Mitochondria are rod-shaped organelles that are found in all cells except red blood cells and terminal keratinocytes. The size, shape, and number of mitochondria depend on specific functions performed by the cell.



Size of mitochondria

The width of mitochondria is 0.5 to 1 micrometer which is fairly constant in all cells but the length is highly variable and in some cells, they may be 10 micrometers long.

Shape of mitochondria

they exist in different shapes which are
thread like


The number is variable in different cells. Mitochondria are more abundant in cells that are engaged in protein synthesis, membrane transport, and cellular contraction.


they are distributed in the cytoplasm of the cell but in some cells, they are found in those areas of the cell where the demand for the supply of energy is higher. For example, Proximal convoluted tubules of a nephron.

Structures of mitochondria


An electron microscope shows that it is surrounded by a double membrane in which the outer membrane is smooth while the inner membrane invaginates as a series of characteristic extensions called cristae.
Cristae are shelf like in shape but are tubular in those cells in which synthesis of steroid hormones occur. The Cristae main function is to increase the surface area of the inner mitochondrial membrane. The number of cristae in mitochondria also depends on the specific function of the cell.
For example
Cristae are more abundant in those cells which are engaged in high metabolic activity.
Space is present between the double membrane of mitochondria which is known is inter-membrane space.
The outer membrane of it contains transmembrane proteins known as porins. These porins form the channels through which ions and small molecules can enter to inter-membrane space.
The inner membrane contains cardiolipin phospholipid. These phospholipids make the inner membrane impermeable ions.
The inner membrane also contains biochemical enzymes for the electron transport chain and oxidative phosphorylation. ATP synthase is the most important biochemical enzyme in the inner membrane of mitochondria.
ATP synthase has a globular head and racquet-shaped structure. The head of ATP synthase is 10 nm in diameter.


The internal cavity of it is filled with a mitochondrial matrix which is electron-dense material. The matrix contains proteins that represent the enzymes that are involved in the beta-oxidation of the fatty acids, amino acids oxidation, and citric acid cycle.

The Matrix Also contains the ribosomes, transfer RNA, messenger RNA, and a circular molecule of DNA. and also contains electron dense bodies that are known is matrix granules. These granules are composed of phospholipoproteins. These granules can store calcium and other cations.


The DNA of it is double-stranded and circular in shape. The strands of DNA are synthesized in the matrix and their duplication is independent of nuclear DNA replication. Mitochondrial encodes only 13 proteins which are synthesized by mitochondrial ribosomes, messenger RNA, and transfer RNA.
But most of the mitochondrial proteins are encoded by the nuclear DNA.
they are self-replicating organelles which means that mitochondria reproduce and increase in number by fission. They enlarge in size and replicate their DNA and divide into two equal halves.

History of mitochondria

Richard Altman was the first who established mitochondria as a cell organelle and named them bioblasts. Carl Benda gives the name mitochondria to it in 1898.

In 1952 high-resolution electron micrographs appeared that shows that mitochondrial is surrounded by a membrane and also showed the second membrane inside the mitochondria.

In 1957 Philip coined the term powerhouse of the cell to mitochondria.

In 1967 ribosomes were discovered in the mitochondria.

In 1968 methods for mapping the mitochondrial genes were developed.


The main function of the mitochondria is the production of energy for cell activities and therefore also known as the powerhouse of the cells. Mitochondrian synthesizes ATP which is supplied to the cell for energy.
Another function of mitochondria is to keep the cytoplasmic concentration of calcium ions at a low level.
when the concentration of calcium ion rises the matrix granules pump the calcium ions into mitochondria to maintain its level.

Mitochondrial diseases

In a range of human diseases, damage or dysfunction of mitochondria is an important factor due to its influence on cell metabolism. These diseases often present themselves as neurological disorders.

Mutation in the mitochondrial DNA causes diseases such as Kearns-Sayre syndrome, Melas syndrome, and Leber’s hereditary optic neuropathy.  in many cases, these diseases are transmitted by a female to his offspring.

Nuclear gene defect can cause dysfunction of mitochondrial proteins. The cases in which the dysfunction of mitochondrial proteins occurs are Friedreich’s ataxia, hereditary spastic paraplegia, and Wilson diseases.

DNA repair

Oxidative DNA damage can be repaired by mitochondria by a mechanism that is analogous to those occurring in the nucleus. The DNA repair in mitochondria is done through many pathways.

The pathways through which DNA repair occur include base excision repair, double-strand break repair, a direct reversal repair, and mismatch repair.

Sometimes the DNA damages are bypassed by translesion synthesis.

The base excision repair pathway includes many enzymes catalyzed steps. The enzyme-catalyzed steps in the base excision repair are,

1.  Recognition of damaged DNA base
2.  Excision of damaged DNA base
3.  Removal of the resulting a basic site
4.  End processing
5.  Gap filling
6.   Ligation.

Mitochondria and aging

they are sometimes also known as with the name the powerhouse of the cell. While performing the function of powerhouse there may be a leakage of high energy electron in the respiratory chain that forms reactive oxygen species. This process results in oxidative stress in the mitochondria.

hypothesized links are present between  Oxidative stress and aging. The effect of oxidative stress is shown by the mitochondrial free radical theory of aging.


it is an important cytoplasmic organelle present in the cell, It is present in all the cells of the body except some cells such as red blood cells. The shape and size of the mitochondria depend on the type of cell. the size of this organelle ranges from 0.1 to 1 micrometer. This organelle exists in many shapes such as spherical, cylindrical, and thread-like. the function of mitochondria is to produce energy for various cellular activities.