Question

The type of ribosomes found in prokaryotic cells are
(a) 100S
(b) 80S
(c) 60S
(d) 70S

Answer 1

Hint: Ribosomes are macromolecular cell organelles that perform biological protein synthesis. They are the smallest known organelles. They can be of the 70S, 80S, 100S, etc types where ‘S’ is the sedimentation coefficient. Prokaryotic ribosomes have a lower sedimentation coefficient than eukaryotic ribosomes.

Complete answer:
Ribosomes are found in the cytoplasm of the cells. While they are found only in free form in the cytoplasm of prokaryotes, they are also found bound to the endoplasmic reticulum and outer nuclear membrane in the eukaryotes. They are also found inside some cell organelles like chloroplasts and mitochondria.
In eukaryotic cells, the ribosomes present are of 80S type, whereas in prokaryotic cells they are of 70S type.
Each ribosome is formed of two unequally sized subunits, which join temporarily at the time of protein synthesis.
In 70S ribosomes the larger subunit is 50S and the smaller subunit is 30S; while in the 80S ribosomes, the larger subunit is 60S and the smaller subunit is 40S.
The cohesion of subunits depends on the molar concentration of Mg++(0.001M) ions. If the concentration decreases the subunits detach. Conversely, if the concentration increases beyond 0.001M to ten folds, two ribosomes unite to form dimers.
So, the correct answer is, “70S”.

Note: Chloroplasts and Mitochondria are believed to have been developed from prokaryotic cells.A supportive factor for this theory is the presence of 70S ribosomes in these cell organelles. Endosymbiotic hypothesis: It is the hypothesis behind the origin of mitochondria and chloroplast, which states that they were originally specialized non-sulphur bacteria that survived endocytosis. They developed a symbiotic relationship with the host cell and became incorporated into the cells.