Membrane Function II

Content Overview

  • Topics to be Covered:

    • Introduction of plasma membrane

    • History of research on bio-membrane

    • Structure of bio-membrane

    • Function of bio-membrane (I & II)

    • Osmosis

    • Water Channel

    • Ion Channel

Plasma Membrane

  • Definition: The plasma membrane is selectively permeable; it allows hydrophobic molecules and small polar molecules to diffuse through the lipid layer while preventing ions and large polar molecules from passing.

  • Functional Role of Integral Membrane Proteins:

    • Enable ions and large polar molecules to pass through the membrane through passive or active transport.

Transport Mechanisms

Summary of Transport

  • Types of Transport:

    • Passive Transport:

      • Simple Diffusion

      • Facililated Diffusion

    • Active Transport:

      • Requires ATP

Osmosis

Definition

  • Osmosis: The net flow of solvent molecules through a semipermeable membrane, where solute molecules cannot pass. If a solution containing both solute and solvent molecules is on one side of the membrane, and pure solvent is on the other side, solvent will flow into the solution side.

Characteristics of Solutions

  • Homeostasis of Water Movement:

    • Hypertonic Solution: Higher concentration of solute outside the cell; cells will lose water.

    • Isotonic Solution: Equal concentration of solute inside and outside the cell; no net water movement.

    • Hypotonic Solution: Lower concentration of solute outside the cell; cells will gain water.

Osmosis' Impact on Cells

  • Effects on Plant and Animal Cells:

    • In hypotonic solutions, animal cells may lyse (burst), while plant cells do not due to their rigid cell walls.

    • In isotonic solutions, animal cells prefer this state to maintain their normal shape, while plant cells become flaccid.

    • In hypertonic solutions, both plant and animal cells will shrivel due to water loss (plasmolyzation in plants).

Osmotic Pressure

  • Osmotic Pressure Definition: The pressure applied to a solution to prevent inward water flow across a semipermeable membrane.

  • Mechanism: Increasing hydrostatic pressure on the solution side helps to nullify osmosis by increasing the solvent’s escaping tendency until equilibrium is reached.

Water Channels

Aquaporins

  • Function: Integral/transmembrane proteins facilitating rapid water movement across cell membranes while preventing ions and other solutes from passing.

  • Structure: Composed of six membrane-spanning alpha-helical domains; protects water movement through the narrow channel which restricts entry of larger molecules.

  • Mechanism of Water Movement:

    • Water's polar molecules need reorientation to pass through the aquaporin channel, with conserved asparagine residues helping in this process.

Ion Channels vs. Ion Pumps

Definitions and Functional Differences

  • Ion Channels: Allow passive flow of ions down their concentration gradient.

  • Ion Pumps: Actively transport ions against their concentration gradient, consuming energy (typically ATP).

Sodium-Potassium Pump Mechanism

  • Description: A transmembrane ATPase that exchanges 3 Na⁺ for 2 K⁺, utilizing ATP for energy to maintain ion gradients across the cell membrane:

    • Steps:

      1. Na⁺ binds.

      2. ATP hydrolysis occurs, leading to a conformational change in the pump.

      3. Na⁺ is ejected, and K⁺ binds.

      4. Conformation resets and releases K⁺ into the cell.

Classifications of Ion Channels by Type

  • Ion Channel Categories:

    • Chloride Channels

    • Potassium Channels

    • Sodium Channels

    • Calcium Channels

    • Proton Channels

    • Non-selective Cation Channels

Structural and Functional Overview of Channels

  • Voltage-gated Ion Channels: Activated by membrane potential changes, altering conformation for specific ion flow.

  • Resting Membrane Potential: Result of Na⁺ and K⁺ concentration differences influencing neuronal firing when action potentials occur.

Definitions and Concepts Clarified

  • Plasma Membrane vs. Cell Membrane: Often used interchangeably, but technically distinct; the plasma membrane typically refers to the membrane surrounding organelles, whereas the cell membrane encompasses the whole cell.

  • Osmotic Pressure Formula: = iVRT, where = osmotic pressure, R = 0.0821 ext{ L atm / mol K}, T = ext{temperature in Kelvin}, i = van 't Hoff factor, V = volume of solution, and M = molar concentration of solute.

Key Terms and Definitions

  • Plasma Membrane: Selectively permeable membrane allowing hydrophobic molecules and small polar molecules to diffuse through the lipid layer while preventing ions and large polar molecules from passing.

  • Integral Membrane Proteins: Proteins embedded in the membrane that enable ions and large polar molecules to pass through via passive or active transport.

  • Passive Transport: Movement of substances across the membrane down their concentration gradient, not requiring ATP.

  • Simple Diffusion: A type of passive transport where molecules pass directly through the lipid bilayer.

  • Facilitated Diffusion: A type of passive transport where molecules move across the membrane with the help of membrane proteins.

  • Active Transport: Movement of substances against their concentration gradient, requiring ATP.

  • Osmosis: The net flow of solvent molecules through a semipermeable membrane where solute molecules cannot pass, typically from an area of higher solvent concentration to lower.

  • Hypertonic Solution: A solution with a higher concentration of solute outside the cell, causing cells to lose water.

  • Isotonic Solution: A solution with an equal concentration of solute inside and outside the cell, resulting in no net water movement.

  • Hypotonic Solution: A solution with a lower concentration of solute outside the cell, causing cells to gain water.

  • Osmotic Pressure: The pressure applied to a solution to prevent the inward flow of water across a semipermeable membrane due to osmosis.

  • Aquaporins: Integral/transmembrane proteins that facilitate rapid water movement across cell membranes while preventing the passage of ions and other solutes.

  • Ion Channels: Membrane proteins that allow the passive flow of specific ions down their electrochemical gradient.

  • Ion Pumps: Membrane proteins that actively transport ions against their concentration gradient, consuming energy (e.g., ATP).

  • Sodium-Potassium Pump: A transmembrane ATPase that actively transports 3 Na⁺ ions out of the cell for every 2 K⁺ ions pumped into the cell, utilizing ATP.

  • Voltage-gated Ion Channels: Ion channels that open or close in response to changes in the membrane potential.

  • Resting Membrane Potential: The electrochemical potential difference across the cell membrane at rest, primarily influenced by Na⁺ and K⁺ concentration differences.

  • Cell Membrane: The membrane encompassing the whole cell, often used interchangeably with plasma membrane.

  • Osmotic Pressure Formula: \Pi = iVRT