5. Cell Structure And Organization - part 08 - Lysosomes

 


5. Cell Structure And Organization - part 08 - Lysosomes


Lysosomes :

  • Lysosomes can be considered as dismantling and restructuring units of a cell.
  • These are membrane bound vesicles containing hydrolytic enzymes. 
  • The enzymes in lysosomes are used by most eukaryotic cells to digest (hydrolyse) macromolecules. 
  • The lysosomal enzymes show optimal activity in acidic pH. 
  • Lysosomes arise from Golgi associated endoplasmic reticulum.
  • The list of lysosomal enzymes includes all types of hydrolases viz, - 
  1. amylases
  2. proteases
  3. and lipases. 
  • These enzymes are in inactive state and are activated only when a lysosome comes in contact with another particular organelle to form a hybrid structure.
  • After the action of enzymes is over, the lysosome is reformed and re-used. 
  • Lysosomes are thus found in various structural forms and carry out various functions for the cell.
  • Lysosomes are polymorphic in nature.
  • We can classify lysosomes as - 
  1. Primary lysosomes
  2. Secondary lysosomes
Primary lysosomes -  

  • They are nothing but membrane bound vesicles in which enzymes are in inactive state.
Secondary lysosomes or hybrid lysosomes - 

  • They are formed by fusion of lysosome with endocytic vesicle containing materials to be digested, represented as heterophagic vesicle. 
  • This is larger in size than primary lysosome.
  • Residual body is the vesicle containing undigested remains left over in the heterophagic vesicle after releasing the products of digestion in the cytosol.
  • Lysosomes which bring about digestion of cells own organic material like a damaged cell organelle, are called autophagic vesicles (or suicide bags). 
  • An autophagic vesicle essentially consists of lysosome fused with membrane bound old cell organelle or organic molecules to be recycled. 
  • Remember, every week, a human liver cell recycles half of its macromolecules.
  • Lysosomes bring about intracellular and extracellular digestion. 
  1. Intracellular digestion :
  • The intracellular digestion is brought about by autophagic vesicle or secondary lysosomes which contain foreign materials brought in by processes like phagocytosis. 
  • e.g. Food vacuole in amoeba or macrophages in human blood that engulf and destroy harmful microbes that enter the body.

Always Remember :

  • Lysosomal enzymes do not digest their own membrane proteins. 
  • Three-dimensional shape of these proteins probably protects the membrane.
  • Accidental release of lysosomal enzymes in limited amount does not harm the cell because pH of cytosol is near neutral.
  • Any insufficiency in secretion of lysosomal enzymes leads to disorders.
  • e.g. in genetic disorder, Tey Sach’s disease, due to insufficiency of lipase, brain gets impaired due to accumulation of fats.


Extracellular digestion :

  • It is brought about by release of lysosomal enzymes outside the cell. 
  • e.g. acrosome, a cap like structure in human sperm is a modified lysosome which contain various enzymes like Hyaluronidase.
  • These enzymes bring about fertilization by dissolving protective layers of ovum.
  • During metamorphosis process found in many organisms, lysosomal enzymes help in reusing the tissues of redundant organs. 
  • They also help in destruction of malignant cells. e.g. T-lymphocytes.




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