can resemble the standard sausage-shaped organelle pictured to the right, ranging from 1 to 4 m in length. At the other end of the spectrum, mitochondria can appear as a highly branched, interconnected tubular network. Observations of fluorescently labelled mitochondria in living cells have shown them to be dynamic organelles capable of dramatic changes in shape. Finally, mitochondria can fuse with one another, or split in two.

The outer boundary of a mitochondrion contains two functionally distinct membranes: the outer mitochondrial membrane and the inner mitochondrial membrane. The outer mitochondrial membrane completely encloses the organelle, serving as its outer boundary. The inner mitochondrial membrane is thrown into folds, or cristae, that project inward. The cristae surface houses the machinery needed for aerobic respiration and ATP formation, and their folded form increases that capacity by increasing the surface area of the inner mitochondrial membrane.

The membranes of the mitochondrion divide the organelle into two distinct compartments: one within the interior of the mitochondrion, called the matrix, and a second between the inner and outer membranes, called the intermembrane space.

The mitochondrial membranes

The outer and inner membranes 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 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 a an unusual phospholipid, cardiolipin , which is usually characteristic of bacterial plasma membranes.

The outer mitochondrial membrane contains numerous integral proteins called porins, which contain a relatively large internal channel (about 2-3 nm) and allow ions and small molecules to move in and out of the mitochondrion. Large molecules, however, cannot traverse the outer membrane. 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 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 important metabolic tasks, such as:

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 Krebs cycle

Main article: Krebs cycle

Each pyruvate molecule produced by glycolysis is actively transported across the inner mitochondrial membrane, and into the matrix where it is combined coenzyme A to form acetyl CoA. Once formed, acetyl CoA is fed into the Krebs cycle, also known as the tricarboxylic acid (TCA) cycle or citric acid 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 Krebs cycle are dissolved in the mitochondrial matrix.

NADH and FADH2: the electron transport chain

Main article: electron transport chain

This energy from NADH and FADH2 is transferred to oxygen (O2) in several steps involving 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). 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 (Pi). 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.

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.

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).

The endosymbiotic theory

Main article: endosymbiotic theory

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 theory — 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.

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