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Unformatted text preview: Chapter 8 Cytoplasmic Membrane Systems: Structure, Function, And Membrane Trafficking An Overview of the Endomembrane System and Its Dynamic Nature I. Before the 20th century - stained tissue sections hinted at an extensive membrane network in cytoplasm A. 1940s - EM revealed diverse array of membranous structures in cytoplasm of most eukaryotes 1. Membrane-bound vesicles of varying diameter; containing material of different electron density 2. Long channels bounded by membranes that radiate through cytoplasm; form an interconnected network of canals 3. Stacks of flattened, membrane-bound sacs ( cisternae ) B. These studies & subsequent biochemical studies showed that eukaryotic cell cytoplasm was subdivided into a variety of distinct membrane-bound compartments 1. Saw distinct organelles in diverse cells from yeast to higher plants and animals 2. The organelles may appear as stable structures, but, in fact, they are dynamic compartments that are in continual flux 3. These organelles have distinct structures & functions but together form an endomembrane system; the individual components function as part of coordinated unit C. Mitochondria & chloroplasts are not part of this interconnected system D. Current evidence suggests that peroxisomes have a dual origin 1. The basic elements of the boundary membrane probably arise from the endoplasmic reticulum, 2. But most of the membrane proteins & soluble internal proteins are taken up from the cytoplasm II. These organelles are part of dynamic, integrated network; materials are shuttled between parts of cell A. Transport vesicles shuttle things between organelles; form by budding from donor compartment 1. Vesicle implies a spherical-shaped carrier; cargo may also be transported in irregular or tubular shaped membrane-bound carriers 2. But the term vesicle is often used, keeping in mind that they are not always spherical B. Transport vesicles move in directed manner, often pulled by motor proteins operating on tracks formed by microtubules & microfilaments of the cytoskeleton C. When they reach their destination, they fuse with acceptor compartment, which receives vesicles' soluble cargo & membrane wrapper D. Exhibit repeated cycles of budding & fusion that move a diverse array of materials along numerous pathways traversing the cell III. Several distinct pathways through cytoplasm have been identified; they fall into two groups: a biosynthetic (secretory) pathway & an endocytic pathway IV. Biosynthetic (secretory) pathway synthesis in ER (protein) or Golgi (lipid, carbohydrate); altered as pass through Golgi, sent from there to various locations (membrane, lysosome, large plant cell vacuole, etc....
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