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A cross-section of a mitochondrion under an electron microscope

Mitochondria (singular- Mitochondrion) are membrane bound organelles (double membrane structure) that can be found in animal and plant cells, that carry out oxidative phosphorylation, to produce ATP. What is more, mitochondria produce the majority of ATP used by eukaryotic organisms and are often referred to as the powerhouses of the cell. Furthermore, due to the fact that mitochondria are the site ATP synthesis, there is often a linear relationship between the number of mitochondria in a cell and the cells ATP requirements e.g. a muscle cell uses vast amounts of ATP and thus often contains many mitochondria to adhere to this requirement and maintain function. A further point that must be brought to attention is that mitochondria contain their own DNA (mostly circular), referred to as mtDNA. The size of mitochondrial DNA and its percentage of total cellular DNA varies between species. In mammalian cells, only about 1% of the total cellular DNA is composed of mitochondrial DNA whereas in other organisms (for example in the egg cells of amphibians) there is a much higher percentage of mitochondrial DNA. Human mitochondrial DNA consists of 16,569 base pairs coding for 13 proteins. In humans, mitochondrial DNA is inherited from the mother because an egg cell has many more mitochondria than a sperm cell. Mitochondria are semiautonomous organelles, depending on the host cell for their existence[1][2]. The outer membrane of the mitochondria contains porins which are small protein membrane channels which are permeable to most small molecules so molecules which are 10 kDa or smaller can diffuse through.

Diagram of ATP
ATP is hydrolysed to ADP and inorganic phosphate (Pi) in mitochondria. This process is essential to provide the 'energy currency' of cells and is maximised by the mitochondria's specialised structure.




The mitochondrion is the site of the Krebs cycle and the electron transport chain in eukaryotic organisms. It has a variable diameter from 0.5 to 1 micrometer thus can be easily seen under a light microscope. Using time-lapse micro-cinematography, it has been established that mitochondria can alter their shape continuously, and are also able to fuse and separate with other mitochondria[4]. It is surrounded by two phospholipid membranes: the outer and inner membrane. The inner membrane is folded inwards to form cristae and it is the location where  the electron transport chain occur. On the other hand, the outer membrane is the envelope that holds all the organelles and it is relatively permeable to small molecules.


Mitochondrial diseases


A type of Cardiovascular disease which is associated with gene abnormalities in mtDNA and/or nulcear DNA (which encode some of the mitochondrial proteins) is Familial Dilated Cardiomyopathy[11]. This disease is caused as the result of lack of metabolite passage across the inner membrane space[12], due to a non-silent mutation which results in the transport proteins having a changed genomic sequence.


mtDNA also plays a key role in carcinogenesis.

mtDNA structure

mtDNA is composed of 16 regions:15 coding and one non-coding, without introns. The non-coding region of mtDNA genome is called D-loop region and is 1124 bp in size. Since D-loop region is a hot spot for different mutations such as insertion, deletion, point mutation, it may play a major role in the developing of a great number of cancerous diseases such as gastric tumours, breast cancer, ovarian carcinomas, prostatic cancer, lung and head cancer etc.


Whilst the exact origins of the Mitochondrion we see in Eukaryotes today are not entirely clear, and the evidence to move any hypothesis into theory is likely lost to time. However the hypothesis that seems to be the most popular is that Mitochondria are endosymbiotes, originally proto-bacteria that have been incorporated into other cells[14]


  3. Bruce Alberts (et al)-2007: pg815
  5. Berg J.M, Tymoczko J.L, Stryer (2012) Biochemistry, Seventh Edition, New York: WH Freema.. pg 543
  6. Alberts, Johnson, Lewis, Raff, Roberts, Walter (2008) Molecular Biology of the cell, Fifth Edition, pg 815
  7. B Alberts, A Johnson, J Lewis, D Morgan, M Raff, K Roberts, P Walter. Molecular Biology of the cell. Sixth Edition. Printed in the United States of America. Garland Science. 2015. Chapter 14.
  8. NCBI. P F. Chinnery. Gene Reviews. Mitochondrial Disorders Overview. June 8, 2000. Last update: August 14, 2014. 19/11/2016.
  10. Mitochondrial Disease (MD, Mitochondrial Myopathies (MM) MedicineNet, 2016, [cited 04/12/2016]
  14. Woese C, Kandler O, Wheelis M. Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the National Academy of Sciences. 1990;87(12):4576-4579.
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