Ribosomes

 


RIBOSOMES( Palade Particle)

Ribosomes  are naked ribonucleoprotein protoplasmic particles ( RNP) with a lenght of 200_ 340 A° and diameter  of 170 _ 240 A° which function as the sites for protein of polypeptide synthesis. Many ribosomes occur in a cell e.g.,a single cell of E.coli has 20000__30000 ribosomes. In Eukaryotic cell their number is several times more .Ribosomes are known as  Protein factories. They are subspherical in outline and have no covering membrane. Each ribosome consists of two unequal subunits,larger dome shaped and smaller ovoid. The larger subunit has a protuberance,  a ridge and a stalk. The smaller subunit which is about half the size of larger subunit possesses a platform, cleft, head and base.The smaller subunit fits over the larger one at one end like a cap. Mg²+ is required  for binding  the two subunits. The two subunits dissociate below 0.0001 M Mg²+ while above this strength , the ribosomes  can come together  to form dimers. A role has also been attributed to Ca²+,Mn²+ and Co++.

     
The ribosomes are noticeable with electrons microscope only.






Discovery.

The ribosomes were noted by Romanian_ born electron microscopist and Nobel Laureate George Palada in animal cells in 1953. He named them ribosomes in 1954.They are also  called  Palade particles. 

Location:

The ribosomes are found in all cells, prokaryotic as well as eukaryotic. In prokaryotic cells, they float freely in the cytoplasm. In eukaryotic cells, they free in the cytoplasmic matrix and also attached to the outer ( cytoplasmic) surface of the Rough endoplasmic reticulum and nuclear envelop.Esherichia  coil cell may have 20,000 to 30,000 ribosomes and a eukaryotic cell may contain up to half a million ribosomes. The ribosomes are especially numerous in actively synthesizing cells, such as liver cells, pancreatic cells, endocrine cells, lymphocytes, yeast cells, and meristematic cells. Cells active in protein synthesis have more prominent nucleoli,which act as sites for ribosome assembling. Ribosomes are fewer in less active and starved cells. As expected, the cancer cells.have numerous ribosomes. 


     Bound and free ribosomes are similar in structure and are interchangeable. The cell can adjust the relative number of each type as its metabolism changes.



   The ribosomes are also found in the  matrix of mitochondria and the stroma of plastids in the eukaryotic cells. These are sometimes called  organeller ribosomes distinguish them form the cytoplasmic ribosomes, or cytoribosomes. The organeller ribosomes of mitochondria and plastids are termed  mitoribosomes and plastidoribosomes respectively. 

Ribosomes Types:

The ribosomes are of two types:70S and 80S¹

(a).70S:

The 70S ribosomes are found in the prokaryotic cells and in the mitochondria and plastids of the eukaryotic cells. This shows that the eukaryotic ribosomes are in some way related to those of the prokaryotes. The 70 S ribosomes are 200 to 300 A° long 170 to 210 A° wide.Each consists of a large 5p S subunit and a small 30 S subunit. Each subunit is composed of rRNA and several basic proteins. The 50S subunit has two types of rRNA: 23 S and 5S, and about 34 different types of proteins. The 30 S subunit has only one kind of rRNA, i.e., 16 S, and about 21 different proteins 

(b) 80S

The 80S ribosomes occur in the cytoplasm of eukaryotic cells. They are 300 to 350A° long 200 to 240 A° wide. Each consists of a large 60 S subunit and a small 40 S subunit. Each subunit is composed of rRNA and several specific basic proteins. The 60 S subunit has three types of  rRNA : 28 S, 5.8S and  5 S, and over 45  different ribosomal proteins. The 40S subunit had only kind of rRNA, i.e., 18 S and over 33 different ribosomal proteins 



Note:  S is Svedberg unit, a measure of particle size dependent on the speed with which the particles sediment in the ultracentrifuge, ( S= 1× 10-¹³ second). 



Structure:

Both types of ribosomes are similar in structure. They are minute, irregular, hydrated organelles without a membrane around them. Each ribosome consists of two subunits: one larger and the other smaller .The subunits occur separately in the cytoplasm, and join to form ribosomes only at the time of protein synthesis. At the time of active protein synthesis, when several copies of a particular polypeptide are to be formed, many ribosomes line up and join an mRNA chain.Such a string of ribosomes is known as polyribosome, or polysome.  The number of ribosomes in a polysome varies with the size of protein molecule being synthesized. Energy is needed for maintaining a polyribosome. After the synthesis of protein, the ribosomes leave that mRNA  chain and dissociate into subunits. Mg++ ions are through to bind the subunits to form a ribosome when required ¹.

Note: Concentration   of Mg++ ions ( 0.001 M) binds the subunits together, a lower concentration separates them.

Functions:

The ribosomes have the following functions __ 

( i) The ribosomes provide space for the synthesis of proteins in the cell. Hence, they are known as the "Protein factors "of the cell. The ribosomes bound to the membranes generally synthesize proteins for export as secretions by exocytosis, or for incorporation into membranes, or for inclusion into lysosomes. Such cells include pancreatic cells, liver cells, chief  cells in the gastric glands,  mammary glands cells, plasma cells, etc. The free ribosomes generally produce enzymic proteins for use in the cell itself.Such cells include developing muscle cells, skin cells, erythroblasts.Haemoglobin is an example of a protein made by free ribosomes in the young red blood corpuscles. Free ribosomes form extra amount of protein required when growth occurs. 


(ii) Ribosomes also furnish enzymes and factors needed for the formation of polypeptides. 

( iii) Ribosomes also provide sites for the attachment of tRNAs and mRNA which participate in protein synthesis. 

( iv) Newly formed polypeptide is protected from degradation by cytoplasmic enzymes in a tunnel of the large ribosomes subunit before releasing it into RER lumen.

( v) Groove between the two subunits furnishes the site for the synthesis of polypeptide. 

Ultrastructure:

A eukaryotic ribosomes shows a groove at the junction of the small and large subunits.From this groove, a tunnel extends through the large subunit and  opens into a canal of the endoplasmic reticulum. It is held that polypeptides are synthesized in the groove between the two ribosomes subunits and pass through the tunnel of the large subunit into the endoplasmic reticulum. 



Chemical Composition  :

A ribosomes consists of a highly folded rRNA molecule and many attached proteins, and Mg2+ ions. The distribution of rRNA molecules and protein molecules . The Mg²+ ions, as stated earlier,  help in binding the subunits together.  The proportion of rRNA and proteins is variable in both types of ribosomes,namely, 70S and 80S. The ratio of RNA to protein in prokaryotic and eukaryotic ribosomes is 60; 40 and 50 : 50 by weight respectively. The rRNA content of the ribosomes gives them their name.

      The ribosomes proteins are basic and of many kinds. Some are structural, others enzymic in function.  The enzyme,  peptidyl transferase,which brings about the actual formation of a peptide bond, is an integral part of the large subunit. The rRNA is through to be on the inside of the ribosome, almost entirely covered by the proteins.The ribosomes are, thus, ribonucleoprotein particles ( RNP).
       
     The ribosomes RNA forms about 80% of the total cellular RNA.


Origin:

In prokaryotic cells, both ribosomes RNAs and proteins are synthesized in the cytoplasm. In eukaryotic cells, ribosomes RNAs are synthesized in the nucleolus, except 5 S rRNA that is formed elsewhere in the nucleus. The ribosomes proteins are synthesized in the cytoplasm and shift to the nucleolus for the formation of ribosomal subunits by complexing with rRNA . The subunits pass out into the cytoplasm through the nuclear pores.


Q: Different Between RER and SER

 RER 

(1): Consists mainly of cisternae. 

(2): Bears ribosomes on its cytoplasmic surface.

(3) : Usually lies deep in the  cytoplasm. 

(4): Takes part in the synthesis of proteins. 

( 5): Arises from the nuclear envelope by outfolding.

( 6): Gives rise to SER.


(7): Contains ribophorins for binding ribosomes. 

( 8): Has pores to let polypeptides synthesized on ribosomes pass into E.R.lumen.

SER:

1)  Consists mainly of tubules and vesicles.

2). Is free of ribosomes. 

3). Usually lies near the cell membrane. 

4). Takes part in the synthesis of lipids, steroids, glycogen. 

(6). Gives rise to Golgi apparatus, lysosomes and vacuoles.

7). Ribophorins are lacking. 

8). Lacks pores. Materials synthesized on SER are released into the cytoplasmic matrix.


Q: Different Between Prokaryotic and Eukaryotic Ribosomes: 

70S Ribosomes:

1. Occur in prokaryotic cells, and in mitochondria and plastids of eukaryotic cells. 

2: Are smaller in size, measuring 200_ 300 A° × 170 _ 210 A° , and heavier in weight.

3:  Exist free in cytoplasmic matrix, mitochondrial matrix and plastid stroma.

4: Have 50 S and 30 S subunits.

5: Large ( 50S) subunit has 23S and 5 S rRENs and 34 different basic proteins. 

6: Small ( 30 S ) subunit has 16 S rRNA and 21 different  basic proteins. 

7: RNA : proteins = 60: 40.

8. Are synthesized in the cytoplasm. 

9: Sedimentation or Svedberg coefficient is 70.

80S Ribosomes:

1): Occur only in cytoplasm of eukaryotic cells.

2): Are larger in size, measuring 300_ 350 A° × 200_ 240 A°, and lighter in weight. 

3): Exist free in cytoplasmic matrix, and also attached to ER and outer surface of nuclear envelope.

4): Have 60 S and 40 S subunits.

5): Large ( 60 S) subunit has 28S , 5.8S and 5 S rRNAs and 45 different basic proteins. 

6): Small ( 40 S) subunit has 18 S rRNA and 33 different basic proteins. 

7): RNA : protein= 50:50.


8): Are synthesized in the nucleolus. 

9): Sedimentation or Svedberg coefficient is 80.


















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