Lysosomes were discovered by Duve (1955).† They are also called suicidal bags.† They can be identified as dense, granular membranous sacs capable of digesting various substances.† The marker enzyme for this organelle is acid phosphotase.†
The size of lysosomes varies from 0.2 to 0.8 nm.† The number also varies from cell to cell and it depends upon the metabolic state of the cells.† Yeast cells under glucose repression produce large number of lysosomes which autophagically digest mitochondria.† Similarly during metamorphosis of tadpole into young frog a great number of lysosomes appear in the tail of tadpole and they remain active till the tail is resorbed.† Lysosomes are classified into primary lysosomes and secondary Lysosomes.† The former are directly derived from Golgi membranes and the latter are the products of fusion between primary lysosomes and endocytotic vesicles or phagosomes.
Isolated single lysosome; http://faculty.muhs.edu/
Lysosomes originate within the Golgi apparatus. The digestive enzymes found in the lysosome are manufactured in the rough endoplasmic reticulum of the cell and transported to the Golgi apparatus where lysosomes are created.
Biogenesis of lysosomes; faculty.sau.edu.sa
Lysosomes are spherical structures bounded by a single unit membrane within which dense granular enzymatic components are found.† Though hydrolysing enzymes are found within the lysosomes, its own membrane is not digested because the membrane is highly modified; more over the enzymes are rendered inactive by electrostatic binding forces.† However lysosomal membranes are sensitive to many labilizers and stabilizers.† Vitamins like A, B, K and hormones like progesterone, B-esterdiol and some metal ions like Ag2+, Hg2 +, Cu2 labilize the membrane sand allow the enzymes to diffuse out.† But cholesterol, cortisones, etc stabilize the membranes.† Thus intacellular conditions control and regulate the release of the lysosomal enzymes into cellular fluid.
More than forty different lysosomal enzymes have been recognized, ex; Nuclease, Acid phosphotase, Lipases, Proteases (Cathapsin A, B, C, D & E), Glycosidase, Lysozyme, Sulfotases, etc.† Among them acid phosphotase is a marker enzyme for lysosomes.† All the enzymes need not be present in all lysosomes at all times.† When lysosomes originate from Golgi bodies they may contain only one set of enzymes but fusion with other lysosomes of different composition produces lysosomes with different enzyme composition.
Lysosomes release the enzymes on activation by intracellular environment. In plants cells, particularly at the time of seed germination, lysosomal enzymes degrade macromolecules like starch and reserve proteins into glucose and amino acids respectively.† In some cases, lysosomal activity results in intercellular or intracellular digestion leading to the formation of schizogenous or lysogenous cavities.† Laticiferous cavities found in many Euphorbiaceae and other members are due to the activity of lysosomal enzymes.††† †
In animal cells, the primary lysosomes fuse with food vacuoles derived from phagocytosis and produce phagosomes. After digestion, the residual body is thrown out of the cell by exocytosis.† Even the incoming endocytotic vesicles fuse with primary lysosomes into secondary lysosomes.† During certain stage of development lysosomes become very active and digest cellular components by autophagy.
In almost all cases, lysosomes take their origin from golgi-endoplasmic reticulum-lysosomal complex called GERL complex.† The RER synthesizes lysosomal proteins on its membrane (polysome bound) then the proteins are transported through the lumen into SER where the membranes are pinched off into vesicles at the cis surface of Golgi body. Lysosomal proteins are marked by mannose-p. The vesicles fuse with one another into flat membranous sacs.† At the lateral regions of golgi membranes the lysosomal enzymes are segregated and packed.† Then many such small primary lysosomes containing similar enzymes or different enzymes fuse with endosomes or endocytotic or phagocytotic vesicle to produce secondary lysosomes.
Under conditions of nutrient availability, lysosomes are maintained at the periphery of the cell through a microtubule-dependent mechanism involving kinesins (KIF1B-β and KIF2A) and the monomeric GTPase ARL8B. mTORC1, which is bound to the cytoplasmic surface of lysosomes, is activated by upstream signalling when at the cell periphery. In the absence of nutrients, the increase in cytoplasmic pH prevents the transfer of lysosomes to a peripheral location through the release of kinesins and ARL8B from microtubules, resulting in the accumulation of lysosomes in the perinuclear area and inactivation of mTORC1. A major consequence of mTORC1 inactivation is the stimulation of autophagosome formation and the fusion of autophagosomes with lysosomes in the perinuclear area. http://www.nature.com/
Transcription factor EB (TFEB) controls lysosomal biogenesis by regulating the level of lysosomal enzymes, lysosomal acidification and the number of lysosomes. TFEB also controls autophagy by regulating the number of autophagosomes and the fusion between autophagosomes and lysosomes. Finally, TFEB regulates the docking and fusion of lysosomes to the plasma membrane in the process of lysosomal exocytosis. The concerted action of these three processes leads to cellular clearance. Carmine Settembre, Alessandro Fraldi, Diego L. Medina & Andrea Ballabio;www.nature.com
Biogenesis of †lysosomes; www.circresearch.com
††††††††††††††††† Some acid hydrolases; http://ztopics.com/
Plant Cell central Vacuoles;
They act as animal lysosomes. They can breakdown large molecules under acidic conditions for the vacuole sap is acidic.† Plant central vacuole develops from smaller provacuolar structures and Golgi vesicles. As plant cell expands the central vacuole increase in size and occupies more than 90% of the cell space and the all cell organelles pushed to peripheral region.† The Vacuole acts as storage organ, also store toxic components, it provides pH and ionic homeostasis; it also acts a defense organelle.
The central vacuole is bound by a membrane called Tonoplast, which contains many transporters. It contains several enzymes such as acid hydrolases, proteases, ribonucleases and glycosidases.
Storage disorders, lack of specific lysozymal enzymes can create problems, it can lead organ disorders, cause disease called Gaucherís disease (lack lipids) and Tay-Sachs disease- lack of glucocerebrosidase.