LYSOSOMES

 

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.

Structure:

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.

Lysosomal Enzymes:

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.

Functions:

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.

Biogenesis of lysosomes

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.