Mitochondria (singular: mitochondrion) are double membrane-bound cell organelles with a typical size of 0.75-3 μm². They are found in most mammalian cells, with notable exceptions including mature erythrocytes. Classically referred to as the ‘powerhouse of the cell’, they are the site of the majority of ATP synthesis and are therefore exceptionally important to function both microscopically and macroscopically.
In this article we will look at the structure and function of mitochrondria, and consider some clinical relevance.
Structure
Mitochondria have an inner and outer membrane, with an intermembrane space between them. The outer membrane contains proteins known as porins, which allow movement of ions into and out of the mitochondrion. Enzymes involved in the elongation of fatty acids and the oxidation of adrenaline can also be found on the outer membrane.
The space within the inner membrane of the mitochondrion is known as the matrix, which contains the enzymes of the Krebs (TCA) and fatty acid cycles, alongside DNA, RNA, ribosomes and calcium granules.
The inner membrane contains a variety of enzymes. It contains ATP synthase which generates ATP in the matrix, and transport proteins that regulate the movement of metabolites into and out of the matrix.
The inner membrane is arranged into cristae in order to increase the surface area available for energy production via oxidative phosphorylation.
Fig 1 – Electron micrograph of a mitochondrion
Function
The mitochondrion is the site of ATP synthesis for the cell. The number of mitochondria found in a cell are therefore a good indicator of the cell’s rate of metabolic activity; cells which are very metabolically active, such as hepatocytes, will have many mitochondria.
Mitochondria also have a role to help maintain the intracellular environment. They:
- Store caspases responsible for triggering apoptosis.
- Are able to transiently store calcium contributing to calcium homeostasis.
In brown adipose tissue mitochondria have an alternative function of heat production using the electron transport chain.
Fig 2 – A mitochondrion with its main features
DNA and Inheritance
Mitochondria replicate their DNA by a process called binary fission and can use this to make multiple copies in one mitochondrion.
Their DNA has maternal lineage which means their DNA is passed from mother to child with little change.
Clinical Relevance – Leber’s Hereditary Optic Neuropathy
As a result of how crucial mitochondria are to the survival of the cell, mitochondrial disorders are rare.
Some general features of mitochondrial disease include exercise intolerance, myopathy and muscle weakness.
The most common mitochondrial disease is Leber’s Hereditary Optic Neuropathy (LHON) which affects the optic nerve causing blurring of central vision and loss of colour vision and carries a risk of developing blindness.
Fig 3 – Mitochondria in the optic nerve
Fig 1 - Electron micrograph of a mitochondrion[/caption]
Fig 2 - A mitochondrion with its main features[/caption]
Fig 3 - Mitochondria in the optic nerve[/caption]