Cell Organelles | Ribosome

Ribosome- Protein factory of a cell

Ribosome are non-membrane bound organelle made of Ribo-nucleic acid (RNA) and proteins.

Discovered by the George Palade (1953), so also called the Palade particle.

Ribosome are the smallest cell organelle that’s why they are even present in the other organelle so that they are also called “Organelle within organelle”.

They are sometimes associated with the ‘Endoplasmic Reticulum (ER)’ to carry out the protein synthesis that’s type ER is called RER (Rough endoplasmic reticulum).

So, ribosome are also called the “Protein factory of the cell” or “Engine of the cell”.

Ribosome are universal organelle present in all the living cells except the mature RBC’s of the mammals.

Note- One might think viruses also lack the ribosome, well they are living only in the host cell.

And the viruses in biology and the hydrogen in the chemistry are the two things which we are unable to understand till now.

Structure of a Ribosome

Chemically ribosome are a type of nucleoprotein, consists of nucleic acid and protein. The nucleic acid in it is essentially the rRNA.

Ribosome are made up of two subunits one small and one large.

Both these subunits are dispersed in the cytoplasm randomly at a given time.

But at the time of the protein synthesis, both subunits associate together to form the fully functional ribosomes.

0.001M concentration of Mg+2 is required to complete this action.

Both these subunits contain the highly coiled RNA molecules of different sedimentation coefficient or Svedberg unit(S).

Types of the Ribosomes

There are two types of ribosomes



70 s Ribosome-

  1. Found in Prokaryotes.
  2. Have a small subunit 30s and a large subunit 50s.
  3. Small unit (the 30s) contain 16s rRNA.
  4. Large unit (the 50s) contain 5s rRNA and 23s rRNA.
  5. Location- Cytoplasm of prokaryotes, Mitochondria, and chloroplast.

80s Ribosome-

  1. Found in Eukaryotes.
  2. Have a small subunit 40s and a large subunit 60s.
  3. Small unit (the 40s) contain 18s rRNA.
  4. Large unit (the 60s) contains 5s rRNA, 28s rRNA, and 5.8s rRNA.
  5. Location- Cytoplasm of Eukaryotes.

Note– Don’t do the math here the ribosome sedimantion coefficient is not the addition of their subunits sedimentation coefficient. Their ‘S’ is calculated separately based on their mass and density.

To known more about the ‘S’ read this Wikipedia article.

Functions of the Ribosomes

Protein synthesis is the sole purpose of the ribosomes.

To initiate and complete protein synthesis ribosomes have to bind with the m-RNA (it contains the code for protein synthesis).

Ribosomes often form the polysome (polyribosome) to make the whole process faster.

Polysome is the structure where many ribosomes are bound to single mRNA.

Now, tRNA brings the amino acids and rRNA forms the peptide bond between them forming a Polypeptide chain. 

Many polypeptide chains combine with each other with the help of Disulphide bonds.

Now, Polypeptide is coiled in a specific manner to form the Quaternary structure of the protein (which is the active form of protein).

Important points-

  • rRNA is often called is the Ribozyme, due to its enzymatic activities.
  • Ribozyme was the first enzyme to appear on the earth.
  • In rRNA ‘r’ stand for the ribosomal RNA and in m-RNA ‘m’ stands for the messenger RNA.
  • During the association of the subunits if the concentration of the Mg+2 increased by 10 times then instead of a functonal ribosome a dimer is formed.
  • The dimer is the combination of the ribosomes with each other forming either 100s ribosome (by 70s+70s) or 120s ribosome (by 80s+80s) which are useless for the cell and are destroyed.

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