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Mitochondria are the power plants of animal and plant cells. They convert bloodborne NADH and NADPH into ATP (adenosine triphosphate), the common energy currency of cellular machinery. The singular of mitochondria is mitochondrion. It is strongly suspected that mitochondria derive from early symbiotic cells living in cooperation with other cells. These organelles have their own DNA, and evolution has already spent millions of years incrementally transferring DNA from the mitochondria to the nucleus of the cell, where the rest of the DNA resides.
An average cell has 2,000 mitochondria, which occupy about 20% of the total volume. The interior of a mitochondrion, observable under a microscope when stained, contains a highly folded membrane. These folds are called cristae. The cristae exist to maximize the surface area of this membrane, making high throughput of ATP possible. The mitochondria is one of few organelles to have a double-membrane structure. Its outer membrane uses phospholipid bilayers and proteins to keep out any molecules of an atomic weight greater than 5000. Special proteins larger than this can only reach the interior of the mitochondrion through active transport. The inner membrane is the most impermeable of all, only letting atoms in and out through active transport.
Mitochondria play critical roles in many aspects of metabolism, and may have specialized functions depending on the cell. For example, in the liver, specialized mitochondria process ammonia, a cellular waste product. When certain mitochondria break down due to genetic mutations, mitochondrial diseases result.
Mitochondria are unusual because instead of inheriting half their genetic material from the father and half from the mother, mitochondrial inheritance is exclusively maternal. Instead of just two copies of the genes, as in the cellular nucleus, mitochondrial genes come in five to ten copies. Although a mitochondrion contains about 3000 proteins, its degenerate genome only can code about 37. Mitochondrial degeneration plays an important role in the aging process, and is referred to as one of the seven primary causes of aging. As such, some anti-aging therapies propose "evacuating" mitochondrial DNA and moving it to the cellular nucleus, a process that evolution has started but not completed.
There are only a few eukaryotic (nucleus-possessing) cells which lack mitochondria — the microsporidians, metamonads, and archamoebae. Studies have gone over prokaryotic (nucleus-lacking) cells looking for possible present-day versions of ancient free-floating mitochondrial cells, but the mitochondria have evolved so extensively in their time in eukaryotic cells that they are probably indistinguishable from their modern-day cousins.
Mitochondria are not the only organelles with their own DNA. Chloroplasts also have their own DNA. Interestingly, in most flowering plants (unlike pines) the chloroplast DNA does not come from pollen -- the male line. So where does it come from?