Most secretory polypeptides are glycoproteins proteins to which a carbohydrate

Most secretory polypeptides are glycoproteins

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Most secretory polypeptides are glycoproteins, proteins to which a carbohydrate is attached. Secretory proteins are packaged in transport vesicles that carry them to their next stage. Rough ER is also a membrane factory. Membrane-bound proteins are synthesized directly into the membrane. Enzymes in the rough ER also synthesize phospholipids from precursors in the cytosol.
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As the ER membrane expands, membrane can be transferred as transport vesicles to other components of the endomembrane system. The Golgi apparatus is the shipping and receiving center for cell products. Many transport vesicles from the ER travel to the Golgi apparatus for modification of their contents. The Golgi is a center of manufacturing, warehousing, sorting, and shipping. The Golgi apparatus is especially extensive in cells specialized for secretion. The Golgi apparatus consists of flattened membranous sacs—cisternae—looking like a stack of pita bread. The membrane of each cisterna separates its internal space from the cytosol. One side of the Golgi, the cis side, is located near the ER. The cis face receives material by fusing with transport vesicles from the ER. The other side, the trans side, buds off vesicles that travel to other sites. During their transit from the cis to the trans side, products from the ER are usually modified. The Golgi can also manufacture its own macromolecules, including pectin and other noncellulose polysaccharides. The Golgi apparatus is a very dynamic structure. According to the cisternal maturation model, the cisternae of the Golgi progress from the cis to the trans face, carrying and modifying their protein cargo as they move. Finally, the Golgi sorts and packages materials into transport vesicles. Molecular identification tags are added to products to aid in sorting. Products are tagged with identifiers such as phosphate groups. These act like ZIP codes on mailing labels to identify the product’s final destination. Lysosomes are digestive compartments. A lysosome is a membrane-bound sac of hydrolytic enzymes that an animal cell uses to digest macromolecules. Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids. These enzymes work best at pH 5. Proteins in the lysosomal membrane pump hydrogen ions from the cytosol into the lumen of the lysosomes. Rupture of one or a few lysosomes has little impact on a cell because the lysosomal enzymes are not very active at the neutral pH of the cytosol. However, massive rupture of many lysosomes can destroy a cell by autodigestion. Lysosomal enzymes and membrane are synthesized by rough ER and then transferred to the Golgi apparatus for further modification.
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