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

  • 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. Note that large molecules are excluded from entering this membrane.
  • The inner membrane, folded with cristae (giving it a very large surface for chemical reactions to take place on), encloses the matrix (the internal fluid of the mitochondrion). It contains several protein complexes. Stalked particles are found on the cristae and it is here that the energy store is formed.
  • The intermembrane space between the two membranes contains enzymes that use ATP to phosphorylate other nucleotides.
Figure 1 : Mitochondrion.

1. Inner membrane. 2. Outer membrane. 3. Cristae. 4. Matrix.

Mitochondrion literally means 'thread granule', which is what they look like under a microscope; tiny rod-like structures present in the cytoplasm of all cells. The matrix contains soluble enzymes that catalyze (enable) the respiration of pyruvic acid and other small organic molecules. Parts of the Krebs Cycle occur in Mitochondria. It also contains several copies of the DNA (5-10 circular DNA molecules per copy) of the mitochondria, 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 considered to be highly modified symbiotic bacteria, probably belonging to the alpha-proteo bacteria (with the closest known candidate being Rickettsia, the causing agent of typhus), and are believed to have been incorporated only once (compare chloroplast).

Energy conversion
The energy from the food molecules (e.g., glucose) was converted to NADH and FADH molecules in glycolysis and the Citric acid cycle. That energy is transferred to oxygen (O2) in several steps. The protein complexes in the inner membrane (NADH dehydrogenase, Cytochrome C reductase, Cytochrome C oxidase) which perform the transfer use the released energy to pump protons (H+) against a gradient (the concentration of protons in the matrix is higher than that in the intermembrane space). 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 (Pi). 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 discovery of the working mechanism of ATP synthase.