Lysosomes



LYSOSOMES: 

Lysosomes are small vesicles which are bounded by a single membrane and contain hydrolytic enzymesAbout 40  enzymes  occur in them with different  sets of enzymes in different types of lysosomes. The important  enzymes  include acid phosphatases, sulphatases, proteases, nucleases, lipases and glycosidases.They are also called acid hydrolases because  these digestive enzymes usually function in acidic medium. Acidic conditions are maintained inside by pumping of H+ into the lysosomes. The membrane of lysosomes keeps the hydrolytic  enzymes  out of contact from the  cellular contents but they become fragile in the absence of the oxygen,  or the presence of excess of vitamins A and E, male and female hormones, bile salts, X_ rays and ultra _ violet Rays.  The membrane is protected from these agencies by cortisol, chloroquine and a type of cholesterol. Substances causing stability of lysosomes membrane are known as stabilizers while those causing instability are known as labilizers.Lysosomes are called suicide bags because of the presence of a larger number  of digestive enzymes, happens to get broken, the various cellular  constituents would undergo lysis.

  Lysosomes are generally  rounded  but can be irregular in outline.  The interior of lysosomes may be almost solid or differentiated into outer denser region and a central less dense mass with granular content. They occur in all animal cells with the exception  of red blood corpuscles. In animals, lysosomes are abundant in leucocytes, macrophages, Kuffer's cells and similar cells with phagocytic activity.They are present  in fungi, Euglena, root tip cells of maize, cotton and pea seeds. In other plants the function of lysosomes is shared by spherosomes, aleuronegrains and vacuoles. 


   Lysosomes   are formed by the joint activity of endoplasmic reticulum   Golgi  complex. The precursors of hydrolytic enzymes are mostly  synthesised  at the rough endoplasmic reticulum which transfers them to the forming face of Golgi complex through  its vesicles  where are pinched off as lysosomes from the maturing face. Some portions of smooth ER which lies near the   Golgi apparatus may also bud off lysosomes directly. 

   Lysosomes  do not normally  burst in the cytoplasm. All materials  which are to be acted upon by lysosomes enzymes are usually enclosed inside vacuoles and the vacuoles fuse with the lysosomes for digestion of materials. Though lysosomes take part in intracellular digestion of various  types of materials of endogenous or exogenous  origin, extracellular digestion  can be performed  by them under certain  conditions. 

  Lysosomes  pass through  various  stages in the same cell. The phenomenon is called polymorphism ( existence of more than one morphological form) . 

The lysosomes are noticeable with electron microscope only. They contain digestive enzymes ( G.lyso = digestive, soma= body).

 



Discovery: 

The lysosomes were first reported by a Belgian  cytologist and biochemist Christain se Duve in 1955.His finding was based on biochemical studies rather than on actual observation. In 1956, Novikoff observed lysosomes in the cell well electron microscope. This confirmed  Duve's Conclusion. The term lysosomeswas coined by Novikoff.


Occurrence:

The lysosomes occur practically in all animal cells and protozoans. A few cell types, such as the mammalian red blood corpuscles, however, lack lysosomes. The lysosomes have also been found in fungi and certain plant cells, such  as yeasts, fungi, root _ tip cells of maize, cotton and pea seeds. However, they are less prevalent in plant cells than in animal cells. Prokaryotic  cells are without lysosomes. 



Animal Lysosomes:

Number:

 Number of lysosomes in a cell varies with the cell type. They are particularly a abundant in certain white blood corpuscles macrophages, Kupffer's cells  and in secretory cells of pancreas, spleen, liver, kidneys. Cells of degenerating or regressing tissue, e.g., tadpole's tail, also contain many lysosomes. 


Morphology:

The lysosomes are generally spherical in form. Certain meristematic cells of plant roots, however, have irregular lysosomes. The lysosomes vary in size from 0.2 to 0.8 ųm. However, some cells, such as leucocytes, may have upto 5 ųm wide lysosomes. A lysosomes is a tiny sac bounded by a single unit membrane of lipoprotein. It contains a dense, finely granular fluid.The latter consists of glycoproteins hydrolytic ( digestive) enzymes called acid hydrolases. These include proteases, lipases, nucleases, glycosidases, sulphatases, acid phosphatases, etc. The human lysosomes contain more than 40 different  digestive  enzymes. However, all the enzymes do not occur in the same lysosomes. There are different sets of enzymes  in different lysosomes. The lysosomes enzymes can break down all major biological   macromolecules present in the cells or entering the cells from outside into their buildings block subunits  by addition of water. The lysosomes  enzymes  are active in acid medium, at about pH 5, hence their name. Acidic medium is maintained by pumping protons into the lysosomes. 

       Lysosomes store the hydrolysing enzymes of the cell. Their membrane prevents the  enzymes from escaping into the  cytoplasm  and destroying it. The materials needing hydrolysis must enter the lysosomes because enzymes remain confined within them. In injured and dead the lysosome membrane ruptures spontaneously, releasing the enzymes that lyse ( dissolve) the weakened cells. This process is postmortem degenerationIn the intact living cells, certain compounds, such as cholesterol and cortisone, prevent the rupturing of the lysosomes membrane. 




Kinds:


The lysosomes change the nature of their contents at different times in the same cell. This variation is referred to as polymorphism.  On the basis of their contents, four types of lysosomes are recognised : primary lysosomes, secondary lysosomes, residual bodies, and autophagic vacuoles. 


(i) Primary Lysosomes:newly formed lysosomes contain enzymes only. It is called the primary lysosomes. Its enzymes are probably in an inactive state.

 (ii) Secondary Lysosomes: When some material to be digested enters a primary lysosomes,  the latter is named the secondary lysosomes,or phagolysosome or digestive vacuole, or heterophagosome. This commonly occurs by fusion of a primary lysosomes with a vacuole ( pinosome or phagosome) or a secretory granule.

(iii) Residual Bodies:  In a secondary lysosomes,the enzymes digest the incoming materials.  The products of digestion pass through the lysosomes membrane into the cytoplasmic matrix for use as a source of nutrition or energy. Indigestible matter remains in the secondary lysosomes. A secondary lysosome containing indigestible matter is known as the residual body or tertiary lysosomes .latter meets the cell membrane and the residue is released from the cell by exocytosis ( ephagy).Residual bodies may be stored in the cells,e.g.,heart muscle cells, liver cells ( hepatocytes). This contributes to aging process.

 (iv) Autophagic VacuolesA cell may digest it's own organelles,  such as  mitochondria and ER. This process is called autophagy, or autolysis .Primary lysosomes fuse together about the damaged or unwanted organelles, forming a large sac known as an autophagic vacuole, or autophagosome, or autolysosome. The enzymes of lysosomes digest the organelles thus enclosed. Therefore, the lysosome are sometimes called disposal units.The products of digestion enter the cytoplasm and are reprocessed into new molecules. This gives the lysosomes the name of recycling units.




Origin:


Lysosomes arise from the Golgi complex. Their membrane and hydrolytic enzymes are synthesised on the Rough ER and are transported in transport vesicles to the Golgi complex for modification and packaging. Secretory vesicles filled with lysosomes enzymes but off from the trans face of the Golgi complex as primary lysosomes. Proteins of the inner surface of the lysosomes membrane escape enzymatic breakdown probably by having three_ dimensional conformations that protect the vulnerable bonds from  enzymes attack.







Functions:


The lysosomes enzymes take part in several normal decomposition processes in a cell. 

(i) Digestion of Useful MaterialsThe organic substances ( food particles) taken up by the cells in vacuolesb ( pinosomes or phagosomes) from the environment are digested in the lysosomes. This is called intracellular digestionIt is a regular feature in protozoans, sponges and coelenterates.

(ii) Digestion of Harmful  Materials: The foreign particles ( viruses, bacteria and toxic molecules) are disposed of by hydrolysing them in certain leucocytes and macrophages. This is called natural defence of the body. This activity of lysosomes is characteristic of higher animals. 

 (iii) Digestion of Unwanted Materials: The dead cells and debris that accumulate at the sites of injury are destroyed in some leucocytes. This is called natural scavenging of the body.

 (iv) Renewal of Cell and organellesThe old worn out cells and cell organelles are broken down to make the component molecules available for the formation of new  cells and cell organelles. Thus, the lysosomes facilitate the turnover of cells in tissue and of organelles in normal cells. A human liver cell recycle half of its macromolecules each week.

(v) Feeding of Starving AnimalsFood to a starving animal is provided by digestion the stored food materials ( proteins,  lipids, glycogen) and even the cells. This is called autophagy.

(vi) AutolysisAutolysis caused by the lysosomes enzymes plays a role in the disappearance of tail during the metamorphosis of frog's tadpole.The tail cells are digested, and the molecules released are absorbed and reused by other cells of the body. Lysosomes  enzymes also cause softening of gum tissue to allow eruption of teeth in vertebrates. Lysosomes breakdown the matrix of cartilage for ossification. They also breakdown the matrix of the bone during  remodeling of bone that can occur in response to injury, new stresses, etc. For autolysis, enzymes of primary lysosomes are released from the cell.

  (vii) Aid in fertilization: The lysosomes of sperms release their enzymes to dissolve the egg membranes for the entry of the sperm into the ovum in fertilization. This is called  extracellular digestion. 

 (viii) Initiation of Mitosis: The lysosomes are supposed to initiate mitosis in cells by removing the agents that repress it. 

 (ix) Thyroxine FormationIn the thyroid gland, thyroglobulin is hydrolyzed by lysosome enzymes to form active thyroxine.

 (x) Cell's Protection:  The lysosomes membrane protects the cell contents from autolysis by lysosomal enzymes. This shows the importance of compartmental organization of the cell to its function.

Many inherited disorders result from lack of certain hydrolytic enzymes in the lysosomes. In Tay__ Sachs disease, for example, absence of a lipid__ digesting enzymes impairs the brain due to accumulation of  lipids  in the cells. 




Popular Names:

The lysosomes may be called " "suicide bags" of the cell in view of their autolytic role, or "disposal units" of the cell because they digest the incoming food materials and remove the foreign bodies, toxic molecules, and debris; or " "recycling centers" they break down worn out cells and cell organelles to components molecules for building new organelles and cells. 





Plant Lysosomes:


Plant lysosomes may contain certain reserve food materials besides the digestive enzymes. They are of 3 kinds :
● Spherosome 
● Aleurone grains
●  Vacuoles. 

1. Spherosome:

The Spherosomes were discovered by Perner in 1953.They are, spherical bodies, about 0.5_ 1ųm wide and enclosed by a single unit membrane. They contain granular contents rich in  lipids  but also have some proteins. They occur in most plant cells but are abundant in the endosperm cells of oil seeds. Spherosomes, arise from the endoplasmic reticulum. In certain tissues, such as maize root tip, the spherosome have hydrolytic enzymes. 


2. Aleurone Grains: 

The Aleurone grains are membrane _ bound, spherical bodies containing proteins and phosphate.They occur in the cells of endosperm and cotyledon of seeds.


3. Vacuoles:

The Vacuoles are somewhat sacs bounded by a single unit membrane. They show little internal structure, but contain a variety of hydrolytic enzymes. 


What are lysosomal storage diseases?
Ans: The lysosomal diseases cause accumulation of materials awaiting digestion in the lysosomes. The firest disease was identified in 1963 by the Belgian investigator H.G.Hers. it was named glycogen storage disease. The cells of the person with this disease lack the lysosomal enzyme, œ_ glycosidase, which degrades glycogen. As a result, the cells of liver, muscles and other tissues contain large glycogen filled lysosomes. In another lysosomal storage disease, lipids accumulate in the lysosomes due to lack of lipid_ degrading enzyme.


Functions:

1): They  take part in digestion of food obtained through phagosomes known as intracellular digestion. 

2): Lysosomes  release  their enzymes  to the outside through  exocytosis to perform extracellular  digestion. 

3): Lysosomes  of leucocytes  devour foreign  proteins, toxic substances,  bacteria  and other microorganisms thus taking part in natural  defence of the body.

4): In the metamorphosis of many animals  (e.g., amphibians,  tunicates) certain embryonic parts like tail, gills, etc. are digested through the agency of lysosomes. The digested food is used in the growth of other parts.

5): Lysosomes destroy the obstructing structures. 

6): Lysosomes  provide nourishment during starvation by rapidly hydrolysing the organic foods stored in the cells ( carbohydrates,  fats and proteins). Extra nourishment may also be got  by digesting some cellular  organelles  and cells.

7): Lysosomes  perform intracellular  scavenging in long_ lived cells by removing  old or useless  organelles. 


8): In sperms lysosomes provide enzymes  for breaking limiting membrane of eggs.


9): Lysosomes cause breakdown  of aging and dead cells.


10): In thyroid, active hormone  Thyroxine is formed through hydrolysis  of thyroglobulin by the agency of lysosomes. 


11): They seem to be essential  for cell division  perhaps by overcoming  agents that cause repression of mitotic cycle.



12): Lysosomes  may harm genetic  material through the release  of nucleases which may result in mutations,  breakage  of chromosomes and other abnormalities. Blood cancer may be the result of such an activity. 


13): Lysosomes  usually  remove carcinogens by engulfing and separating them.But, when the carcinogen is in excess, lysosomes may harm the living cells e.g., in lung fibrosis caused by silicosis or asbestosis. 


14): Leucocyte granules  are derived  from lysosomes. 

15): Lysosomes  of osteoclasts, at the time of formation  of bones from cartilage and during remodeling of the bone, cause breakdown  of the existing  matrix  which may be replaced by the new one.





































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