Ribosomes are the protein-synthesizing machines of the cell.

They translate the information encoded in messenger RNA (mRNA) into a polypeptide.

Link to a description of the process

Ribosomes are

  • roughly spherical.
  • With a diameter of ~20 nm, they can be seen only with the electron microscope. The image on the right is an electron micrograph showing clusters of ribosomes. These clusters, called polysomes, are held together by messenger RNA (mRNA). (Image courtesy

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a system of membrane-enclosed sacs and tubules in the cell. Their lumens are probably all interconnected, and their membranes are continuous with the outer membrane of the nuclear envelope. All the materials within the system are separated from the cytosol by a membrane.

The endoplasmic reticulum is the site where the cell manufactures

The Golgi Apparatus

In this cell (from a bat), the Golgi apparatus (boxed in red) is used for the final stages in the synthesis of proteins that are to be secreted from the cell. (Courtesy of Keith R. Porter.)

The Golgi apparatus is a cell structure mainly devoted to processing the proteins synthesized in the endoplasmic reticulum (ER).