Ribosomes

Ribosomes are often described as the protein factories of the cell -- they use the genetic information carried to them to synthesise proteins. They were first observed as dense particles in electron micrographs of cells by the scientist Palade in 1953. A ribosome is made up of ribosomal RNA together with proteins, and it has no membrane covering of its own.

In a eukaryotic cell, ribosomes are found in mitochondria, in plastids, and in the cytosol. Cytoplasmic ribosomes are either found attached to the outer surface of the rough endoplasmic reticulum and the nuclear membrane, or floating freely in the cytoplasm; both kinds are structurally similar and are of the 80S type, each made up of a large subunit and a small subunit (as opposed to the smaller 70S ribosomes -- made of 50S and 30S subunits -- found in prokaryotes, in mitochondria and in plastids).

Bound ribosomes generally make proteins that are destined to be transported out of the cell after processing in the ER and Golgi body -- for example, the bound ribosomes of the acinar cells of the pancreas produce the pancreas's digestive enzymes. Free ribosomes, by contrast, generally make enzymatic proteins that stay and work within the cytoplasm itself, such as the enzymes needed for the breakdown of sugars; free ribosomes often cluster together into chains called polyribosomes (or polysomes) while carrying out protein synthesis. Both bound and free ribosomes are able to interchange position and function as needed, and cells that are actively engaged in protein synthesis tend to have a higher number of ribosomes overall. …