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Transcript
4.5 THE ENDOMEMBRANE SYSTEM The endomembrane system is a series of functionally connected organelles in which lipids are assembled and new polypeptide chains are modified. Its products are sorted and shipped to different destinations. Figure 4.13 shows how its organelles—the ER, Golgi bodies, and vesicles—interconnect with one another. Endoplasmic Reticulum The functions of the endomembrane system begin with endoplasmic reticulum, or ER. In animal cells, the ER is continuous with the nuclear envelope, and it extends through the cytoplasm. Its membranes appear rough or smooth, depending on whether ribosomes are attached to the membrane facing the cytoplasm. 5 Vesicles budding from the Golgi membrane transport finished products to the plasma membrane. The products are released by exocytosis. 4 Proteins and lipids take on final form in the space inside the Golgi body. Different modifications allow them to be sorted out and shipped to their proper destinations. assorted vesicles Golgi body smooth ER 3 Vesicles bud from the ER membrane and then transport unfinished proteins and lipids to a Golgi body. 2 In the membrane of smooth ER, lipids are assembled from building blocks delivered earlier. We typically observe rough ER arranged into stacks of flattened sacs with many ribosomes attached. Every new polypeptide chain is synthesized on ribosomes. But only the newly forming chains having a built-in signal can enter the space within rough ER or become incorporated into ER membranes. (The signal is a sequence of fifteen to twenty specific amino acids.) Once the chains are in rough ER, enzymes may attach oligosaccharides and other side chains to them. Many specialized cells secrete the final proteins. Rough ER is abundant in such cells. For example, in your pancreas, ER-rich gland cells make and secrete enzymes that end up in the small intestine and help digest your meals. Smooth ER is free of ribosomes and curves through cytoplasm like connecting pipes. Many cells assemble most lipids inside the pipes. Smooth ER is well developed in seeds. In liver cells, certain drugs as well as toxic metabolic wastes are inactivated in it. Sarcoplasmic reticulum, a type of smooth ER in skeletal muscle cells, functions in muscle contraction. Some vesicles form at the plasma membrane, then move into the cytoplasm. These endocytic vesicles might fuse with the membrane of other organelles or may remain intact, as storage vesicles. Other vesicles bud from ER and Golgi membranes, then fuse with the plasma membrane. The contents of these exocytic vesicles are thereby released from the cell. rough ER 1 Some polypeptide chains enter the space inside rough ER. Modifications begin that will shape them into the final protein form. SECRETORY PATHWAY DNA instructions for building polypeptide chains leave the nucleus and enter the cytoplasm. The chains (green) are assembled on ribosomes in the cytoplasm. Golgi Bodies In Golgi bodies, enzymes put the finishing touches on proteins and lipids, sort them out, then package them inside vesicles for shipment to specific locations. For example, an enzyme in one Golgi region might attach a phosphate group to a new protein, thereby giving it a mailing tag to its proper destination. Commonly, a Golgi body looks vaguely like a stack of pancakes; it is composed of a series of flattened membrane-bound sacs (Figure 4.14). In functional terms, the last portion of a Golgi body corresponds to the top pancake. Here, vesicles form as patches of the membrane bulge out, then break away into the cytoplasm. Figure 4.13 Endomembrane system, a membrane system in the cytoplasm that synthesizes, modifies, packages, and ships proteins and lipids. Green arrows highlight a secretory pathway by which some proteins and lipids are packaged and released from many types of cells, including gland cells that secrete mucus, sweat, and digestive enzymes. Figure 7.4 Page 1 Van_Blerkom.indd 4 2/6/08 2:53:51 PM internal space budding vesicle 0.25 μm Figure 4.14 Sketch and micrograph of a Golgi body from an animal cell. A Variety of Vesicles Vesicles are tiny, membranous sacs that move through the cytoplasm or take up positions in it. The lysosome, a common type, buds from Golgi membranes of animal cells and some fungal cells. Lysosomes are organelles of intracellular digestion. They hold enzymes that digest complex carbohydrates, proteins, nucleic acids, and some lipids. Often, they fuse with vesicles formed earlier from patches of plasma membrane that surrounded bacteria, molecules, and other items that docked at the membrane receptors. Lysosomes also digest entire cells and cell parts. For example, as a tadpole is developing into an adult frog, its tail slowly disappears. Lysosomal enzymes are responding to developmental signals and are helping to destroy cells that make up the tail. Peroxisomes, another type, are sacs of enzymes that break down fatty acids and amino acids. A product of the reactions, hydrogen peroxide, is toxic, as the Chapter 5 introduction describes. But enzyme action converts it to water and oxygen or uses it to break down alcohol and other toxins. Drink alcohol, and the peroxisomes of liver and kidney cells will degrade nearly half of it. In the ER and Golgi bodies of the endomembrane system, many proteins take on final form and lipids are assembled. Lipids, proteins (such as enzymes), and other items become packaged in vesicles destined for export, storage, membrane building, intracellular digestion, and other cell activities. Figure 7.4 Page 2 Van_Blerkom.indd 5 2/6/08 2:53:59 PM