Ribosomes and Endoplasmic Reticulum

About this topic

Ribosomes are tiny structures made of RNA and protein that build proteins by reading mRNA. They float free in the cytoplasm or attach to the rough endoplasmic reticulum (RER), a network of membranes studded with ribosomes that makes proteins for export or membranes.

Smooth ER, lacking ribosomes, makes lipids, detoxifies chemicals and stores calcium. Proteins made on the RER are folded and sent in vesicles to the Golgi. Cells that secrete lots of protein, like pancreatic enzyme cells, have extensive RER.

Take insulin as an example. Ribosomes on the rough ER read the insulin mRNA and thread the growing protein into the ER interior, where it folds and is checked. It then moves in a vesicle to the Golgi for finishing and finally out of the pancreas cell into the blood. Cells that secrete large amounts of protein, like antibody-producing plasma cells, have an enormous rough ER.

Smooth ER is important in liver cells, where it helps break down alcohol and drugs, and in muscle cells, where a specialised version stores calcium for contraction. Students often think ribosomes are organelles with a membrane; they are not, which is why prokaryotes have them. The name rough comes only from the ribosomes studded on its surface under the electron microscope.