Regions of ER that lack ribosomes are called smooth ER.
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The smooth ER is involved in lipid metabolism. Most cells have very little smooth ER, but cells specializing in the production of steroid hormones or lipoproteins may have significant amounts of smooth ER. For example, the hepatocyte (liver cell) has abundant smooth ER–containing enzymes (P450) responsible for the manufacture of lipoproteins as well as the detoxification of harmful lipid-soluble compounds, such as alcohol. The cellular smooth ER can double in surface area within a few days if large quantities of drugs or toxins enter the circulation. Cells in the adrenal cortex and gonads that produce steroid hormones also have abundant smooth ER. In addition to synthetic functions, the ER sequesters calcium ions by pumping them from the cytoplasm. In response to specific signals, the ER releases calcium ions as part of important second-messenger cascades. Muscle cells have extensive smooth ER (sarcoplasmic reticulum) dedicated to the sequestration of calcium. When the cell is stimulated, the sarcoplasmic reticulum releases the calcium ions needed to accomplish muscle contraction.
FIG 3.10 Schematic and micrographs of three major types of cytoskeletal proteins. A, Microfilaments shown are composed of actin proteins. B, Intermediate filaments are a large group of various types of proteins. C, Microtubules. (From Patton KT, Thibodeau GA: Anatomy & physiology, ed 9, St. Louis, 2016, Mosby, p. 89. Micrographs from Pollard T, Earnshaw W: Cell biology, revised reprint, international edition, Philadelphia, 2004, Saunders.)
CHAPTER 3 Cell Structure and Function 33
fusing with a lysosome. The final products of lysosomal digestion are simple molecules, such as amino acids, fatty acids, and carbohydrates, which can be used by the cell or secreted as cellular waste at the cell surface.
Discovery of the mechanism for sorting and transport of lysosomal enzymes was aided by studying patients suffering from lysosomal storage diseases. Patients with I-cell (inclusion cell) disease, for example, accumulate large amounts of debris in lysosomes, which appear as spots, or “inclusions,” in the cells. These lysosomes lack nearly all of the hydrolases normally present and thus are unable to perform lysosomal digestion. However, all the hydrolases missing from the lysosomes can be found in the patient’s bloodstream. The abnormality results from “mis-sorting” by the Golgi apparatus, which erroneously packages the enzymes for extracellular secretion rather than sending them to the lysosomes. Studies of this rare genetic disease resulted in the discovery that all lysosomal enzymes have a common marker, mannose- 6-phosphate, which normally helps target the enzymes to the lysosomes. Persons with I-cell disease lack the enzyme needed for configuring this marker.
Peroxisomes (microbodies), like lysosomes, are membrane-bound bags of enzymes that perform degradative functions. They are particularly important in liver and kidney cells, where they detoxify various