Mitochondria are organelles that convert NADH and NADPH into ATP, have their own DNA, and are suspected to derive from early symbiotic cells. They have a highly folded membrane with ridges called cristae to maximize ATP productivity. Mitochondria play critical roles in metabolism and have specialized functions depending on the cell. Mitochondrial inheritance is exclusively maternal, and mitochondrial degeneration plays a role in aging. Only a few eukaryotic cells lack mitochondria, and studies have not found modern-day versions of ancient free-floating mitochondrial cells.
Mitochondria are the powerhouses of animal and plant cells. They convert blood-borne NADH and NADPH into ATP (adenosine triphosphate), the common energy currency of cellular machinery. The singular of mitochondria is mitochondrion. Mitochondria are strongly suspected to derive from early symbiotic cells living in cooperation with other cells. These organelles have their own DNA, and evolution has already taken millions of years to gradually transfer the DNA from the mitochondria to the cell nucleus, where the rest of the DNA resides.
An average cell has 2,000 mitochondria, which take up about 20% of the total volume. The inside of a mitochondrion, visible under a microscope when stained, contains a highly folded membrane. These folds are called ridges. The cristae exist to maximize the surface area of this membrane, making high ATP productivity possible. Mitochondria are one of the few organelles to have a double membrane structure. Its outer membrane uses phospholipid bilayers and proteins to keep out any molecules with 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, allowing atoms to enter and exit only through active transport.
Mitochondria play critical roles in many aspects of metabolism and can have specialized functions depending on the cell. For example, in the liver, specialized mitochondria process ammonia, a cellular waste product. When some mitochondria break down due to genetic mutations, mitochondrial disease results.
Mitochondria are unusual in that instead of inheriting half of their genetic material from the father and half from the mother, mitochondrial inheritance is exclusively maternal. Instead of just two copies of genes, as in the cell nucleus, mitochondrial genes have five to ten copies available. Although a mitochondrion contains approximately 3000 proteins, its degenerated genome can encode only 37. Mitochondrial degeneration plays an important role in the aging process and is listed as one of the seven primary causes of aging. Therefore, some anti-aging therapies propose to “evacuate” mitochondrial DNA and move it into the cell nucleus, a process that evolution has started but not completed.
There are only a few eukaryotic cells (having a nucleus) that lack mitochondria: microsporidia, metamonads, and archamoebae. Studies have looked at prokaryotic (nucleus-free) cells for possible modern-day versions of ancient free-floating mitochondrial cells, but mitochondria have evolved so extensively in their time in eukaryotic cells that they are likely indistinguishable from their modern cousins.
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