* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download .. Golgi Bodies
Survey
Document related concepts
Extracellular matrix wikipedia , lookup
Cell nucleus wikipedia , lookup
Cell culture wikipedia , lookup
Tissue engineering wikipedia , lookup
Cellular differentiation wikipedia , lookup
SNARE (protein) wikipedia , lookup
Cytokinesis wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
Cell encapsulation wikipedia , lookup
Cell membrane wikipedia , lookup
Signal transduction wikipedia , lookup
Proteolysis wikipedia , lookup
Transcript
.. THE CYTOMEMBRANE SYSTEM The cytomembrane system is a series of organelles in which lipids are assembledand new polypeptide chains are modified into final proteins. Its products are sorted and shipped to different destinations. Figure 4.13shows how its organelles-the ER, Golgi bodies, and various vesicles-functionally interconnect with one another. Endop lasrnic Reticul urn The functions of the cytomembrane system begin with endoplasmic reticulum, or ER. In animal cells, the ER is continuous with the nuclear envelope and extends through cytoplasm. Its membrane regions appearrough or smooth, depending mainly on whether ribosomes are attached to the membrane facing the cytoplasm. .1 We typically observerough ER arranged into stacks of flattened sacswith many ribosomesattached). 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 string 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 secreteenzymes that end up in the small inte~tine and help digest your meals. SmoothER 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, some drugs and toxic metabolic wastes are inactivated in it. Sarcoplasmic reticulum, a type of , smooth ER in skeletal muscle cells, I functions in muscle contraction. Golgi Bodies In Goigi bodies, enzymes put the finishing touches on proteins and lipids, sort them out, and 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-boundsacs(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 Cytomembrane system, a membrane system in the cytoplasm that assembles, modifies, packages, and ships proteins and lipids. Green arrows highlight a secretory pathway by which certain proteins and lipids are packaged and released from many types of cells, including gland cells that secrete mucus, sweat, and digestive enzymes. ~ I A Variety of Vesicles ROUGH ER SMOOTH ER GOLGlBODY Vesicles are tiny, membranous sacs that move through the cytoplasm or take up positions in it. A common type, the lysosome, buds from Golgi membranes of animal cells and certain fungal cells. Lysosomes are organelles of intracellular digestion. They contain a potent brew, rich with diverse enzymes that speed the breakdown of proteins, complex carbohydrates, nucleic acids, and some lipids. Often, lysosomes fuse with vesicles that formed at the plasma membrane. The vesicles typically contain molecules, bacteria, or other items that docked at the plasma membrane. Lysosomes even digest whole cells or 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 tiny sacs of enzymes that break down fatty acids and amino acids. Hydrogen peroxide, a potentially harmful product, forms during the reactions. Enzyme ~ction converts it to water and oxygen or channels it into reactions that break down alcohol. After someone drinks alcohol, nearly half of it is degraded in peroxisornes of liver and kidney cells. Many proteins take on final fonn and lipids are synthesized in the ER and Golgi bodies of the cytomembrane system. Figure 4.14 Sketch and micrograph of a Golgi body from an animal cell. Lipids, proteins (such as enzymes), and other items become packaged in vesicles destined for export, storage, membrane building, intracellular digestion, and other cell activities. Text material adaptedfrom C. Starr and R. Taggart,Biology: The !Oks/Cole,2001),pp. 64-65. lrce: