An organelle, found in all eukaryotic cells, that provides an efficient apparatus for the production of ATP. The number present varies considerably, being greatest in metabolically active cells. Mitochondria vary in form being generally spherical or threadlike and there is consistency of form in any one cell type. They are approximately 1 - 3 μm across and are generally freely distributed in the cytoplasmic matrix, though tending to concentrate in regions of the cell where the demand for ATP is high. The mitochondrion is surrounded by two membranes. The outer one is smooth but the surface area of the inner one is very greatly increased by infoldings that extend to varying distances into the central compartment, forming shelflike structures or cristae. The central compartment contains a colloidal matrix with a fine fibrillar structure. Ri-bosomes of the smaller 70S type and circular DNA molecules, both characteristic of prokaryotic cells (see serial endosymbiotic theory
), are present, as are DNA polymerase enzymes and enzymes of the TCA cycle. In cell cultures, mitochondria display twisting and wriggling movements and the cristae change their shape when respiratory activity is stimulated.
The inner surface of the crista has many knoblike structures uniformly distributed in the membrane. These are composed of a protein, F1 capable of transferring phosphate to ADP (i.e. an ATP synthase enzyme). Electron transfer compounds, required for the eventual formation of water from hydrogen ions and oxygen, are situated within the membrane of the crista. The energy made available by electron transfer is incorporated into ATP molecules formed by the activity of the F1 protein particles. The outer membrane is freely permeable to water and soluble ions but the inner membrane has limited permeability and contains carrier compounds that transfer specific metabolites. One such specific carrier allows a molecule of ADP to enter only if a molecule of ATP passes in the reverse direction. Another concentrates calcium ions in the matrix. Mitochondria can only arise by fission of preexisting ones and therefore must be transmitted to daughter cells during 'mitosis. The DNA and ribosomes they contain enable them to synthesize many of their constituent molecules.