The smooth endoplasmic reticulum may be recognized in electron micrographs as areas of bare membrane tubules in the cytoplasm. The tubules may branch extensively and may be observed to be continuous with areas of rough endoplasmic reticulum.

An electron micrograph of smooth endoplasmic reticulum in a liver cell, posted on the UTMB website, laboratory of Gwen V. Childs, Ph.D.

The smooth endoplasmic reticulum (SER) serves numerous functions, a major example of which is to synthesize fatty acids and phospholipids for use in all of the cellular membranes. In liver cells, it contains enzymes that metabolize glycogen, and those that detoxify lipophilic drugs and transform bile pigments. The SER has an important impact on the half-lives of life-saving medications as well as the modification of toxins so that they can be detoxified and expelled in the urine.

In muscle the SER is called the sarcoplasmic reticulum and it is the major cellular storage and release organelle for ionized calcium, the direct stimulus for muscle contraction.

In steroid-secreting cells such as those in the adrenal cortex, ovarian follicles, and testes, the SER is a major cellular organelle, with multiple enzymatic systems specialized for certain steps in the synthesis of steroid hormones such as cortisol, testosterone, and estrogens.

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