Lecture 3-4: Ion Channels and Membrane Potential

Ion Channels & Membrane Potential

• Overview of how ion channels contribute to membrane potential in cells.

Ionic Gradients in Cells

• Plant and animal cells utilize distinct ionic gradients, which influences cellular functions.

  • Animal Cells: Primarily use sodium (Na+) ions.

  • Plant Cells: Utilize proton (H+) gradients.

Examples of Ion Pumps

Transport Mechanisms

• Na+-driven pumps:

  • Active import of glucose through glucose-Na symport using sodium gradient.

  • Na+-H+ exchanger for pH regulation and ion balance.

  • Na+-K+ ATPase: active transport of Na+ out and K+ into cells maintaining ion balance.

• Ca2+ pumps:

  • In eukaryotic cell membranes for calcium export crucial for muscle function.

• H+ pumps (H+ ATPase):

  • Active export of H+ ions in lysosomal membranes (animal cells) and vacuoles (plant and fungi).

Ion Channels and Selectivity

• Ion channels exhibit selectivity based on size and charge of ions.

  • Selectivity Filters:

    • Sodium channels prevent potassium ions from crossing due to size difference.

    • Potassium channels designed to interact with K+ ions due to larger radius allows interaction with carbonyls in the filter.

Channel Dynamics

• Ion channels oscillate between open and closed states influenced by protein conformation and external signals.

  • Rates of transition affect the overall ion flow and responses in the cell.

Membrane Potential Creation

• Membrane potential arises from unequal ion distribution across membranes.

  • A few ion movements can significantly impact membrane potential; e.g., 6000 K+ ions crossing can result in a 100mV change.

Nernst Equation

• Used to calculate equilibrium potential for individual ions, reflecting no net movement when forces balance.

  • Positive potential indicates more positive charges inside, negative means more negative charges inside.

Equilibrium Potential of Ions

• Understanding how equilibrium potentials correlate with ion concentrations inside and outside the membrane (Table Data). Examples include:

  • K+:

    • Equilibrium potential ( V_K ) calculated with Nernst equation: VK = 61.5mV log([K+]_out/[K+]_in).

• Net movement direction determined by the difference in resting membrane potential and equilibrium potential.

Potassium Leak Channels

• Major drivers of membrane resting potential due to passive K+ ion movement.

  • Operating mostly independently of Na+ channels but influences potential significantly.

  • Functional dynamics create a K+-driven resting potential around -70mV.

Patch Clamp Technique

• Technique to measure activity of individual ion channels through isolated patches of membrane.

  • Allows researchers to observe channel functions under varying conditions and treatments.

Various Ion Channel Mechanisms

• Different stimuli lead to channel opening:

  • Voltage-gated channels respond to membrane potential changes.

  • Ligand-gated channels open when bound by specific molecules.

  • Mechanically-gated channels react to physical changes (e.g., touch).

Cell Movement and Ions in Plants

• Certain plants employ a mix of mechanically and voltage-gated channels for movement.

  • Venus Flytrap: Detects prey, triggering electrical signals that close its leaves.

  • Mimosa pudica: Responsive to touch via propagating electrical signals causing leaf folding.

Key Concepts Summary

• Ions establish electrical gradients; sodium in animals vs. protons in plants.

• Ion channels' selectivity due to amino acid arrangements.

• Membrane potentials described relative to inner and outer leaflets (e.g., -70mV indicates inner is more negative).

• Nernst equation defines equilibrium potentials with implications for ion movement.

• K+ leak channels greatly influence resting membrane potential, maintained through ion pumps.