BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.10. Cellular Structures

5.10.3. Endoplasmic Reticulum (ER)

One of The most significant discoveries made using the Electron microscope was the detection of a complex membrane system permeating the Cytoplasm of all Eukaryotic Cells. This network of membranes, named the Endoplasmic reticulum (ER; from the Latin reticulum, meaning network), is well-developed within The Cell yet lies beyond the resolution limit of the Light Microscope. Frequently, these membranes are studded with tiny particles known as Ribosomes.

In ultrathin sections, the ER appears as numerous pairs of parallel lines (membranes) located within the cytoplasm (Fig. 5.24). Occasionally, however, a section allows us to view The surface of these membranes directly, revealing that in three dimensions the ER possesses a sheet-like rather than purely tubular Structure. A model of the three-dimensional STRUCTURE OF THE ER is shown in Fig. 5.26. As can be seen, the ER consists of flattened membrane-bound sacs called cisternae. The ER cisternae may be studded with ribosomes, in which case it is referred to as rough or granular ER; when ribosomes are absent, it is called smooth or agranular ER (which tends to have a more tubular structure).

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Fig. 5.26. Three-dimensional model of The endoplasmic reticulum.

The Functions of the rough ER are associated with the transport of Proteins synthesized by the ribosomes on its surface. Protein Synthesis is described in detail in Ch. 23. For now, suffice it to say that the growing protein molecule—namely, the chain of Amino Acids, or the so-called polypeptide chain—remains attached to the ribosome until its synthesis is complete. A receptor in the ER membrane forms a channel through which the newly synthesized protein passes into the ER cisternae.

As it is transported through the cisternae, the protein typically undergoes substantial modifications along the way. For instance, it may be converted into a glycoprotein. The typical pathway for a protein leads from the rough ER to the Golgi apparatus, from which it is either secreted outside the cell or delivered to other Organelles within the same cell. The Enzymes contained in Lysosomes reach them via this exact pathway (see also Fig. 5.29).

One of the primary functions of the agranular ER is the synthesis of Lipids. For example, in the intestinal epithelium, the agranular ER synthesizes lipids from the Fatty acids and glycerol absorbed by the intestine, and subsequently transfers them to the Golgi apparatus for export. Steroids—a specific class of lipids—are also synthesized in the agranular ER. Steroids include several Hormones, such as corticosteroids synthesized in the adrenal cortex, or the Sex Hormones testosterone and estrogen. Muscle cells contain a specialized, modified form of the agranular ER known as the sarcoplasmic reticulum.