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): 379–389.

Kornfeld, Stuart, and Ira Mellman. "The Biogenesis of Lysosomes." Annual Review of Cell Biology 5 (1989): 483–525.

Mellman, Ira. "Endocytosis and Molecular Sorting." Annual Review of Cellular and Developmental Biology 12 (1996): 575–625.

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).

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

Lysosomes

Lysosomes are small, round bodies containing digestive enzymes that break down large food molecules into smaller ones. They are found in the cytoplasm, or jelly-like fluid, of all eukaryotic cells (cells with a distinct nucleus). Lysosomes are the main site where digestion takes place inside a cell.

As organelles or specialized, membrane-bound structures inside a cell that have a certain job to do, lysosomes contain very powerful enzymes called "hydrolases" that are capable of breaking down many different types of substances. These enzymes work on food molecules such as proteins, carbohydrates, and fats and quickly break them down into smaller particles that can be easily used by the cell. The powerful enzymes in lysosomes are also sometimes put to another use by the cell when it needs to rid itself of a damaged or defective organelle. In such a case, the lysosomes attack an organelle and quickly break it down and destroy it. At other times, a cell may use lysosomes to actually destroy itself. This process is known as "autolysis" (auto means "self" and lysis means "destruction"). This usually happens for a very good reason, as in metamorphosis when an animal has to entirely reshape its tissues (as when a caterpillar changes into a butterfly). While biologists know that lysosomes are used by the cell to digest the food it takes in, they do not yet fully understand how the lysosome membrane itself avoids being broken down by the enzymes it carries. This is especially puzzling since the membrane is made of the same compounds that the enzymes easily destroy.

[See alsoCells; Enzymes ]

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.