# 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.

## Definition

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.[1] Some mitochondria are spherical (globular); the transition to spherical/globular shape generally happens due to unusual circumstances such as the loss of membrane potential.[2]
Size ${\displaystyle 0.5-3\mu m}$ diameter per mitochondrion. In some cells with significant energy needs (such as human heart cells), they could together take up to 1/4 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.[3][4]
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
Biogenesis See mitochondrial biogenesis for more. Mitochondria divide by mitochondrial fission. This is a type of 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. Mitochondria can also fuse together (mitochondrial fusion); the balance of fission and fusion determines the mitochondrial dynamics, i.e., the evolution of mitochondrial networks.
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.
Quality control See mitochondrial quality control for more.

## 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)

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

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: ${\displaystyle 0.5\mu m}$ tubular radius and ${\displaystyle 2\mu m}$ diameter).[1] NOTE: "diameter" in this case refers to the maximum length, which corresponds to the cylinder's height, not the diameter of its tubular cross-section.

### Surface area

The surface area of mitochondria (area of the outer mitochondrial membrane) is about ${\displaystyle 0.75}$ to ${\displaystyle 3}$ ${\displaystyle \mu m^{2}}$. 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 ${\displaystyle 10-100\mu m}$ 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 ${\displaystyle 0.4-0.7\mu m}$, 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 ${\displaystyle 0.2\mu m}$) but their internal structures cannot be clearly identified. Electron microscopes (that cannot be used on live cells) need to be used to study the structure of mitochondria well.

### Comparison with other organelles

Mitochondria are among the bigger of the cellular organelles. The mitochondrion, nucleus, and Golgi bodies are the cellular organelles big enough to be identified using a light microscope, and for that reason are among the oldest organelles to be identified and studied (even prior to the advent of electron microscopes).

## 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

## Interconnection, propagation, and dynamics

### Mitochondrial networks, fission and fusion

In most cells, the mitochondria are interconnected as a network called the mitochondrial network. The mitochondria in the network align along their tubular axis. The network could be a single curve (without branching) or it could have branches, depending on details specific to the cell.

Mitochondria are constantly undergoing mitochondrial fission (one mitochondrion splitting into two, that then go on to form adjacent nodes in the mitochondrial network) and mitochondrial fusion (two adjacent mitochondria in the mitochondrial network fusing into one).

The relative rates of fission and fusion determine how the mitochondrial network evolves over time.

## Laboratory analysis

### Analysis of mitochondria in living cells using a light microscope (in situ, in vivo)

The size of a mitochondrion (${\displaystyle 0.5-3\mu m}$) is slightly higher than the best possible resolution of light microscopes (about ${\displaystyle 0.2\mu m}$). The internal structures of mitochondria are too small to be visible under light microscopes (for instance, the intermembrane space is 20 nm in thickness, which is 1/10 of the resolution that light microscopes afford).

In order to make the mitochondria stand out clearly under the light microscope, a potential-sensitive dye such as J1c, TMRE, or TMRM. The dye picks up on the electrochemical potential gradient across the inner mitochondrial membrane and so each mitochondrion shows up as a dot with the color of the dye (under the light microscope).

The light microscope and dye can be used for mitochondria in live cells, and in particular can be used to look at mitochondrial networks and mitochondrial dynamics (the change to mitochondrial networks over time).

### Analysis of internal structure of mitochondria (outside living cells) using an electron microscope

An electron microscope is needed to achieve the resolution necessary to study the internal structures of the mitochondrion. Electron microscopes tend to destroy living cells, so they cannot be used to study the dynamics of mitochondria in living cells.