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Lysosomes

Lysosomes

Lysosomes are membrane-bound organelles that function as the "stomachs" of eukaryotic cells . They contain about fifty different enzymes that break down all types of biological molecules including proteins , nucleic acids, lipids , and carbohydrates . Cells transport material into lysosomes, the material is digested by the enzymes, and the digested molecules are moved back into the cytosol for use by the cell. Both extracellular materials brought into the cell by endocytosis and obsolete intracellular materials are degraded in the lysosome.

Lysosomes vary in size and shape, but have several common features. They are surrounded by a single membrane, have an acidic interior pH level of about 5, and carry a high content of digestive enzymes. All of the digestive enzymes found in the lysosome require an acidic environment to function properly and are called acid hydrolases. The low pH of the lysosome is maintained by membrane proteins that pump protons (H+ ions ) from the cytosol into the lysosome.

In addition to the proton pumps, the lysosomal membrane contains many other proteins that transport the digested molecules out of the lysosome and into the cytosol. Although it may seem dangerous for cells to contain enzymes that can digest most biological molecules, the contents of the cell are doubly protected from the digestive enzymes of the lysosome. First, the enzymes are enclosed in the lysosomal membrane and second, even if the enzymes were to leak out of the lysosome, they would not be active at the neutral pH of the cytosol.

Extracellular materials to be degraded in the lysosome are brought into the cell by either pinocytosis or phagocytosis. Pinocytosis, which occurs in all eukaryotic cells, is the internalization of extracellular fluid and small macromolecules by means of small vesicles that pinch off the inside of the plasma membrane. These small vesicles carrying endocytosed molecules are initially delivered to membranous organelles called endosomes. It is not precisely clear how molecules to be degraded progress from endosomes to lysosomes. Endosomes may actually mature into lysosomes when newly made acid hydrolases are delivered to the endosome.

Phagocytosis, which occurs in only specialized cell types, is the ingestion of large particles such as cell debris or whole microorganisms. Phagocytic cells engulf large particles by forming a large intracellular vesicle containing the engulfed particle. The large vesicle then fuses with a lysosome, resulting in a single membranous organelle in which the digestive enzymes break down the ingested particle.

Intracellular materials, such as old organelles, are brought into a lysosome by a process called autophagy. For example, when a mitochondrion comes to the end of its ten-day life, it is engulfed by membrane derived from the endoplasmic reticulum . The newly enclosed mitochondrion then fuses with a lysosome, resulting in its degradation by the acid hydrolases.

A group of genetic disorders caused by defective lysosomal enzymes demonstrates the importance of lysosomes. Called lysosomal storage diseases, these disorders are characterized by the harmful accumulation of undigested substances. The accumulated materials impair or kill the affected cells, resulting in skeletal or muscular defects, mental retardation, or even death.

see also Endocytosis; Endoplasmic Reticulum; Enzymes; Mitochondrion

Cynthia K. Damer and Scott N. Daigle

Bibliography

Hunziker, Walter, and Hans J. Geuze. "Intracellular Trafficking of Lysosomal Membrane Proteins." Bioessays 18 (1996): 379389.

Kornfeld, Stuart, and Ira Mellman. "The Biogenesis of Lysosomes." Annual Review of Cell Biology 5 (1989): 483525.

Mellman, Ira. "Endocytosis and Molecular Sorting." Annual Review of Cellular and Developmental Biology 12 (1996): 575625.

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Lysosome

Lysosome

Lysosomes are small membranous bags of digestive enzymes found in the cytoplasm of all eukaryotic cells (those with true nuclei). As the principle site of intracellular digestion, they contain a variety of enzymes capable of degrading proteins, nucleic acids, sugars, lipids, and most other ordinary cellular components. These enzymes hydrolyze (break down) their target compounds best under acidic conditions. Although lysosomes vary considerably in size even within a single cell, the normal range is usually slightly smaller than the average mitochondrion.

The membrane enclosing lysosomes appears to be similar to that of other cellular organelles, but it has several unique properties. First, hydrogen pumps in the membrane acidify the lysosomal interior to a pH of five, an optimal level for the activity of its internal enzymes. The membrane has docking sites on its exterior that allow both materials to be digested and the enzymes to carry out the job to be transferred into the lysosome from transport vesicles derived from the Golgi apparatus, the endoplasmic reticulum, or from endocytosis by the plasma membrane. The lysosomal membrane also has transport complexes that allow the final products of digestion such as amino acids, simple sugars, salts, and nucleic acids to be exported back into the cytoplasm, where they can be either excreted or recycled by the cell into new cellular components. Finally, by mechanisms that are not yet fully understood, the lysosomal membrane is able to avoid digestion by the enzymes it contains even though it is composed of the same compounds that those enzymes routinely destroy.

See also Cell membrane transport

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lysosome

lysosome A membrane-bound sac (organelle) found in animal cells and in single-celled eukaryotes. It contains hydrolytic enzymes that degrade aged or defective cell components or material taken in by the cell from its environment, such as food particles or bacteria. The lysosomal enzymes are adapted to work in the acid conditions of the lysosome interior, which has a pH of about 4.8. This means that should the enzymes escape from the lysosome they are inactivated by the neutral pH of the cell cytosol, and so will not attack the cell contents. Primary lysosomes do not contain debris, but fuse with vesicles or organelles containing material for disposal, forming a secondary lysosome in which digestion takes place. In plant cells, the vacuole contains hydrolytic enzymes equivalent to those in the lysosome and can degrade materials in a manner similar to a lysosome.

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lysosome

lysosome A membrane-bound vesicle, commonly 0.1–0.5 μm in diameter, in a cell that contains numerous acid hydrolases capable of digesting a wide variety of extra- and intracellular materials. The membranes of lysosomes apparently originate from the Golgi body and the hydrolases are synthesized in the rough endoplasmic reticulum. The functions of lysosomes are: (a) the digestion of material taken in by endomitosis, usually by fusing with the material and discharging their contents into a vacuole; (b) the destruction of redundant organelles (see autophagy); (c) the storage of indigestible residues (see residual body); (d) the destruction of cellular materials during autolysis; and (e) the release of enzymes outside the cell (e.g. during the resculpturing of bone).

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lysosome

lysosome A membrane-bound vesicle in a cell that contains numerous acid hydrolases capable of digesting a wide variety of extra- and intracellular materials. Lysosomes apparently originate from the Golgi body, and function by fusing with and discharging their contents into a vacuole containing the material to be digested. Lysosomes have been difficult to isolate from plant cells, but a number of biochemical studies have demonstrated clearly that plant cells contain a wide range of particles, varying in size and internal contents, which may be termed lysosomes. They are all surrounded by a single membrane and contain hydrolytic enzymes. In many plants, the main cell vacuole may assume the functions of lysosomes.

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"lysosome." A Dictionary of Plant Sciences. . Encyclopedia.com. 23 May. 2017 <http://www.encyclopedia.com>.

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lysosome

lysosome (ly-sŏ-sohm) n. a particle in the cytoplasm of cells that contains enzymes responsible for breaking down substances in the cell. Lysosomes are especially abundant in liver and kidney cells.

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