The scanning electron microscope (SEM) is a tool for studying and researching about the fine, detailed structure of a biological specimen. The resolution of this instrument and specimen preparation techniques are continuously undergoing improvement. In 1985, an ultra-high resolution Scanning Electron Microscope (UHS-T1) with a resolution of 0.5nm was developed. The osmium-DMSO-osmium method is used for revealing specimen intracellular structures due to its high effectiveness (Tanaka, 1989).
Mitochondria are essential organelles in the cells of eukaryotes because they are the the powerhouse of the cell.' They exist as the dynamic, extended and interconnected network of tubules that are integrated with other compartments of the cell (Nunnari & Suomalainen, 2012). The outer membrane of the mitochondria has the permeability to oxygen, ATP, and other molecules. Its thickness is 4nm. The cristae are the invaginations that form the inner membrane of the mitochondria, and they vary in shape and number in adaptation to the cellular requirements. For instance, cristae can be prism-shaped as in a nerve cell or whorl-like shaped as in a photoreceptor cone cell. They host a system which facilitates the conversion of energy to produce adenosine triphosphate (ATP). This system is known as Oxidative Phosphorylation System (OXPHOS) (Jacobs & Wurm, 2014).
Studies were done by Schmidt et al. have shown a clustered distribution of mitochondrial outer membrane (TOM) complexes by use of Tom20 as the subunit to highlight this distribution (2008). A study involving more than 1000 cells by Wurm et al. (2011) demonstrated that this clustering is adjusted to cellular growth conditions. The mitochondrial inner membrane consists the cristae and the inner boundary membranes exhibiting different protein composition (Vogel, Bornhovd, Neupert & Reichert, 2006). Mitochondria have nucleoids located in the aqueous matrix, where their genome (mtDNA) is packaged. mtDNA encodes 13 proteins in humans essential for OXPHOS. Inconclusive estimates done by (Kukat & Larsson, 2013), show an average range of 1.4-10 molecules of mtDNA in a single nucleoid. According to Brown et al. nucleoids have an ellipsoidal shape, 85nm 108nm 146nm. However, this shape may show a strong variation and may depend on the interaction with the inner membrane (2011).
Brown, T. A., Tkachuk, A. N., Shtengel, G., Kopek, B. G., Bogenhagen, D. F., Hess, H. F., & Clayton, D. A. (2011). Superresolution fluorescence imaging of mitochondrial nucleoids reveals their spatial range, limits, and membrane interaction. Molecular and cellular biology, 31(24), 4994-5010.
Jakobs, S., & Wurm, C. A. (2014). Super-resolution microscopy of mitochondria. Current opinion in chemical biology, 20, 9-15.
Kukat, C., & Larsson, N. G. (2013). mtDNA makes a U-turn for the mitochondrial nucleoid. Trends in cell biology, 23(9), 457-463.
Wurm, C. A., Neumann, D., Lauterbach, M. A., Harke, B., Egner, A., Hell, S. W., & Jakobs, S. (2011). Nanoscale distribution of mitochondrial import receptor Tom20 is adjusted to cellular conditions and exhibits an inner-cellular gradient. Proceedings of the National Academy of Sciences, 108(33), 13546-13551.
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