Ribosomes are large non membranous RNA-protein complexes which are necessary for protein synthesis. These are dense granules of 150 to 200 A diameter (as revealed by electron microscope) and found either in free state or attached to the outside of cytoplasmic membrane just like that of ER or nuclear membrane etc., through ribophorins. These are also called Palade particles as in animal cells, first of all observed by Palade in 1955. In plants, however, these are reported two years early ie. In 1953 by Robinson and Brown in bean roots. Other all three cell organelles ie, Lysosomes, (single layered) mitochondria (double layered) and peroxisome (single layered) are bound by lipid protein (unit membrane ) covering.