By Dilyara Sabirova - Biological Natural Sciences Student @ St John's College, Cambridge
In 1957, in his article for the Scientific American, a cell biologist Philip Siekevitz coined the term “powerhouse of the cell” to describe the function of mitochondria. However, this assignment of their cellular function is vastly simplified, as the organelle is involved in a wide variety of cellular processes beyond energy metabolism. Whilst acknowledging the great functional importance of mitochondria in energy generation (in the end, mitochondrial oxidative phosphorylation does generate 18 times more ATP per glucose molecule compared to cytosolic glycolysis), this essay will instead focus on organelle’s participation in other cellular tasks, with the aim of highlighting its role in a wide range of physiological processes.
Contrary to the common depiction of mitochondria as isolated bean-like organelles, in most cells, mitochondria form a highly dynamic network, where they continually undergo fusion and fission, mediated by large GTPases. Such morphological transitions are thought to play a role in mitochondrial quality control. Evidence further shows that there is bidirectional communication between mitochondrial reticula and the rest of the cell, making mitochondria an integral part of cell signaling cascades, that link organelle’s function and dynamics to the regulation of cell metabolism, development and cell death. This evidence, therefore, defines mitochondria as a central platform in the execution of diverse cellular events.
Furthermore, mitochondria are central players in thermoregulation. Chemiosmotic hypothesis, proposed by Peter Mitchell in 1961, states that mitochondrial electron transport is coupled to ATP synthesis via the proton gradient. However, uncoupling proteins can short-circuit the proton gradient and release energy in the form of heat. This principle is used for heat generation in brown adipose tissue, high levels of which are found in human newborns and hibernating mammals. The same principle is used in plants: plant mitochondria have 3 additional complexes in their electron transport chain, which are uncoupled. The heat released is exploited to attract pollinators and to melt a covering layer of snow.
Moreover, mitochondria play a key role in the activation of the intrinsic apoptotic pathway. Apoptosis, also known as programmed cell death, is a vital process in development and a critical pathway in elimination of damaged cells. In response to an intracellular stress signal (e.g. DNA damage), pro-apoptotic Bax and Bak proteins are released from anti-apoptotic proteins Bcl2 and Bcl-XL; they homodimerize and form pores in the outer mitochondrial membrane, resulting in the release of molecules such as cytochrome c. These activate a cascade of caspase proteases, which digest cellular organelles and proteins, leading to apoptosis.
The description of the full scope of mitochondrial function is beyond the scope of this essay, as mitochondria also serve other cellular functions, in addition to the roles mentioned above. The aim of this short essay was merely to show that mitochondrial function extends far beyond its most well-known role of an ATP generator, and to inspire the reader to pursue this further. Mitochondrial dysfunction is widely implicated in aging and a variety of diseases such as uniparentally inherited mitochondrial genetic disorders as well as cancers, diabetes, and Parkinson’s. Therefore, fully understanding the complex interconnected nature of mitochondria and cellular physiology will have far-reaching implications in many areas of biology and medicine.
· Lane, N. (2018). Power, sex, suicide: mitochondria and the meaning of life.
· Know, L. (2018). Mitochondria and the future of medicine: the key to understanding disease, chronic illness, aging, and life itself.
· Peer-reviewed research papers on PubMed on the recent advances in mitochondrial research https://pubmed.ncbi.nlm.nih.gov/