endocytosis. exocytosis

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  • Exocytosis, Endocytosis
    • Principles
      • membrane bound vesicle carry cargo from donor to recipient
      • vesicles bud from specialised coated regions of the donor
      • coat is shed, allowing the vesicle to fuse with the recipient
      • Transport
        • vesicles have surface markers to identify origin and cargo
        • complementary receptors on target membrane
        • (vesicular) v-SNARES and (target) t-SNARES
          • wrap together forming trans-SNARE (docked)
        • Rab proteins also act as markers
          • recognised by Rab effector proteins
    • Protein coats
      • COPI
        • golgi to ER
      • COPII
        • ER to Golgi
      • Clathrin
        • Golgi to lysosomes and cell membrane
        • each subunit has 3 large and 3 small polypeptides
        • 3 legged triskellion
        • adaptin needed to attach
        • budding vesicle pinched off by dynamin
    • membrane fusion
      • very close- no water between
      • trans-SNARE complex can act as a winch
      • use energy as helices wrap to expel water
    • Exocytosis
      • constitutive secretory pathway
        • default pathway
        • vesicles bud from trans golgi network
      • regulated secretory pathway
        • cells store secretory proteins in secretory granules
        • acidic conditions of golgi cause secretory products to aggregate
        • trans golgi network loosely bound immature secretory vesicles
        • concentration of secretory proteins increases in vesicles
          • retrieval of vesicles from golgi
          • increased acidity causing more aggregation
        • secretory vesicle protein may be processed by proteolysis
          • enables short peptides to be produced
          • protects cell from own hydrolytic enzymes
        • may require trigger
    • Endocytosis
      • pinocytosis
        • clatherin coated pits form vesicles which lose coat and fuse with early endosomes
        • delivered to lysosome for digestion
      • receptor mediated endocytosis
        • LDLs react with LDL receptors
        • reacts with adaptin, guides to pits
        • in endosome, LDL released from receptor, deivered to lysosome
    • Phagocytosis
      • macrophages
      • neutrophils
      • dendritic cells
      • large particles bind to surface of these phagocytes
      • recognised by cell receptors
      • stimulates cell to extend pseudopods which engulf
      • phagosomes fuse with lysosome


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