Lysosome

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Lysosomes are membrane bound organelles found in eukaryotic cells. They were first discovered by the Belgian cytologist Christian Rene de Duve in the 1950s[1]. They are around 0.5-1.0 µm in diameter[2].

Lysosomes, the digestive system of the cell, break down substances from outside the cell as well as digesting material from inside the cell. The organelle, which is mosly sperical in shape, varies in size and shape, depending on the material that is being broken down[3]. Lysosomes contain up to 40 different types of hydrolytic enzymes which control the digestion of macromolecules, such as nucleic acids and proteins, old cell parts, and other foriegn pathogens. Some of the most common lysosomal enzymes include proteasesnucleases, glycosidases, lipases, phospholipases, phosphatases and sulfatases which are all acid hydrolases[4]. These enzymes optimum pH is at 4.5 - 5.5 which, compared to the cytosols pH of 7.2, is very acidic. The acidic pH of the lumen is maintained by a vacular ATPase proton pump which pumps H+ Into the lysosome. These conditions help protect the cell from possible 'leakages', as the hydrolytic enzymes would not be able to function in the neutral pH of the cytosol.

Lysosomes are linked to a number of pathways, mainly endocytosis, autophagy and phagocytosis.

Endocytosis

In endocytosis, molecules are taken in from the outside the cell and packaged into vesicles. These vesicles containing macromolecules and other small substances fuse with organelles inside the cell called early endosomes, which mature into late endosomes by becoming more acidic (roughly pH 6) due to the ATPase. It is here, in the late endosomes, where the digestion of the macromolecule starts. Late endosomes are sometimes known as multivesicular bodies (MVBs) as during the mauturing stage, some molecules are sorted into smaller vesicles, forming lumenal vesicals in the endosome lumen. Before the late endosomes fuse with a lysosome, they replace the RAB5 with RAB7[5].

Autophagy

Autophagy or autophagocytosis, is a process involving the use of lysosomes to digest intracellular components of the cell. There are two different autophagic pathways: selective and nonselective autophagy. Selective autophagy is the specific degradation of certain damaged organelles or protein aggregates in order to maintain homeostasis. Non-selective autophagy involves the random sequestration of cytoplasm to then by degraded in a lysosome so that the cytoplasm can be recycled - the resulting metabolites are needed by the cell for survival since this type of autophagy occurs when the cell is put under stress (e.g. starvation)[6].

The autophagic pathway involves the formation of an autophagosome, a double membrane structure, around the unwanted molecule. The autophagosome then fuses with a lysosome, the contents of which are digested by the hydrolytic enzymes. Permeases in the lysosomal membrane allow the resulting amino acids to be translocated into the cytoplasm where they can then be used to synthesize new proteins[7].

Autophagy plays an important role in maintaining the basic cell contents, but also plays a role in protecting the cell against certain pathogens and also maintaining the cells nutrient levels when starvation occurs making sure the cells vital processes can continue.

References

  1. http://www.britannica.com/EBchecked/topic/174733/Christian-Rene-de-Duve?anchor=ref23999
  2. Hardin, J., Bertoni, G. and Kleinsmith L. (2011), Becker’s World of The Cell, 8th edition, San Francisco, Pearson Education Inc.
  3. Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Lysosomes. Available from: http://www.ncbi.nlm.nih.gov/books/NBK9953/
  4. Bruce Alberts [et al] "Molecular biology of the cell" 5th edition.
  5. Cell, Volume 122, Issue 5 2005, Rab Conversion as a Mechanism of Progression from Early to Late Endosomes: http://www.cell.com/retrieve/pii/S0092867405006975
  6. Lodish H, Berk A, Kaiser CA, Krieger M, Bretscher A, Ploegh H, Amon A, Scott MP. Molecular Cell Biology. 7th edition. New York: Macmillan Higher Education; 2013
  7. Lodish H, Berk A, Kaiser CA, Krieger M, Bretscher A, Ploegh H, Amon A, Scott MP. Molecular Cell Biology. 7th edition. New York: Macmillan Higher Education; 2013
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