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Mitochondrion

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Mitochondria (singular, mitochondrion), are rod-shaped organelles that can be considered the power generators of the cell, converting oxygen and nutrients into adenosine triphosphate (ATP). ATP is the chemical energy "currency" of the cell that powers the cell's metabolic activities. This process is called aerobic respiration and is the reason animals breathe oxygen. Without mitochondria, higher animals could probably not exist because their cells would only be able to obtain energy from anaerobic respiration, a process much less efficient than aerobic respiration. In fact, mitochondria enable cells to produce 15 times more ATP than they could otherwise, and complex creatures, including humans, need large amounts of energy in order to survive.

Dysfunction of mitochondria has been implicated in a number of genetic diseases, such as Alzheimer's disease and diabetes, as well as a number of rare metabolic disorders.

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, which includes plants, animals, fungi, and protists, are large enough to be observed with a light microscope and were first discovered in the 1800s.

Contents

Etymology

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.

Structure

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 over a certain size. 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 they 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.

Many of the critical metabolic steps of cellular respiration are catalyzed by enzymes that are able to diffuse through the mitochondrial matrix, while 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.

Mitochondrial DNA

The mitochondrion is different from most other organelles because it has its own circular DNA (similar to the ring-shaped DNA plasmids of prokaryotes) and reproduces independently of the cell in which it is found; an apparent case of endosymbiosis. Each mitochondrion may contain multiple copies of this circular genome, which is not associated with histone proteins like nuclear DNA is. Evolutionists hypothesize 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.

However, of the 83 proteins contained in human mitochondria, only 13 are coded for by the mitochondria's own DNA; the rest are provided by the cell containing the mitochondria. As such, mitochondria are unable to exist independently outside the eukaryotic cell, contrary to the evolutionary speculation. Evolutionists therefore must propose that genes from the mitochondrion have been transferred to the nuclear genome of the host eukaryote over time. The same principle applies for non-human mitochondria, although the size of the mitochondrial genome is variable among eukaryotes; some non-human mitochondria possess genomes that code for the majority of their own proteins, while others have smaller genomes that only code for two or three proteins.

Mitochondrial DNA is localized to the matrix, which also contains a host of enzymes, as well as ribosomes for protein synthesis. The ribosomes of mitochondria are different from those of the eukaryotic cell; in structure and function they more closely resemble the ribosomes of prokaryotes like bacteria. Protein synthesis in the mitochondrion may be turned off with the use of drugs that deactivate bacterial protein synthesis, while leaving protein synthesis in the host cell unaffected.

The DNA 'code' of the mitochondrion, which determines which three-base sequences correspond to which amino acid or control function, differs slightly from that of the eukaryotic cell. As an example, while methionine is the starting amino acid in eukaryotic protein synthesis, in bacteria and mitochondria the same start codon codes for n-formylmethionine. Among different eukaryotic mitochondria there are different exceptions to the 'standard' DNA code. In mammals, for example, the UGA codon is a 'stop' codon, but in mammalian mitochondria the UGA codon codes for tryptophan. If all the mitochondria of a eukaryotic cell are destroyed, the cell cannot produce new replacements.

Mitochondrial DNA is also used in forensic science as a tool for identifying corpses or body parts. While such analysis cannot provide the highly specific match that comparison of nuclear DNA can provide, this is balanced by the fact that while each eukaryotic cell normally carries only one copy of its genome, it carries multiple copies of its mitochondrial genome, making chemical extraction and analysis easier.

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 separately within the cell.

Mitochondria Inheritance

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. Also, most animal mitochondria in the sperm are tagged with a marker that results in their destruction if they do enter the egg. 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, so it barely changes through generations, which is useful for the study of the lineages among species that have this type of inheritance.

Ethnic Differences

Because of the matrilineal inheritance of 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."[1]

References

  1. Fall, Christopher, et. al. Computational Cell Biology. Boone, NC: Appalachian State University Press, 2002. 314.
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