• Endoplasmic means "within the plasm" and reticulum means "network".
• The endoplasmic reticulum is a network of folded membranes that form channels. They extend from the cell membrane through the cytoplasm to the nuclear membrane.
• The ER was discovered in 1945 by Albert Claude in Belgium and Keith Porter at Rockefeller Institute. It was first noticed in the cytoplasm of chick embryo cells.
• All eukaryotic cells have endoplasmic reticulum except the red blood cells of mammals.
• The membranes of the ER are concentrated in the inner region of the cell, the endoplasmic region.
• The membranes surround and inner cavity called the lumen. The lumen is basically a compartment for storage of substances that must be kept separate from the cytoplasm.
• The main function of the ER is to transport materials through the cell.
• It also serves as the site of biochemical reactions.
• There are two types of endoplasmic reticulum, rough ER and smooth ER.

Rough Endoplasmic Reticulum
(RER)

• The RER is dotted with ribosomes that form granules on the surface to give it a "rough" appearance.
• The RER is involved in the synthesis of proteins.
• The ribosomes on the RER make the protein with a specific amino acid sequence that allows it to enter the inner ER before it obtains its tertiary structure.
• Because the protein cannot travel through the lumen in its 3-D shape, it is pushed out into the smooth ER.
• Areas of the ER membrane break off to form a sac around the protein called a vesicle. A vesicle is a transport vehicle for the protein.
• Because most proteins are not mature when they leave the lumen, the vesicles take them to a golgi complex or other cellular organelle.
• In the mid 1950's, George Palade discovered that the amount of RER in a cell corresponds to the quantity of protein a cell exports.
• Example: White blood cells that produce infection fighting immune system proteins called antibodies have highly developed RER

Smooth Endoplasmic Reticulum
(SER)

• Not dotted with ribosomes.
• Involved with the synthesis of fatty acids and membrane components such as lipids.
• Contains enzymes that help detoxify and process chemicals, like drugs (especially downers) and alcohol.
• Found in LIVER CELLS (makes chemicals water soluble)