Functions of Smooth ER
Appears smooth in the electron microscope because its cytoplasmic surface lacks ribosomes. Smooth ER functions in diverse metabolic processes:
1. Synthesizes lipids, phospholipids and steroids.
 
            · For example, mammalian sex hormones and steroids secreted by the adrenal gland.

            · Cells that produce and secrete these products are rich in smooth ER (e.g. testes, ovaries, skin oil glands).

2. Participates in carbohydrate metabolism.

            · Smooth ER in liver contains an embedded enzyme that catalyzes the final step in the conversion of
              glycogen to glucose (removes the phosphate from glucosephosphate).

3. Detoxifies drugs and poisons.
 
            · Smooth ER, especially in the liver, contains enzymes which detoxify drugs and poisons.

            · Enzymes catalyze the addition of hydroxyl groups to drugs and poisons. This makes them soluble in the
              cytosol so they may be excreted from the body.

            · Smooth ER in liver cells proliferates in response to barbiturates, alcohol and other drugs. This, in turn,
              may increase drug tolerance.

4. Stores calcium ions necessary for muscle contraction.

            · In a muscle cell, the ER membrane pumps Ca++ from the cytosol into the cisternal space.

            · In response to a nerve impulse, Ca++ leaks from the ER back into the cytosol, which triggers muscle cell
              contraction.