Membrane transport

• 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