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Old Friday, June 05, 2009
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Default Cell Biology Lysosome


Lysosomes are the cells' garbage disposal system. Lysosomes are membrane bound vesicles containing various hydrolytic enzymes necessary for digesting certain material in a cell. Lysosomes contain an ionic pump, which maintains a highly acidic pH
They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. The membrane surrounding a Lysosomes 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. The name Lysosomes 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.


Lysosomes sizes, microscopic appearances, and other properties vary among different cell types and circumstances owing, in part, to differences in their functions and states. Typical Lysosomes are roughly spherical or elongate bodies with largest dimensions of 0.1–1 micrometer or greater; tens to hundreds are present in a single cell.
Each Lysosomes is bounded by a membrane and contains several dozen different species of digestive enzymes, each of which can sever particular chemical bonds found in natural materials. Most lysosomal enzymes function best in an acid environment. This acidification is accomplished by a proton pump, built into the membrane surrounding the Lysosomes, which affects the transport of hydrogen ions into the Lysosomes.


Lysosomes were discovered by the Belgian cytologist Christian de Duve in 1955.He discovered them when he homogenized some animal cells into various components by running them through an ultracentrifuge. After a few days the level of a few enzymes rose significantly implying that they were segregated in the cell and had not attacked any part of the cell before they were broken down.

Lysosomes have 2 main roles:
  1. To digest macromolecules, which enter the cell-, they are like sacs, which contain around forty digestive enzymes. The Lysosomes infuse with vesicles of engulfed material and release the digestive enzymes to break up the material. The large molecules of food are broken down into smaller particles. The products diffuse through the Lysosomes' membrane and are distributed throughout the rest of the cell. The products serve as building blocks of new materials.
  2. To breakdown old nonfunctioning organelle that out lived their usefulness- Cells fail to restore them causing them to age. Because of these Lysosomes are needed to rid the cell of these unneeded materials that are occupying space in the cell. In stressed or dying cells' membrane, this component of the cell denigrates material, releasing the destructive lysosomal enzymes into the cytoplasm. Here they digest all organelles and speed up the cells' death by this process of autolysis (self-digestion). The products are recycled and reused to compose new parts of cells.
In heterophagy, the cell takes up particles or molecules by the process of endocytosis, engulfing them in membrane-bounded vesicles or vacuoles that are formed at the cell surface. The endocytosed material enters Lysosomes via intermediate membrane-bounded compartments known as endosomes. In higher animals, heterophagy is most prominently used by leukocytes and macrophages. These specialized cells endocytose invasive microorganisms and use endocytosis in clearing debris and disposing of dead or senescent cells.


In autophagy, cells segregate regions of their own cytoplasm within compartments that come to be bounded by single membranes and to receive lysosomal enzymes. Autophagic lysosomes take part in the remodeling of cells as part of the processes of development and during stressful circumstances. They also participate, along with nonlysosomal enzymes and heterophagic Lysosomes, in normal turnover of the body's constituents—the balanced synthesis and destruction through which new molecules replace most molecules of most cells.


To accomplish the tasks associated with digestion, the lysosomes use some 40 different types of hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum and modified in the Golgi apparatus.

Some important enzymes found within lysosomes include:Lysosomal enzymes are synthesized in the cytosol and the endoplasmic reticulum, where they receive a mannose-6-phosphate tag that targets them for the Lysosomes. Aberrant lysosomal targeting causes inclusion-cell disease, whereby enzymes do not properly reach the Lysosomes, resulting in accumulation of waste within these organelles.

Clinical relevance

There are a number of lysosomal storage diseases 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.

Genetic defects in lysosomal enzymes and related proteins are known to be associated with a large number of rare disorders in humans and animals (such as Tay-Sachs disease and Niemann-Pick disease type C). Defective lysosomal function leads to storage of particular classes of molecules that cannot be degraded and, in long-lived cells such as neurons, to complex pathogenic cascades with widespread impact on endosomal-lysosomal function, membrane trafficking, and signal transduction. Such disorders are most often fatal. Lysosomes or prelysosomal structures also have been “adopted” as intracellular homes by certain pathogenic microorganisms that avoid or survive the attacks of the lysosomal system. Some strains of viruses, and toxins such as the one responsible for diphtheria, may use endosomes as their route of entry into the cell, penetrating through the endosomal membrane into the surrounding cytoplasm.

Lysosomes are used for the digestion of macromolecules from phagocytosis (ingestion of other dying cells or larger extracellular material, like foreign invading microbes), 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.

In 1965, Christian de Duve and his colleagues found other enzymes containing organelle. They called these peroxisomes because they seemed to generate and break down hydrogen peroxide.
Peroxisomes, also known as microbodies, and they are self-replicating cells containing oxidative enzymes. They are similar to lysosomes. Their enzymes have two functions; to convert fats to carbohydrates and to detoxify potentially harmful molecules which form in the cell.
Peroxisomes, in contrast to lysosomes, are produced only on the smooth ER system. They are found in the cytoplasm of many eukaryotic cells as well as prokaryotic cells, microorganisms, and plant cells. They are very active in yeast, protozoans, kidney cells, and mammailain cells.
Peroxisomes are permeable. This allows many small molecules to enter easily.
The enzymes of peroxisomes remove hydrogen atoms from small molecules and join them to oxygen creating hydrogen peroxide. They consume up to twenty percent of the oxygen in the liver cell. The peroxisomal enzyme catalase uses this oxygen to convert hydrogen peroxide to water and oxygen neutralizing it. In the liver this method is used to break down molecules of alcohol into substances that can be eliminated from the body.
Diseases: The rare fatal genetic disorder Zellweger's syndrome is the result of malformed peroxisomes. This indicates peroxisomes do have an important role in healthy cells.
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