Describe history, location, types of ribosomes.
History. Ribosomes were first seen as granules by Cluade in 1941. Originally they were named as microsomes but the term ribosome was introduced by Palade in 1955.

Location. The ribosomes are the only organelles which are universally found in all cells i.e. prokaryotic and eukaryotic cells. In prokaryotic cells they are found freely floating in the cytoplasm, while in eukaryotic cells they are are usually found attached at the outer surface of endoplasmic reticulum. 

Shape and size. They are oblate spheroidal in form and have size of 18nm in bacteria, chloroplasts and mitochondria and 30 nm in eukaryotic cell.

Types. Ribosomes are of two basic types, 70S and 80S ribosomes. The ‘S’ refers to Svedberg units . 80S ribosomes are found in eukaryotes.  while the 70S is found in prokaryotes. The 70S ribosome is has 50S and 30S units whereas 80S is made up of 60S and 40S units.