Lysosome

From Freepedia

Lysosomes contain digestive enzymes (acid hydrolases) to digest macromolecules. They are found in both plant and animal cells, and are built in the Golgi apparatus. 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. The digestive enzymes need the acidic environment of the lysosome to function correctly. 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. 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 protected from digestion due to its unique internal 3-D structures which prevent enzymatic action^[1].

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.

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.

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.

The constant pH of 4.8 is maintained by hydrogen proton pumps and Cl- ion channels.

References

• 1. ^  Campbell, Neil A. and Reece, Jane B. (2002). Biology 6th ed. Benjamin Cummings. San Fransisco. ISBN 0805366245
• This article contains material from the Science Primer published by the NCBI, which, as a US government publication, is in the public domain [2].

Organelles of the cell
Chloroplast | Mitochondrion | Centriole | Endoplasmic reticulum | Golgi apparatus | Lysosome | Myofibril | Nucleus | Peroxisome | Ribosome | Vacuole | Vesicle