(a) ribosomes bind to the er membrane during co-translational translocation. (B) By contrast, cytosolic ribosomes complete the synthesis of a protein and release it prior to post-translational translocation. in both cases, the protein is directed to the er by an er signal sequence (red and orange).
200 nm 0.5 µm
Figure 12–33 The rough and smooth ER. (a) an electron micrograph of the rough er in a pancreatic exocrine  cell that makes and secretes large amounts of digestive enzymes every day. The cytosol is filled with closely packed sheets of er membrane that is studded with ribosomes. at the top left is a portion of the nucleus and its nuclear envelope; note that the outer nuclear membrane, which is continuous with the er, is also studded with ribosomes. (B) abundant smooth er in a steroid-hormone-secreting cell. This electron micrograph is of a testosterone-secreting leydig cell in the human testis.
(C) a three-dimensional reconstruction of a region of smooth er and rough er in a liver cell. The rough er forms oriented stacks of flattened cisternae, each having a lumenal space 20–30 nm wide. The smooth er membrane is connected to these cisternae and forms a fine network of tubules 30–60 nm in diameter. The er lumen is colored green. (D) a tomographic reconstruction of a portion of the er network in a yeast cell. membrane-bound ribosomes (tiny dark spheres) are seen in both flat sheets and tubular regions of irregular diameter, demonstrating that the ribosomes bind to er membranes of different curvature in these cells. (a, courtesy of lelio orci; B, courtesy of Daniel s. friend; C, after r.V. Krstic´, Ultrastructure of the mammalian Cell. new York: springer-Verlag, 1979; D, from m. West et al., J. Cell Biol. 193:333–346, 2011. With permission from rockefeller University press.) signals, as discussed in Chapter 15. A Ca2+ pump transports Ca2+ from the cytosol into the ER lumen. A high concentration of Ca2+-binding proteins in the ER facilitates Ca2+ storage. In some cell types, and perhaps in most, specific regions of the ER are specialized for Ca2+ storage. Muscle cells have an abundant, modified smooth ER called the sarcoplasmic reticulum. The release and reuptake of Ca2+ by the sarcoplasmic reticulum trigger myofibril contraction and relaxation, respectively, during each round of muscle contraction (discussed in Chapter 16).
To study the functions and biochemistry of the ER, it is necessary to isolate it. This may seem to be a hopeless task because the ER is intricately interleaved with other components of the cytoplasm. Fortunately, when tissues or cells are disrupted by homogenization, the ER breaks into fragments, which reseal to form small (~100–200 nm in diameter) closed vesicles called microsomes. Microsomes are relatively easy to purify. To the biochemist, microsomes represent small authentic versions of the ER, still capable of protein translocation, protein glycosylation (discussed later), Ca2+ uptake and release, and lipid synthesis. Microsomes derived from rough ER are studded with ribosomes and are called rough microsomes. The ribosomes are always found on the outside surface, so the interior of the microsome is biochemically equivalent to the lumen of the ER (Figure 12–34A).