Membranes

Endocytosis and Exocytosis

III. Traffic of Large Molecules: Endocytosis and Exocytosis

  1. General Features
    1. Small molecules cross membranes by:
      1. Passing through the phospholipid bilayer.
      2. Being translocated by a transport protein.
    2. Large molecules (e.g. proteins and polysaccharides) cross membranes by the mechanism of exocytosis and endocytosis.
  2. Exocytosis
    1. Process where a cell secretes macromolecules by fusion of vesicles with the plasma membrane.
    2. Vesicle usually budded from the ER or Golgi and migrates to plasma membrane.
    3. Used by secretory cells to export products (e.g. insulin in pancreas, or neurotransmitter from neuron).
  3. Endocytosis
    1. Process where a cell takes in macromolecules by forming vesicles derived from the plasma membrane.
    2. Vesicle forms from a localized region of plasma membrane that sinks inward; pinches off into the cytoplasm.
    3. Used by cells to incorporate extracellular substances.
  4. There are three types of endocytosis: (1) phagocytosis, (2) pinocytosis and (3) receptor-mediated endocytosis.
    1. Phagocytosis
      1. (cell eating) Endocytosis of solid particles.
      2. Cell engulfs particle with pseudopodia and pinches off a food vacuole.
      3. Vacuole fuses with a lysosome containing hydrolytic enzymes that will digest the particle.
    2. Pinocytosis
      1. (cell drinking) Endocytosis of fluid droplets.
      2. Droplets of extracellular fluid are incorporated into small vesicles.
      3. The process is not discriminating. The cell takes in all solutes dissolved in the droplet.
    3. Receptor-Mediated Endocytosis
      1. Endocytosis when coated pits form vesicles when specific ligands bind to receptors on the cell's surface.
      2. More discriminating process than pinocytosis.
      3. Enables cells to acquire bulk quantities of specific substances, even if they are in low concentration in extracellular fluid (e.g. cholesterol).
      4. Membrane-embedded proteins with specific receptor sites exposed to the cell's exterior, cluster in regions called coated pits.
      5. A layer of clathrin, a fibrous protein, lines and reinforces the coated pit on the cytoplasmic side.
      6. A molecule that binds to a specific receptor site of another molecule is call a ligand.