The endoplasmic reticulum is part of the endomembrane system and accounts for over half of the total membrane in many eukaryotic cells. "Endoplasmic" means "within the cytoplasm" and "reticulum" is from the Latin term for "network". ER is indeed a network, one composed of membranous tubules and sacs called cisternae (Latin: "cisterna" means "box" or "chest"). The membrane separates the ER material compartment--the cisternal space--from the cytosol.

There are two different and distinct types of ER that have individual forms and functions: smooth ER and rough ER (shown in the illustration above at left). Smooth ER does not have attached ribosomes, whereas rough ER does--hence the name and appearance.

SMOOTH ER

This form of endoplasmic reticulum functions in diverse metabolic processes, including the synthesis of lipids, metabolism of carbohydrates, and detoxification of drugs and poisons. Smooth ER also produces steroids, such as sex hormones and steroid hormones secreted by adrenal glands. Ovary and testicle cells have a large quantity of smooth ER. Liver cells store carbohydrate as glycogen, which is hydrolyzed to release glucose from the cell to regulate sugar concentration in the blood. When it is hydrolyzed, its glucose phosphate (an ionic sugar that cannot enter the blood) is removed by smooth ER. Smooth ER detoxifies drugs in the liver, usually by adding a hydroxyl group or groups to make the substance more soluble and therefore easier to be expelled from the body. Barbiturates, alcohol, and many other drugs lead to the need and creation of more smooth ER and its detoxification enzymes. While this may seem like a good thing, what is happen is the body is increase its tolerance to the drugs, meaning higher "doses" are necessary to achieve the drug's intended effect. While the liver is working to detoxify excessive amounts of alcohol, it could also be detoxifying helpful drugs, such as antibiotics.

In muscle cells, the smooth ER membrane pumps calcium ions from the cytosol into the cisternal space. When a nerve impulse stimulates the muscle cell, the calcium moves back across the ER membrane into the cytosol quickly, triggering the contraction of the muscle cell.

ROUGH ER

As a polypeptide chain is forming from a bound ribosome, it moves into the cisternal space and folds into its protein conformation. Most secretory proteins are glycoproteins, which are bonded covalently to carbohydrates, attached to the protein when in the ER. The ER membrane keeps secretory proteins separate from proteins that will remain in the cytosol (those produced by free ribosomes). A specialized region in the ER called transitional ER buds out vesicles of secretory proteins to transport them away.

Rough ER is a membrane factory, building itself by adding on proteins and phospholipids. Polypeptides growing from ribosomes are inserted into the ER membrane. Enzymes in the ER membrane build phospholipids as well, piecing together snippets from the cytosol. ER membrane expands and can be transported to other parts of the endomembrane system via transport vesicles.