Mitochondrion

This article describes an organelle, a cell component with its own distinctive structure and function. In eukaryotic cells, this is bounded by its own membrane, which is a lipid bilayer made of phospholipid.
Unless otherwise specified, information about this organelle is about its in situ occurrence in vivo, i.e., its occurrence in its usual location in living cells.

Mitochondrion (plural mitochondria, also historically called bioblast) is an organelle found in eukaryotic cells whose primary function is to carry out aerobic respiration, i.e., convert energy from a relatively more hard-to-use form (pyruvates) to energy stored in the form of ATP.

Summary

Item	Value
Type of organisms whose cells contain mitochondria	Eukaryotic cells only, including plant cells, animal cells, and the cells of protists and fungi; some of the more primitive eukaryotic cells (only in unicellular organisms) lack mitochondria; some of them have other similar (and likely evolutionary related) structures such as mitosomes or hydrogenosomes. For more, see most eukaryotic organisms have mitochondria in most of their cells.
Type of cells within the organisms that contain mitochondria	All cells except red blood cells in mammals (other vertebrates do have mitochondria in their red blood cells). For more, see most eukaryotic organisms have mitochondria in most of their cells.
Number of mitochondria per cell	1 to 1000s, depending on the energy needs of the cell
Shape	Most mitochondria are tubular (cylindrical, with rounded ends). The tubular radius is usually significantly less than half the diameter, e.g., about 1/4 in one example. Some mitochondria are spherical (globular); the transition to spherical/globular shape generally happens due to unusual circumstances such as the loss of membrane potential.
Size	$0.5-3\mu m$ diameter per mitochondrion. In some cells, they could together take up to 1/5 of the cell volume
Interconnection	Mitochondria are usually networked with each other, forming a mitochondrial network. The extent to which the mitochondria are fused with each other depends on the relative rates of fission and fusion. The study of the shape of mitochondrial networks is called mitochondrial morphology and the study of the change in these over time is called mitochondrial dynamics.
Location within cell	Could be found anywhere in the cell, depending on the cell's energy needs. For instance, in sperm cells, mitochondria are found in the tail to provide power for propulsion.
Structural components	outer mitochondrial membrane, intermembrane space, inner mitochondrial membrane, cristae, mitochondrial matrix
Chemical constituents	lots of proteins
Control of the entry and exit of materials	Membranes (hydrophilic/hydrophobic issues), the TIM/TOM complex
Function	cellular respiration, i.e., ATP synthesis Control of cell cycle Cellular differentiation Cell growth Cell death
Propagation	Mitochondria divide by binary fission, just like bacteria (this is consistent with the endosymbiotic theory of mitochondrial origin). The process may be regulated to be coordinated with the cell cycle. The nature of regulation depends on the organism and cell type.
Evolutionary origin	endosymbiotic theory of mitochondrial origin -- the mitochondria are evolutionary descendants of endosymbionts (organisms living in the cell in a mutually beneficial relationship with their host)
Variation between species	Mammals do not have mitochondria in their red blood cells. In most species, mitochondria is inherited maternally, but there are some species where it is inherited paternally. The shape and structure of mitochondria, and the code of the mitochondrial genome, vary between species.
Variation between individuals within a species	Mitochondria have their own DNA which (in most eukaryotic organisms) is inherited from the mother. In addition, some of the behavior of the mitochondria is controlled by nuclear DNA.
Variation between cells within an organism	The number and location of mitochondria depend on the cell's energy needs.

Size and shape

BACKGROUND INFORMATION ON SIZE MEASURES: Size measures for items related to cells (explains the units and orders of magnitude for various sizes)| Relation between ratios of lengths, areas, and volumes (a general geometric fact relating figures of similar shape and different sizes)

Diameter and tubular radius

The diameter of mitochondria is in the range $0.5-3\mu m$ (microns), where one micron is $10^{-6}m$.

Most mitochondria have a tubular shape, i.e., a cylindrical shape with rounded ends, with the tubular radius well under half the diameter (one measurement of cardiac cells found it to be 1/4 the diameter: $0.5\mu m$ tubular radius and $2\mu m$ diameter). NOTE: "diameter" in this case refers to the maximum length, which corresponds to the cylinder's height, not the diameter of its tube.

Surface area

The surface area of mitochondria (area of the outer mitochondrial membrane) is about $0.75$ to $3$ $\mu m^{2}$. The approximate range can be deduced as the estimated surface area of a capped cylinder, using the estimates for the diameter (height) and tubular radius.

Comparison with cell sizes

Comparison with prokaryotic cells: The mitochondrion size is roughly the lower end of the size range for prokaryotic cells (which is explained by their evolutionary origin; see endosymbiotic theory of mitochondrial origin).

Comparison with eukaryotic cells: Mitochondria are found in eukaryotic cells (not prokaryotic cells) which have a diameter in the $10-100\mu m$ range. Thus, each mitochondrion has about 1/100 to 1/10 the diameter of the whole cell and hence about 1/10^6 to 1/10^3 the volume of the whole cell.

The total volume of the mitochondria depends on the number of mitochondria as well. It could be as large as 1/5 (or 20%) of cell volume.

Comparison with wavelengths of light and implications for visibility under microscopes

The wavelength of visible light is in the range $0.4-0.7\mu m$, which is at the lower end of the diameter range for mitochondria. Thus, mitochondria can be viewed with light microscopes (whose resolution is limited to $0.2\mu m$) but their internal structures cannot be clearly identified. Electron microscopes need to be used to study the structure of mitochondria well.

Physical structure

The mitochondrion has the following structural components:

Component	Thickness	Thickness as percentage of mitochondrial diameter (approx.). -- double this value to account for it being on both sides)
outer mitochondrial membrane	60 - 75 angstrom (6 to 7.5 nm)	0.2-1%
intermembrane space	~200 angstrom (20 nm)	1-4%
inner mitochondrial membrane	70 angstrom (7 nm)	0.2-1%
cristae
mitochondrial matrix