Ribosomes are spherical, ovoid, granular and bulky organelles. It can be triangular or pentagonal. Its diameter is 150-200Å. It is 22nm in width and 20nm in height. Each ribosome has two subunits. The smaller subunit is at the top and the larger subunit is at the bottom. The small subunit consists of three parts. Head, Foot and Stage. The large subunit consists of three parts. Apex, peduncle and swollen center.
In real cells, there is a groove where the two subunits of the ribosome meet. Proteins are synthesized in this cavity. A narrow tunnel runs through the large subunit from the shaft. Endoplasmic reticulum moves through these tunnels. When two subunits of a ribosome are joined, four sites appear. Aaminoacyl site, peptidyl site, exit site and mRNA attachment site. Many ribosomes join together to form polyribosomes. Ribosomes associated with chloroplasts, mitochondria and nuclear membranes are called ribonucleo-protein particles. E. coli bacteria contain 1300-20,000 ribosomes. That is, about 22 percent of the dry weight of E. coli bacteria is ribosomes.