Membrane transport

    Cards (12)

    • methods of passive transport: channels, simple diffusion, through a facilitative transporter
      methods of active transport: Pumps
    • Simple diffusion allows small inorganic solutes such as O2, Co2, and H2O to pass through the bilayer. They also have a high lipid solubility.
      • Hypertonic solution: higher solute conc' outside of the cell
      • Hypotonic sol'n: lower solute con'c outside of cell
    • Simple diffusion through a channel:
      • aquaporin: channel protein that allows water through quicker than just by diffusion through the bilayer
      • ion channels: transmembrane structure permeable to a specific ion or ions.
      • most ion channels are gated: require a change in conformation to open/close
    • Types of ion channels:
      1. Voltage-gated: open/close based on the difference in charge across the 2 sides of the membrane
      2. ligand-gated: open/close depending on the binding of a specific molecule (ligand) that is usually not the solute passing through the channel
      3. Mechano-gated: open/close depending on mechanical forces (eg, stretching, soundwaves, etc)
    • Is a facilitative transporter passive or active transport?
      Passive
    • is transport through a channel or bilayer passive or active?
      passive
    • Facilitative transporter
      Binding of the solute triggers a conformational change in the transmembrane protein that exposes the solute to the other side.
      • the transporter is specific to the molecule transported
      • exhibits saturation-type kinetics because the have to undergo a conformational change
    • Practice question (select letter)
      f
    • Primary active transport: pumps
      three types:
      • P-type pump: pump becomes phosphorylated during active transport
      • V-type pump: transports hydrogen ions across organelles and vacuoles
      • ABC transporter: ATP-binding-cassette transporters; share a similar of ATP binding domain and transport ions, lipids, peptides, and nucleotides.
    • Per ATPin Na+/K+ ATPase
      • 3 Na+ transported out of the cell
      • 2 K+ ions pumped into cell
    • Na+/K+ ATPase
      Step 1: E1 conformation: high affinity for Na+, pump faces the cytosol. ATP is bound.
      Step 2: Ion is bound, protein closes (occluded E1)
      Step 2-3: Hydrolysis of ATP, pump is phosphorylated with phosphate from ATP. ADP and P still bound.
      Step 3-4: Release of ADP triggers change to E2 conformation. Pump faces the extracellular side, low affinity of Na+ and high affinity for K+.
      Step 5-6: Ion is bound, protein closes (occluded E2). Dephosphylation .
      Step 7-8: ATP binds, protein returns to E1 and there is high Na+ affinity
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