The cell's protein synthesis is carried out by ribosomes. Ribosomes are responsible for protein binding in polypeptide chains. Ribosomes are unique in that they occur in both prokaryotic and eukaryotic organisms. Ribosomes are very small, spherical bodies that synthesize proteins and are bound to the endoplasmic reticulum or appear freely suspended in the cytoplasm.

• Discovery: Palade discovered the ribosome for the first time in 1955.
• Structure: These are circular or oval in shape, with a diameter of 100—200 A and a two-layered membrane covering them. 50 percent RNA and 50 percent histone proteins make up each ribosome. Ribosomes are classified as 70S or 80s based on their size and sedimentation index (S = Svedberg constant). The 70S ribosome is subdivided into SOS and 30S subunits once more. The 60S and 40S subunits make up the SOS ribosome.

• Function: The primary function of the ribosome is to synthesize protein. It aids fat metabolism as well.