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
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.
Cynthia K. Damer and Scott N. Daigle
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.
Lysosomes ni organelles membrane-kuunganishwa kwamba kazi kama "matumbo" ya seli eukaryotic. Ina karibu 50 enzymes tofauti ambazo kuvunja aina zote za molekuli za kibiolojia ikiwa ni pamoja na protini, asidi nucleic, lipids na wanga. Chembe huingiza vitu katika lysosomes, vitu hivyo huchemsha na kuyeyushwa na enzymes, na molekuli hizo huchemsha na kuelekezwa kwenye cytosol ili zitumiwe na chembe. Vifaa vyote viwili vya nje ya chembe vilivyoletwa ndani ya chembe kwa endocytosis na vifaa vya ndani ya chembe vilivyopitwa na wakati vinaharibiwa katika lysosome. Lysosomes hutofautiana kwa ukubwa na umbo, lakini zina sifa kadhaa za kawaida. Mimea hiyo imezungukwa na utando mmoja, ina pH ya ndani ya asidi ya 5, na ina kiasi kikubwa cha enzymes za kumeng'enya chakula. Kwa kuongezea pampu za protoni, utando wa lysosomal una protini nyingi ambazo husafirisha molekuli zilizomeng'enywa nje ya lysosome na kuingia katika cytosol. Ingawa inaweza kuonekana kuwa hatari kwa chembe kuwa na enzymes ambayo inaweza kumeng'enya molekuli nyingi za kibiolojia, yaliyomo ya chembe ni mara mbili kulindwa kutoka enzymes digestive ya lysosome. Enzymes ni kuingizwa katika membrane lysosomal, na hata kama kuvuja nje ya lysosome, wao si kuwa hai katika pH neutral ya cytosol. Vifaa vya nje ya chembe vinavyopaswa kuharibiwa katika lysosome huletwa ndani ya chembe kwa pinocytosis au phagocytosis. Pinocytosis ni mchakato wa kuingiza maji ya nje ya chembe na macromolecules ndogo kwa njia ya vesicles ndogo ambazo huondoa membrane ya plasma. Vipuli hivi vidogo vinavyobeba molekuli za endocytosed mwanzoni hutolewa kwa organelles za utando zinazoitwa endosomes. Haijulikani wazi jinsi molekuli zinazopaswa kuharibiwa zinavyosonga kutoka endosomu hadi lysosomu. Endosomes inaweza kweli kukomaa katika lysosomes wakati hydrolases asidi mpya kufanywa ni kutolewa kwa endosome. Phagocytosis ni hali ya kuambukizwa kwa chembe ndogo za chembe, ambayo hutokea katika aina maalum za chembe, na ni kuingizwa kwa chembe kubwa kama vile mabaki ya chembe au viumbe wadogo. Chembe za phagocytic huchukua chembe kubwa kwa kuunda chembe kubwa ya ndani ya chembe inayojumuisha chembe iliyochukuliwa. Kisha, kibofu hicho kikubwa huungana na lysosome, na kutokeza kiungo kimoja cha utando ambacho enzymes za kumeng'enya chakula huvunja chembe hiyo iliyomezwa. Vifaa vya ndani ya chembe, kama vile organelles za zamani, huletwa ndani ya lysosome kwa mchakato unaoitwa autophagy. Mitochondria ya binadamu inapokamilisha maisha yake ya siku kumi, hufunikwa na utando unaotokana na endoplasmic reticulum. Mitochondrion mpya iliyofungwa kisha huungana na lysosome, na kusababisha uharibifu wake na hydrolases asidi. Kikundi cha magonjwa ya urithi yanayosababishwa na enzymes za lysosomal zenye kasoro huonyesha umuhimu wa lysosomes. Magonjwa hayo yanaitwa magonjwa ya kuhifadhi lisosomu, na yanaonyesha kwamba vitu visivyogandishwa hujipanga vibaya. Vifaa vilivyojikusanya huathiri au kuua chembe zilizoathiriwa, na kusababisha kasoro za mifupa au misuli, kushindwa kufikiri, au hata kifo. Cynthia K. Damer na Scott N. Daigle Hunziker, Walter, na Hans J. Geuze. "Uuzaji wa Protini za Mshipa wa Lysosomal Ndani ya Chembe". Bioessays 18 (1996): 379-389. Kornfeld, Stuart, na Ira Mellman. "Biogenesis ya Lysosomes". Annual Review of Cell Biology, toleo la mwaka wa 1989 Mellman, Ira. "Endocytosis na Utaratibu wa Molekuli". Annual Review of Cellular and Developmental Biology, toleo la mwaka wa 1996