A mitochondrion (/ˌmaɪtəˈkɒndriən/;[1] pl mitochondria) is an organelle found in
the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have
a double membrane structure and use aerobic respiration to generate adenosine
triphosphate (ATP), which is used throughout the cell as a source of chemical
energy.[2] They were discovered by Albert von Kölliker in 1857[3] in the voluntary
muscles of insects. The term mitochondrion was coined by Carl Benda in 1898. The
mitochondrion is popularly nicknamed the "powerhouse of the cell", a phrase coined
by Philip Siekevitz in a 1957 article of the same name.[4]
Some cells in some multicellular organisms lack mitochondria (for example, mature
mammalian red blood cells). A large number of unicellular organisms, such as
microsporidia, parabasalids and diplomonads, have reduced or transformed their
mitochondria into other structures.[5] One eukaryote, Monocercomonoides, is known
to have completely lost its mitochondria,[6] and one multicellular organism,
Henneguya salminicola, is known to have retained mitochondrion-related organelles
in association with a complete loss of their mitochondrial genome.[6][7][8]
Mitochondria are commonly between 0.75 and 3 μm2 in cross section,[9] but vary
considerably in size and structure. Unless specifically stained, they are not
visible. In addition to supplying cellular energy, mitochondria are involved in
other tasks, such as signaling, cellular differentiation, and cell death, as well
as maintaining control of the cell cycle and cell growth.[10] Mitochondrial
biogenesis is in turn temporally coordinated with these cellular processes.[11][12]
Mitochondria have been implicated in several human disorders and conditions, such
as mitochondrial diseases,[13] cardiac dysfunction,[14] heart failure[15] and
autism.[16]
The number of mitochondria in a cell can vary widely by organism, tissue, and cell
type. A mature red blood cell has no mitochondria,[17] whereas a liver cell can
have more than 2000.[18][19] The mitochondrion is composed of compartments that
carry out specialized functions. These compartments or regions include the outer
membrane, intermembrane space, inner membrane, cristae, and matrix.
Although most of a eukaryotic cell's DNA is contained in the cell nucleus, the
mitochondrion has its own genome ("mitogenome") that is substantially similar to
bacterial genomes.[20] This finding has led to general acceptance of the
endosymbiotic hypothesis - that free-living prokaryotic ancestors of modern
mitochondria permanently fused with eukaryotic cells in the distant past, evolving
such that modern animals, plants, fungi, and other eukaryotes are able to respire
to generate cellular energy.[21]
Structure
Simplified structure of a mitochondrion
Mitochondria may have a number of different shapes.[22] A mitochondrion contains
outer and inner membranes composed of phospholipid bilayers and proteins.[18] The
two membranes have different properties. Because of this double-membraned
organization, there are five distinct parts to a mitochondrion:
The outer mitochondrial membrane,
The intermembrane space (the space between the outer and inner membranes),
The inner mitochondrial membrane,
The cristae space (formed by infoldings of the inner membrane), and
The matrix (space within the inner membrane), which is a fluid.
Mitochondria have folding to increase surface area, which in turn increases ATP
(adenosine triphosphate) production. Mitochondria stripped of their outer membrane
are called mitoplasts.
Outer membrane
The outer mitochondrial membrane, which encloses the entire organelle, is 60 to 75
angstroms (Å) thick. It has a protein-to-phospholipid ratio similar to that of the
cell membrane (about 1:1 by weight). It contains large numbers of integral membrane
proteins called porins. A major trafficking protein is the pore-forming voltage-
dependent anion channel (VDAC). The VDAC is the primary transporter of nucleotides,
ions and metabolites between the cytosol and the intermembrane space.[23][24] It is
formed as a beta barrel that spans the outer membrane, similar to that in the gram-
negative bacterial membrane.[25] Larger proteins can enter the mitochondrion if a
signaling sequence at their N-terminus binds to a large multisubunit protein called
translocase in the outer membrane, which then actively moves them across the
membrane.[26] Mitochondrial pro-proteins are imported through specialised
translocation complexes.
The outer membrane also contains enzymes involved in such diverse activities as the
elongation of fatty acids, oxidation of epinephrine, and the degradation of
tryptophan. These enzymes include monoamine oxidase, rotenone-insensitive NADH-
cytochrome c-reductase, kynurenine hydroxylase and fatty acid Co-A ligase.
Disruption of the outer membrane permits proteins in the intermembrane space to
leak into the cytosol, leading to cell death.[27] The outer mitochondrial membrane
can ass
