11th Class Biology Cell - The Unit Of Life Ribosome

The ribosomes are smallest known electron microscopic without membrane, ribonucleo–protein particles attached either on RER or floating freely in the cytoplasm and are the sites of protein synthesis. 

Discovery : In 1943 Claude observed some basophilic bodies and named them as microsome. Palade (1955) coined the term ribosome (form animal cell). Ribosomes in nucleoplasm were observed by Tsao and Sato (1959). First isolated by Tissieres and Watson (1958) from E. coli. Ribosomes found in groups are termed as polyribosomes or ergosomes (Rich and Warner 1963 observed first time polyribosomes).

Occurrence : In prokaryotes ribosomes are found only in free form in the cytoplasm. While in the eukaryotes the ribosomes are found in two forms in the cytoplasm, free form and bind form (bound on RER and outer nuclear membrane). These are also reported inside some cell organelles like mitochondria and plastids respectively called mitoribosomes and plastidoribosomes.

Types of ribosomes

(1) 70S ribosomes : Found in prokaryotes, mitochondria and plastid of eukaryotes.

(2) 80S ribosomes : Found in cytoplasm of eukaryotes.

(3) 77S, 60S and 55S ribosomes : Levine and Goodenough (1874) observed 77S ribosomes in fungal mitochondria 60S ribosomes in animal mitochondria and 55S in mammalian  mitochondria.

Structure : Each ribosome is formed of two unequal subunits, which join only at the time of protein synthesis. In 70S and 80S ribosomes, 50S and 30S, 60S and 40S are larger and smaller subunits respectively. Larger subunits is dome shaped and attached to ER by glycoproteins called “ribophorins”.

Smaller subunit is oval shaped and fits as a cap on flat side of larger subunit. Ribosomes are attached to ER through hydrophobic interactions.

Chemical composition : Ribosomes are chemically composed of rRNA and proteins Ribonucleo-Protein (RNP). 70S ribosomes has 60-65% rRNA and \[35-40%\] proteins (ratio is 1.5:1). rRNAs are of three types : 23S type and 5S type rRNAs in 50S and 16S type rRNA in 30S sub-units.

80S ribosome has 45% rRNA and 55% proteins (ratio is about 1 : 1). r-RNA are of four types : 28S, 5S and 5.8S types of rRNAs in 60S and 18S type rRNA in 40S sub-units.

A \[1\times {{10}^{-3}}\,(0.001M)\] molar concentration of \[M{{g}^{++}}\] is needed for the structural cohesion of ribosomes i.e., for holding the two subunits together. If this concentration is increased by ten folds, two ribosomes unite to form a dimer. By decreasing the \[M{{g}^{++}}\] conc. to normal, the dimer breaks into monomers (single ribosomes).

Biogenesis of ribosome

(1) In eukaryotes the ribosomal RNAs like 18S, 5.8S and 28S are synthesized by nucleolus and 5S RNA out of the nucleus.  

(2) In prokaryotes both rRNA and its protein are synthesized as well as assembled by cytoplasm.

Polyribosomes or Polysomes : When many ribosomes (generally \[68\]) are attached at some mRNA strand. It is called polysome. The distance between adjacent ribosomes is of 90 nucleotides. These are functional unit of protein synthesis.

Functions

(1) Ribosomes are also called protein factories of the cell or work branch of proteins.

(2) Free ribosomes synthesize structural proteins and bounded ribosomes synthesize proteins for transport.

(3) Ribosomes are essential for protein synthesis.

(4) Help in the process of photosynthesis.

(5) Enzyme peptidyl transferase occurs in large subunit of ribosome which helps in protein synthesis.

(6) Newly formed polypeptide is protected from degradation by cytoplasmic enzymes in large sub-unit of ribosomes before releasing it into RER lumen.