The Endoplasmic Reticulum
The endoplasmic reticulum (ER) may form up to half of the cell’s total membrane (table 1.1). This proportion is strikingly high in those cells specialised for the export of lipids (such as hepatocytes and cells of the gonads) or of proteins (such as pancreatic cells or certain classes of B lymphocytes). However, almost all cells contain an extensive array of membranes that can be seen under the electron microscope to ramify throughout the cytosol (as in figure 1.3). Biochemical characterisation of he ER has been aided by the tendency of its membranes to fragment into small vesicles during during homogenisation of tissue samples. The resulting ‘microsomes’ can be readily prepared by centrifugation (section 2.2.1). As we shall see, microsomes have been widely used as cell-free model systems for studying many of the biosynthetic processes mediated by the ER.
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