Transporters and Ion Channels

Page 4: Mechanism of Passive Transporters

• Passive transporters move solutes along electrochemical gradients (e.g., glucose transporter).

• Electrical component of glucose's electrochemical gradient

  • Glucose is uncharged, relies on concentration gradient.

• Glucose transporter: random conformational switching

  • Binding site alternates between exterior and interior of the cell.

  • Blood glucose regulation through import/export mechanisms.

Page 5: Types of Pumps

• Pumps transport solutes against their electrochemical gradients:

  • ATP-driven pumps: Utilize ATP hydrolysis energy for uphill transport.

  • Gradient-driven pumps: Use downhill transport of one solute for uphill transport of another.

  • Light-driven pumps: Harness energy from sunlight.

Page 6: Na+-K+ Pump Mechanism

• The Na+ pump uses ATP hydrolysis to transport:

  • 3 Na+ out

  • 2 K+ in

• Benefits of coupling two ions:

  • Prevents energy loss during ATP hydrolysis.

• Maintains cytosolic low Na+ and high K+ concentrations.

• Cycle dependent on sequence of steps; if one fails, the whole cycle halts.

• Effect of ouabain: halts pump by preventing K+ binding.

Page 7: Gradient Potential from Na+-K+ Pump

• Na+-K+ pump generates a steep Na+ concentration gradient.

• Conceptualize this gradient as energy storage, analogous to water behind a dam.

Page 8: Ca2+ Pumps

• Ca2+ pumps maintain low cytosolic Ca2+ similar to Na+-K+ pumps.

• Both are ATPases with similar amino acid sequences.

• Key difference: Ca2+ pump does not require a secondary ion for operation.

Page 9: Gradient-Driven Pump Types

• Gradient-driven pumps utilize energies:

  • Symport: Moves two solutes in the same direction.

  • Antiport: Moves solutes in opposite directions.

  • Uniport: Moves one type of solute (not a pump).

Page 10: Glucose – Na+ Symport

• Glucose transport from gut lumen to blood via symport, regardless of high intracellular concentration.

• Driven by the electrochemical Na+ gradient for active import of glucose.

• Cooperative binding: enhances transport efficiency.

• Prevents Na+ leakage when glucose is absent.

Page 11: Localization of Transporters

• Different transporters are localized at apical and basal sides of cells.

• Glucose symport on the apical side; glucose uniport on the basolateral.

• Uniport releases glucose into bloodstream, aiding in regulation of glucose distribution.

Page 12: Light-Driven Pumps

• Light-driven pumps (e.g., bacteriorhodopsin) generate H+ gradients in some bacteria.

Page 13: Impact of Na+-K+ Pump Inhibition on Glucose Transport

• Question regarding how ouabain impacts glucose transport on the apical membrane:

  • Possible outcomes addressed (options A-D) regarding glucose transport dynamics.

Page 14: Resting Membrane Potential Establishment

• K+ leak channels and Na+-K+ pump critical for resting membrane potential.

• Na+-K+ pump maintains Na+ low and K+ high in cytosol.

• K+ leak contributes to negative membrane potential.

Page 15: K+ Movement Dynamics

• K+ moving down its concentration gradient faces electrical gradient opposition.

• Equilibrium set when K+ concentration gradient balances the membrane potential.

• Variation in resting membrane potential (-20 to -200 mV).

Page 16: K+ Channel Selectivity

• K+ leak channels selectively permit K+ ions.

• Na+ ions, though smaller, cannot pass due to channel structure specifics, favoring K+ interactions.

Page 17: Ion Channel Activity Monitoring

• Ion channels change between open/closed states randomly.

• Patch clamp technique used to measure ion flow in isolated membrane patches.

Page 18: Gated Ion Channels

• Gated mechanisms regulate ion channel states based on:

  • Voltage changes (voltage-gated channels)

  • Ligand binding (ligand-gated channels)

  • Mechanical forces (mechanically-gated channels)

Page 19: Mechanically-Gated Channels in Auditory Cells

• Mechanical stimulation opens channels in auditory hair cells.

• Stereocilia movement from sound vibrations initiates channel opening, allowing ion influx.