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Lysosomes are organelles that contain digestive enzymes (acid hydrolases). They digest excess or worn out organelles, food particles, and engulfed viruses or bacteria. The membrane surrounding a lysosome prevents the digestive enzymes inside from destroying the cell. Lysosomes can fuse with vacuoles and dispense their enzymes into the vacuole, digesting its contents. Lysosomes are found in both animal and plant cells but they are rare in plant cells. They are built in the Golgi apparatus. The name “lysosome” were came from the Greek words “lysis” which means dissolution or destruction and "soma" which means body. They are frequently nicknamed "suicide-bags" by cell biologists due to their role in autolysis. Lysosomes were discovered by the Belgian cytologist Christian de Duve in 1949.
[edit] Acidic Environment
At pH 4.8, the interior of the lysosomes is more acidic than the cytosol (pH 7). The lysosome single membrane stabilizes the low pH by pumping in protons (H+) from the cytosol, and also protects the cytosol, and therefore the rest of the cell, from the degradative enzymes within the lysosome. For this reason, should a lysosome's acid hydrolases leak into the cytosol, their potential to damage the cell will be reduced, because they will not be at their optimum pH.
The constant pH of 4.8 is maintained by proton pumps and Cl- ion channels
[edit] Creation
They are involved in the creation of enzymes that undergo phagocytosis. All these enzymes are produced in the endoplasmic reticulum, and transported and processed through the Golgi apparatus. The Golgi apparatus produces lysosomes by budding. Each acid hydrolase is then targeted to a lysosome by phosphorylation. The lysosome itself is likely to be safe from enzymatic action due to having proteins in the inner membrane which has a three-dimensional molecular structure that protects vulnerable bonds from enzymatic attack[1].
[edit] Enzymes
Some important enzymes in lysosomes are:
- Lipase, which digests lipids,
- Carbohydrases, which digest carbohydrates (e.g., sugars),
- Proteases, which digest proteins,
- Nucleases, which digest nucleic acids.
- Phosphatases, which digest phosphoric acid monoesters
Lysosomal enzymes are synthesized in the cytosol and the endoplasmic reticulum, where they receive a mannose-6-phosphate tag that targets them for the lysosome. Aberrant lysosomal targeting causes inclusion-cell disease, whereby enzymes do not properly reach the lysosome, resulting in accumulation of waste within these organelles.
[edit] Functions
The lysosomes are used for the digestion of macromolecules from phagocytosis (ingestion of cells), from the cell's own recycling process (where old components such as worn out mitochondria are continuously destroyed and replaced by new ones, and receptor proteins are recycled), and for autophagic cell death, a form of programmed self-destruction, or autolysis, of the cell, which means that the cell is digesting itself.
Other functions include digesting foreign bacteria that invade a cell and helping repair damage to the plasma membrane by serving as a membrane patch, sealing the wound. Lysosomes also do much of the cellular digestion required to digest tails of tadpoles and to remove the web from the fingers of a 3-6 month old fetus. This process of programmed cell death is called apoptosis [2]
[edit] Clinical relevance
There are a number of illnesses that are caused by the malfunction of the lysosomes or one of their digestive proteins, e.g., Tay-Sachs disease, or Pompe's disease. These are caused by a defective or missing digestive protein, which leads to the accumulation of substrates within the cell, resulting in impaired cell metabolism.
Broadly, these can be classified as mucopolysaccharidoses, GM2 gangliosidoses, lipid storage disorders, glycoproteinoses, mucolipidoses, or leukodystrophies.
[edit] See also
[edit] External links
[edit] References
- ^ Campbell, Neil A. and Reece, Jane B. (2002). Biology 6th ed. Benjamin Cummings. San Francisco. ISBN 0-8053-6624-5
- ^ Mader, Sylvia. (2007). Biology 9th ed. McGraw Hill. New York. ISBN 9780072464634
- This article contains material from the Science Primer published by the NCBI, which, as a US government publication, is in the public domain
