A large portion of the ER is grouped with ribosomes, which are the factories of protein synthesis. Ribosomes are also enriched with lipids, particularly phospholipids. Ribosomes consist of particles of two sizes: one with a sedimentation constant of 50S and the other with 30S. The two particles combine to give one with a sedimentation constant of 70S. Ribosomes are the site of final stage of protein synthesis.