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. 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 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.
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 of mitochondrion, 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)

As discussed in the later section #Interconnection, propagation, and dynamics, mitochondria are in constant flux. The scope of the current section, however, is to describe their general shape patterns rather than dynamics. For simplicity, assume for this section that we have a snapshot of the mitochondrion when it's not in the middle of fission of fusion.

Size parameters: length and tubular radius

Most mitochondria have a tubular shape, i.e., a cylindrical shape with rounded ends. The mitochondrion's shape and size are roughly described using these two parameters:

• The length or (cylinder) height: This is the length along the mitochondrion's tubular axis. When modeling the mitochondrion as a cylinder, this is the height of the cylinder.
• The tubular radius: This is the radius of the tubular cross-section, which looks like a circular disk. When modeling the mitochondrion as a cylinder, this is the radius of the cylinder.

Note that the term diameter is ambiguous as it could either refer to the length of the mitochondrion (as it is the diameter in the sense of being the maximum possible length within the mitochondrion) or to the diameter of the tubular cross-section (i.e., twice the tubular radius). It is therefore better to use the terms "length" and "tubular radius" that are more unambiguous. If using diameter to refer to the tubular diameter, it is better to say "tubular diameter" to make it clear.

Qualia of shape: spherical / globular shape versus the more typical tubular shape

The shape of the mitochondrion can be roughly gauged by taking the ratio of the length to the tubular radius; this ratio should not be much less than 2.

• A ratio of around 2 indicates that the mitochondrion is close to spherical. Such mitochondria are called spherical or globular mitochondria.
• A ratio greater than 2 indicates a more typical tubular (elongated) mitochondrion shape. A typical ratio is around 4; for instance, that's the ratio seen in cardiac cells in one study.

The shape of a mitochondrion has significance for its functionality. The tubular shape is tied to maintaining membrane potential, which is a key aspect of mitochondrial function, and a spherical/globular shape is often seen as a result of a loss of membrane potential. In general, the tubular radius of a mitochondrion is less subject to change than its length, as most dynamics (such as fission and fusion) happen along the tubular axis.

Value ranges for size parameters

• The length is generally at least 1 $\mu m$ and at most 4 $\mu m$.
• The tubular radius is generally at least 0.5 $\mu m$ and at most 1 $\mu m$.

Cross-sectional surface area

Cross-sectional surface area along the tubular cross-section is important because cross-sections are the things we put under the microscope. The cross-section is approximately a circular disk whose radius is the tubular radius.

The formula for cross-sectional surface area is $\pi r^{2}$ where $\pi$ is the constant pi (about 3.14159) and $r$ is the tubular radius.

Using the above value ranges, we have:

• Low-end estimate using $r=0.5\mu m$: $0.785\mu m^{2}$
• High-end estimate using $r=1\mu m$: $3.142\mu m^{2}$

Rounding, we often say that the cross-sectional surface area of mitochondria is in the range of 0.75 to 3 $\mu m^{2}$.

Total surface area

The surface area of mitochondria (area of the outer mitochondrial membrane) can be modeled by approximating it as a capped cylinder where the radius $r$ is the radius of the tubular cross-section and the height $h$ is the length (i.e., the length along its tubular axis).

The formula for surface area of a capped cylinder is $2\pi r(r+h)$.

Using the value ranges above, we have:

• Low-end estimate using $r=0.5,h=1$: $4.7\mu m^{2}$
• High-end estimate using $r=1,h=4$: $31\mu m^{2}$

Rounding, we can say that the surface area of mitochondria is about 5 to 30 $\mu m^{2}$.

Volume

We can estimate the volume of the mitochondrion as a cylinder with radius equal to the tubular radius and height equal to the length.

The formula for the volume is $\pi r^{2}h$ where $r$ is the radius (tubular radius) and $h$ is the height (length).

We therefore obtain these estimates:

• Low-end estimate using $r=0.5,h=1$: $0.785\mu m^{3}$
• High-end estimate using $r=1,h=4$: $12.57\mu m^{3}$

A cubic micrometer ($\mu m^{3}$) is the same as a femtoliter, or $10^{-15}$ liters.

Mass

Since most cellular matter is approximately as dense as water, we can approximate the density of mitochondria using the density of water, which is about 1 gram per milliliter. Based on the volume estimate above, we get that the mass of a mitochondrion is approximately between 0.785 and 12.57 picograms, where a picogram is $10^{-12}$ grams.

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, the dimensions of an individual mitochondrion are 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 approximately equal to the tubular radius and a little less than the length of the mitochondrion. 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 (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).

Comparison with molecules

A water molecule's size is about $2.75*10^{-10}m$, which is a little under 1/1000 of the low end for the tubular radius of a mitochondrion (0.5 $\mu m$). Since this is in just one dimension (length) we need to square when thinking about cross-sectional area and cube when thinking about volume. Roughly speaking, a cross section has space for the order of millions of water molecules, and the mitochondrion has space for the order of billions of water molecules.

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 of mitochondrion	~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

Forms in which mitochondria are studied in the lab

• Mitochondria may be studied in situ, in vivo in living cells.
• Mitochondria can be studied as isolated mitochondria, removed from the cell but with their outer mitochondrial membrane still intact.
• Mitochondria can be studied as mitoplasts, which are like isolated mitochondria but with the outer mitochondrial membrane removed.

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

The size of a mitochondrion ($0.5-3\mu m$) is slightly higher than the best possible resolution of light microscopes (about $0.2\mu m$). 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 is used. 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.