Ribosomes are the cell organelles which serve as the site for protein synthesis (Translation). These are made up of special proteins and nucleic acids. Ribosomes are found in all living cells. In prokaryotic cells, they float freely in the cytoplasm. Whereas in the eukaryotes they can be found at many places, you can find them freely floating in the cytosol, attached to the endoplasmic reticulum or attached to the nuclear membrane. Ribosomes which float in the cytosol make proteins which are used in the cell. Those which are attached to ER make proteins which are either used inside the cell or are transported outside the cell. Ribosomes attached to nuclear envelope synthesize proteins for release in the perinuclear space.
Structure and Function of Ribosomes
They are made up of two subunits, one is called larger subunit and the other is called smaller subunit. In eukaryotes, the larger subunit is 60S and smaller subunit is 40S.The smaller subunit mainly performs the catalytic function while the smaller one mainly performs the decoding function. In the larger subunit, the ribosomal RNA functions as an enzyme so it is called a ribozyme and the smaller subunit links up with the mRNA and then locks with the larger subunit. While in prokaryotes the larger subunit is 50S and smaller is 30S. Apparently, there seems a small difference in the structure of ribosomes in prokaryotes and eukaryotes, but scientists have used this difference to develop many drugs which can kill disease causing prokaryotic microorganisms.
Major functions of ribosomes are:
(i) They translate the encoded information provided by mRNA in the cell nucleus,
(ii) they link the amino acids which are selected and collected by the tRNA from the cytoplasm (however, the order of linking up of amino acids is determined by mRNA),
(iii) They then export the polypeptide formed in this process to the cytoplasm where it will be used to form a functional protein. Ribosomes exist only temporarily. When they have synthesized the requires polypeptide, the two subunits either break up or are re-used. They can join amino acids at the speed of 200 per minute.