A mitochondrion (plural: Mitochondria) is a rod-shaped organelle that is responsible for the majority of energy metabolism within cells, converting oxygen and nutrients into carbon dioxide and water and synthesizing ATP in the process. ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities. This process of production is called aerobic respiration and is the reason animals breathe oxygen.
The number of mitochondria present in a cell depends upon the metabolic requirements of that cell, and may range from a single large mitochondrion to thousands of the organelles. Mitochondria, which are found in nearly all eukaryotes, including plants, animals, fungi, and protists, are large enough to be observed with a light microscope and were first discovered in the 1800s.
The name of the organelles was coined to reflect the way they looked to the first scientists to observe them, stemming from the Greek words for "thread" and "granule." For many years after their discovery, mitochondria were commonly believed to transmit hereditary information. It was not until the mid-1950s when a method for isolating the organelles intact was developed that the modern understanding of mitochondrial function was worked out.
The elaborate structure of a mitochondrion is very important to the functioning of the organelle. Two specialized membranes encircle each mitochondrion present in a cell, dividing the organelle into a narrow intermembrane space and a much larger internal matrix, each of which contains highly specialized proteins. The outer membrane of a mitochondrion contains many channels formed by the protein porin and acts like a sieve, filtering out molecules that are too big. Similarly, the inner membrane, which is highly convoluted so that a large number of infoldings called cristae are formed, also allows only certain molecules to pass through it and is much more selective than the outer membrane. To make certain that only those materials essential to the matrix are allowed into it, the inner membrane utilizes a group of transport proteins that will only transport the correct molecules. Together, the various compartments of a mitochondrion are able to work in harmony to generate ATP in a complex multi-step process.
Mitochondria are generally oblong organelles, which range in size between 1 and 10 micrometers in length, and occur in numbers that directly correlate with the cell's level of metabolic activity. The organelles are quite flexible, however, and time-lapse studies of living cells have demonstrated that mitochondria change shape rapidly and move about in the cell almost constantly. Movements of the organelles appear to be linked in some way to the microtubules present in the cell, and are probably transported along the network with motor proteins. Consequently, mitochondria may be organized into lengthy traveling chains, packed tightly into relatively stable groups, or appear in many other formations based upon the particular needs of the cell and the characteristics of its microtubular network.
The mitochondrion is different from most other organelles because it has its own circular DNA (similar to the DNA of prokaryotes) and reproduces independently of the cell in which it is found; an apparent case of endosymbiosis. Evolutionists hypothesize through their Endosymbiotic hypothesis that millions of years ago small, free-living prokaryotes were engulfed, but not consumed, by larger prokaryotes, perhaps because they were able to resist the digestive enzymes of the host organism. The two organisms developed a symbiotic relationship over time, the larger organism providing the smaller with ample nutrients and the smaller organism providing ATP molecules to the larger one. Eventually, according to this view, the larger organism developed into the eukaryotic cell and the smaller organism into the mitochondrion. Recent evidence supporting this few demonstrates that the innate mammalian immune system recognises mitochondrial DNA as foreign and mounts an immune response toward it. This, along with mitochondrial DNA genotyping, suggests that its origins are prokaryotic in nature.
Mitochondrial DNA is localized to the matrix, which also contains a host of enzymes, as well as ribosomes for protein synthesis. Many of the critical metabolic steps of cellular respiration are catalyzed by enzymes that are able to diffuse through the mitochondrial matrix. The other proteins involved in respiration, including the enzyme that generates ATP, are embedded within the mitochondrial inner membrane. Infolding of the cristae dramatically increases the surface area available for hosting the enzymes responsible for cellular respiration.
Similarities to Chloroplasts
Mitochondria are similar to plant chloroplasts in that both organelles are able to produce energy and metabolites that are required by the host cell. As discussed above, mitochondria are the sites of respiration, and generate chemical energy in the form of ATP by metabolizing sugars, fats, and other chemical fuels with the assistance of molecular oxygen. Chloroplasts, in contrast, are found only in plants and algae, and are the primary sites of photosynthesis. These organelles work in a different manner to convert energy from the sun into the biosynthesis of required organic nutrients using carbon dioxide and water. Like mitochondria, chloroplasts also contain their own DNA and are able to grow and reproduce independently within the cell.
In most animal species, mitochondria appear to be primarily inherited through the maternal lineage, though some recent evidence suggests that in rare instances mitochondria may also be inherited via a paternal route. Typically, a sperm carries mitochondria in its tail as an energy source for its long journey to the egg. When the sperm attaches to the egg during fertilization, the tail falls off. Consequently, the only mitochondria the new organism usually gets are from the egg its mother provided. Therefore, unlike nuclear DNA, mitochondrial DNA doesn't get shuffled every generation. However, the mt genome has a higher rate of mutation, and consists of only some sixteen thousand base pairs. Hence, it is often more useful to gauge the "genetic distance" between two persons (since sequencing after DNA isolation is much quicker and it provides a finer genetic comb). It is also useful for the study of human evolution. Mitochondrial DNA is also used in forensic science as a tool for identifying corpses or body parts, and has been implicated in a number of genetic diseases, such as Alzheimer's disease and diabetes.
Evolutionary biologists posit the existence of a mitochondrial Eve, the most recent progenitor of all our mitochondrial DNA. That is, this individual is the single individual from whom all persons existing today have a totally matrilineal line of descent. Given two persons A and B, mitochondrial Eve E is "the mother of the mother of ... the mother of A," as well as "the mother of the mother of ... the mother of B" for some numbers n(A) and n(B) of iterations of "the mother of", where n(A) need not equal n(B). This theory does not imply that Eve was the *only* female human existing in her time, just that if a modern human A can trace his ancestry to a contemporary of Eve, F, then any line of descent from F to A must include a male. Evolutionists believe that mitochondrial Eve existed some 160,000 years ago in Africa. Analogously, the Y chromosome is passed only patrilineally, so her male counterpart is referred to as Y-chromosomal Adam.
Because of this slower mutation rate within mitochondrial DNA, human lineages can be more directly traced back through these inherited genes. Computational Cell Biology by Christopher Fall and Eric Marland indicates that this more-direct linkage allows for a correlation to be drawn between descent and certain traits, such as intelligence:
"The lineal descent of this genetic material makes it possible to more directly trace genetic links and indicates a strong correlation with traits, most interestingly intelligence. This indicates a link between genetic strains that can be measured objectively, although many researchers view such endeavors with distaste in fears of encouraging racist prejudices."
- Fall, Christopher, et. al. Computational Cell Biology. Boone, NC: Appalachian State University Press, 2002. 314.