Ribosomes are small globular particles of approximately 250Ao diameter and are nonmembranous. RNA and ribonucleoproteins are found in these in equal amounts. Robinson and Brown (1953) were the first to report ribosomes in bean roots. Later (1955) Palade reported ribosomes in animal cells. They are found in chloroplast, mitochondria and nucleus also. They are situated either on the wall of endoplasmic reticulum or are present in cytoplasm as independent bodies. They are also called RNP (Ribonucleoprotein particles).
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On the basis of sedimentation coefficient they are of two types:
(a) 70s Ribosome : They are smaller in size and their sedimentation coefficient is 70s (found in Bacteria and chloroplast).
(b) 80s Ribosome : Their sedimentation coefficient is 80s and are found in higher plants and animals.
Ribosome are small spherical bodies flat at poles. Each ribosome is made up of two subunits. One smaller and another larger subunit. In 70s ribosome the larger subunit is 50S and the smaller one 30S. In 80S ribosome, the larger subunit is 60S and smaller one 40S. In recent years more details about ribosome structure have become available. According to most recent account of James A.Lake (1981) the smaller sub-units consists of a head, a base and separating from head with the help of a cleft, a platform. Similarly, the larger subunit consists of a ridge, a central protuberance and a stalk. The ridge and the central protuberance are separated with the help of a valley.