5. Cell Structure And Organization - part 12 - Ribosomes

 


5. Cell Structure And Organization - part 12 - Ribosomes


Ribosomes :

  • You are aware that ribosomes are protein factories of the cell. 
  • They use the genetic information to synthesise proteins.
  • Ribosomes were first observed as dense particles in electron micrograph of a cell by scientist Pallade in 1953. 
  • Ribosomes are made up of -
  1. Ribosomal RNA and 
  2. proteins. 
  • They do not have any membranous covering around them.
  • In a eukaryotic cell, ribosomes are present in mitochondria, plastids and in cytosol.
  • Ribosomes in cytoplasm are either found attached to outer surface of Rough Endoplasmic Reticulum and nuclear membrane or freely suspended in cytoplasm. Both are similar in structure and are 80S type. 
  • Each ribosome is made up of two subunits; a large and a small subunit.
  • Bound ribosomes generally produce proteins that are transported outside the cell after processing in ER and Golgi body. 
  • e.g. Bound ribosomes of acinar cells of pancreas produce pancreatic digestive enzymes. 
  • Free ribosome come together and form chains called polyribosomes for protein synthesis.
  • Free ribosomes generally produce enzymatic proteins that are used up in cytoplasm like enzymes required for breakdown of sugar.
  • Both types of ribosomes can interchange position and function. 
  • Number of ribosomes is high in cells actively engaged in protein synthesis.
  • The nucleoplasm or karyolymph contains various substances like- 
  1. nucleic acids
  2. protein molecules
  3. minerals and 
  4. salts.


  • It contains chromatin network and nucleolus.
  • Nucleolus is another component which is not bound by cell membrane.
  • Nucleolus is made up of rRNA and ribosomal proteins and it is best known as the site of ribosome biogenesis. 
  • Depending on synthetic activity of a cell, there are one or more nucleoli present in the nucleoplasm. 
  • For ex: cells of oocyte contain large nucleolus whereas sperm cells contain small inconspicuous one. 
  • They appear as dense spherical bodies present near chromatin network. 
  • They produce rRNA and ribosomal proteins which are then transported to cytoplasm and are assembled together to form ribosomes.

Always Remember :

  • The particle size of ribosomes is measured in terms of Svedberg unit (S). 
  • It is a measure of sedimentation rate of a particle in ultracentrifuge. 
  • It is thus a measure of density and size of a particle. 1S = 10-13 sec.

Know the scientists :

  • Venkatraman Ramakrishnan : Won Nobel Prize in Chemistry in the year 2009, for explaining the structure and working of ribosomes. 
  • He shared the prize with Yonath (Israel) and Thomas Steitz (USA).

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