Biology ENDOMEMBRANE SYSTEM , ENDOPLASMIC RETICULUM, GOLGI BODIE, LYSOSOME, VACUOLES

Lysosome :

`->` These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases -lipases, proteases, carbohydrases) optimally active at the acidic pH (`pH = 5`). These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.

`->` With the exception of mammalian RBC they were reported from all cells.

`->` In plant cells large central vacuole functions as Lysosome. So in higher plants lysosomes are less frequent. But number of lysosomes is high in fungi.

`->` Periplasmic Space:- space between cell wall and cell membrane in bacteria, may play similar role.

`->` Lysosomes are spherical bag like structures (`0.1-0.8 mu` m) which is covered by single unit membrane. They are large sized in Phagocytes (WBC) (`0.8` to `2 mu` m).

`->` Lysosomes are filled with `50` different type of digestive enzymes termed as Acid hydrolases.
These acid hydrolases function in acidic medium(` pH = 5`).Membrane of lysosome has an active `H^+` pump mechanism which produce acidic pH in lumen of lysosome.

`->` Lysosomes are highly polymorphic cell organelle. Because, during functioning, lysosomes have different morphological and physiological states.

TYPES OF LYSOSOMES

(1) Primary Lysosomes or storage granules- These lysosomes store enzyme Acid Hydrolases in the inactive form. (Enzymes synthesized on ribosomes in cytoplasm) these are newly formed lysosome.

(2) Digestive vacuoles or Heterophagosomes - These lysosome formed by the fusion of primary lysosomes and phagosomes. These are secondary Lysosomes.

(3) Residual bodies :- Lysosomes containing undigested material are called residual bodies. These may be eliminated by exocytosis. These are also called as Telolysosomes. (Tertiary lysosomes)

(4) Autophagic Lysosomes or Cytolysosomes or autophagosomes :- Lysosomes containing cell to be digested are known as Autophagosomes.

FUNCTIONS :

(1) Intracellular digestion:-

(a) Heterophagy :-This is digestion of foreign materials received in cell by phagocytosis and pinocytosis

(b) Autophagy :- Digestion of old or dead cell organelles. Autophagy also takes place during starvation of cell.

(2) Extracellular digestion :-

Lysosomes of osteoclast (bone eating cells) dissolve unwanted part of bones.

(Extracellular digestion also occurs by fungallysosomes.)
(3) Crinophagy :-
Excessive secretory granules of hormone in endocrine gland may be digested by lysosomes. This event is called crinophagy. Thyroglobulin stores in thyroid gland with its follicles and after crinophagy by proteases it thyroxine.

(4) Cellular digestion (Autolysis) :- Sometimes all lysosomes of a cell burst to dissolve the cell rr.,cn-r.!at.o!n (so Lysosome called as suicidal bags of cell). Old cells are removed by autolysis. unwanted organs of embryo are destroyed by autolysis Cathepsin of lysosome digests the tail of tadpole of frog during metamorphosis.

`->` Membrane stabilizers are substances, which stabilize the lysosome membrane and stop its rupture, thus prevents autolysis. e.g. cholesten)l, chloroquine, cortisone etc.

`->` Membrane labilizers are substances which make !he lysosome membrane fra~~ile and increase the chance of autolysis

e.g. Prc:JSJesterone, testosterone, Vitamin `A, D, E, K, U.V`. radiations, bile salts etc.

`->` Biogenesis of Lysosome Lyosornes originates from `G E R L- (Golgi associated Endoplasmic Reticulum from
which Lysosomes arise).

VACUOLES

`->` The vacuole is the membrane-bound space found in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell.

`->` The vacuole is bound by a single membrane called tonoplast. In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell.

`->` In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole, hence their concentration is significantly hi~Jher in the vacuole than in the cytoplasm.

`->` In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.