Endoplasmic Reticulum

Most cells contain a system of membranes known as the endo-plasmic reticulum, or ER. The ER may be either of two types: (1) a granular, or rough, endoplasmic reticulum and (2) an agranular, or smooth, endoplasmic reticulum (fig. 3.12). A granular endoplasmic reticulum bears ribosomes on its surface, whereas an agranular endoplasmic reticulum does not. The agran-ular endoplasmic reticulum serves a variety of purposes in different cells; it provides a site for enzyme reactions in steroid hormone

Cell Structure and Genetic Control 59

(a)

Tubule Membrane Ribosome

Tubule Membrane Ribosome

production and inactivation, for example, and a site for the storage of Ca2+ in striated muscle cells. The granular endoplasmic reticulum is abundant in cells that are active in protein synthesis and secretion, such as those of many exocrine and endocrine glands.

The agranular endoplasmic reticulum in liver cells contains enzymes used for the inactivation of steroid hormones and many drugs. This inactivation is generally achieved by reactions that convert these compounds to more water-soluble and less active forms, which can be more easily excreted by the kidneys. When people take certain drugs (such as alcohol and phenobarbital) for a long period of time, increasingly large doses of these compounds are required to achieve the effect produced initially. This phenomenon, called tolerance, is accompanied by growth of the agranular endoplasmic reticulum, and thus an increase in the amount of enzymes charged with inac-tivation of these drugs.

Clinical Investigation Clues

Remember that Timothy's liver cells have an unusually extensive smooth endoplasmic reticulum. Why is his endoplasmic reticulum so well developed, and what beneficial function might this serve? What could he do to determine if this is the cause of his liver problems?

(c)

■ Figure 3.12 The endoplasmic reticulum. (a) An electron micrograph of a granular endoplasmic reticulum (about 100,000x). The granular endoplasmic reticulum (b) has ribosomes attached to its surface, whereas the agranular endoplasmic reticulum (c) lacks ribosomes.

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