![]() The membrane surface of the ER performs several functions for cells. This chapter describes the following: (1) the overall functions and organization of the ER (2) insertion of proteins into and across the ER membrane (3) the mechanisms of folding, assembly, and degradation of proteins in the ER and (4) the synthesis and metabolism of lipids by the ER.Įndoplasmic Reticulum Functions and Organization Erik Snapp, Albert Einstein College of Medicine, Bronx, New York.) E, Fluorescent micrograph of a cell expressing an ER marker tagged with green fluorescent protein (appears white in this image). Wrapping of this network around DNA led to the formation of the nuclear envelope and nucleus. The invaginated membranes then proliferated into a reticular network. A–D, The ER may have originated by invagination of regions of the plasma membrane containing protein translocation channels (red complexes), which transfer newly synthesized proteins across the membrane. The flow results from the large rate of synthesis of lipids and proteins (between 2 million and 13 million new proteins per minute) occurring at the ER membrane, as well as from the continuous export of these molecules into the secretory pathway and their selective retrieval back to the ER from the Golgi apparatus.įigure 20-1 model for origin of endoplasmic reticulum/nuclear envelope and fluorescent image of endoplasmic reticulum distribution within a cell. The ER’s size and shape are maintained over time despite a continuous flow of proteins and lipids into and out of this compartment. This allows the ER to coordinate diverse processes over large regions of the cytoplasm. ![]() Microtubules and their associated motors generate this extended network in animal cells by pulling ER membranes out toward the periphery of the cell. Consisting of an extensive array of tubules or flat saccules called cisternae (cisterna means “reservoir”), the ER forms a continuous three-dimensional network (a reticulum) stretching from the nuclear envelope to the cell surface ( Fig. About one third of all cellular proteins are imported into the lumen of the ER or integrated into its membranes. ![]() It provides both an expanded membrane surface (up to 30 times that of the plasma membrane) for carrying out vital cellular functions, including protein and lipid biosynthesis, and an internal compartment (or lumen) that collects proteins synthesized in the cytoplasm for modification and delivery into the secretory pathway. The ER is thought to have evolved from the prokaryotic plasma membrane by expansion, internalization, and subdivision (Fig. One of the key distinguishing features of eukaryotic cells is the presence of the endoplasmic reticulum (ER), the largest of numerous membrane-delineated intracellular compartments.
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