Mitochondrion

A mitochondrion (plural mitochondria; Fig. 1) is a membrane-enclosed cellular organelle. Mitochondria are distributed through the cytosol of most eukaryotic cells. Their main function is to convert the potential energy (via electron transport) of food molecules into ATP (the universal energy currency of the cell). They are composed of folds called cristae which give a much increased surface area on which chemical reactions can occur.

Table of contents

1 Structure
2 Energy conversion
3 Use in genetic studies

Structure

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 synthetase 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 respiration of pyruvic acid 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

The energy from food molecules (e.g., glucose) is used to produce NADH and FADH₂ molecules in glycolysis and the citric acid 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 (using energy) is required to pump 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.

Use in 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 often used it to draw conclusions about geneaology and evolution. More at mitochondrial Eve.

Mitochondria are unusual among organelles in that they contain some of their own genetic material and some of the apparatus required to express it. This led to the development of the endosymbiosis theory of the origin of the mitochondrion, proposing that they were once independent bacteria that colonized the interior of the ancient precursor of all eukaryotic life.