Lysosome
Lysosome is an organelle of eukaryotic cells that contains hydrolytic enzymes involved in intracellular digestion. This membrane-bound sub-cellular structure contains a unique collection of about 40 hydrolytic enzymes, including proteases, nucleases, lipases, glycosidases, phospholipases, phosphatases, and sulfates, all of which are acid hydrolases, optimally active near pH 5 (Alberts et al. 1989). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. The unique surrounding membrane keeps enzymes out of the cystosol, where they might damage proteins and nucleic acids, although the acid dependence of the enzymes also provides some protection even in leakage given the higher pH (around 7.2) of the cytosol (Alberts et al. 1989).
The name lysosome derives from the Greek words lysis, which means dissolution or destruction, and soma, which means body. They are frequently nicknamed "suicide-bags" or "suicide-sacs" by cell biologists due to their role in autolysis (destruction of a cell through the action of its own enzymes). Lysosomes were discovered by the Belgian cytologist Christian de Duve in 1949.
Overview
(1) nucleolus
(2) nucleus
(3) ribosomes (little dots)
(4) vesicle
(5) rough endoplasmic reticulum (ER)
(6) Golgi apparatus
(7) Cytoskeleton
(8) smooth ER
(9) mitochondria
(10) vacuole
(11) cytoplasm
(12) lysosome
(13) centrioles within centrosome
diverse in size ahdn shape
The membrane surrounding a lysosome allows the digestive enzymes to work at the 4.5 pH they require. Lysosomes fuse with vacuoles and dispense their enzymes into the vacuoles, digesting their contents. They are created by the addition of hydrolytic enzymes to early endosomes from the Golgi apparatus.
At pH 4.8, the interior of the lysosomes is more acidic than the cytosol (pH 7.2). The lysosome's single membrane stabilizes the low pH by pumping in protons (H+) from the cytosol via proton pumps and chloride ion channels. The membrane 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
Enzymes
Some important enzymes in these are:
- Lipase, which digests lipids
- Carbohydrases, which digest carbohydrates (e.g., sugars)
- Proteases, which digest proteins
- Nucleases, which digest nucleic acids
- 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.
Functions
The lysosomes are used for the digestion of macromolecules from phagocytosis (ingestion of other dying cells or larger extracellular material), endocytosis (where receptor proteins are recycled from the cell surface), and autophagy (wherein old or unneeded organelles or proteins, or microbes that have invaded the cytoplasm are delivered to the lysosome). Autophagy may also lead to 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 (or other forms of waste) that invade a cell and helping repair damage to the plasma membrane by serving as a membrane patch, sealing the wound. In the past, lysosomes were thought to kill cells that were no longer wanted, such as those in the tails of tadpoles or in the web from the fingers of a 3- to 6-month-old fetus. While lysosomes digest some materials in this process, it is actually accomplished through programmed cell death, called apoptosis.[1][2]
autolysis may refer to the destruction of a cell through the action of its own enzymes. It may also refer to the digestion of an enzyme by another molecule of the same enzyme. The term derives from the Greek words αυτό ("self") and λύσις ("splitting").
Autolytic cell destruction is uncommon in adult organisms and usually occurs in injured cells or dying tissue. Autolysis in initiated by the cells lysosomes releasing the digestive enzymes they contain out into the cytoplasm. The cell then, in effect, starts to digests itself. Autolysis of individual cell organelles can be lessened if the organelle is stored in ice-cold isotonic buffer after cell fractionation.
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, impairing metabolism.
In the broad sense, these can be classified as mucopolysaccharidoses, GM2 gangliosidoses, lipid storage disorders, glycoproteinoses, mucolipidoses, or leukodystrophies.
Additional images
Proteins in different cellular compartments and structures tagged with green fluorescent protein.
External links
ReferencesISBN links support NWE through referral fees
- ↑ http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/Lysosomes.html
- ↑ Mader, Sylvia. (2007). Biology 9th ed. McGraw Hill. New York. ISBN 978-0072464634
- This article contains material from the Science Primer published by the NCBI, which, as a US government publication, is in the public domain at http://www.ncbi.nlm.nih.gov/About/disclaimer.html.
Alberts et al. 1989.
| Organelles of the cell |
|---|
| Acrosome | Chloroplast | Cilium/Flagellum | Centriole | Endoplasmic reticulum | Golgi apparatus | Lysosome | Melanosome | Mitochondrion | Myofibril | Nucleus | Parenthesome | Peroxisome | Plastid | Ribosome | Vacuole | Vesicle |
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