2.4-Membrane permeability

Cell Membrane Permeability

• Cell membrane permeability refers to the ability of certain substances to pass through the cell membrane at different rates.

• The plasma membrane is composed of phospholipids that are amphipathic, having both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

Selective Permeability

• Not all molecules can cross the cell membrane easily:

  • Nonpolar molecules (e.g., oxygen, carbon dioxide) can pass through the lipid bilayer easily due to their hydrophobic nature.

  • Polar molecules (e.g., water, ethanol) and charged molecules (ions) struggle to diffuse across the membrane without assistance.

Transport Mechanisms

• Transport Proteins: Facilitate the movement of larger molecules or charged ions across the membrane. Types include:

  • Channel Proteins: Provide a hydrophilic channel for specific molecules (e.g., aquaporins for water).

  • Carrier Proteins: Change shape to transport substances such as the sodium-potassium pump.

Types of Transport

• Passive Transport: No energy required; occurs naturally due to concentration gradients.

  • Diffusion: Movement from higher concentration to lower concentration until equilibrium is reached.

    • Example: Dye molecules diffusing in water to equalize concentration on both sides of the membrane.

  • Osmosis: Specific movement of water across a semipermeable membrane, necessary for maintaining cell integrity.

    • Tonicity: Refers to the relative concentration of solutes in a solution.

      • Isotonic: Equal solute concentrations, no net movement of water.

      • Hypotonic: Higher concentration of solute outside the cell, causing the cell to lose water.

      • Hypertonic: Lower concentration of solute outside the cell, causing the cell to gain water.

• Facilitated Diffusion: A form of passive transport that requires proteins to assist in moving molecules across the membrane, like small ions or water through channel proteins.

Active Transport

• Active Transport: Requires energy (usually from ATP) to move substances against their concentration gradient. Types include:

  • Primary Active Transport: Direct use of ATP, exemplified by the sodium-potassium pump.

  • Secondary Active Transport: Indirect use of ATP, such as the hydrogen ion-sucrose cotransporter.

  • Bulk Transport: Movement of large quantities of materials across the membrane.

    • Exocytosis: Secretion of substances (e.g., insulin) from the cell.

    • Endocytosis: Engulfing of substances into the cell, including:

      • Phagocytosis: Engulfing large entities.

      • Pinocytosis: Engulfing liquids and small particles.

      • Receptor-mediated endocytosis: Specific absorption of hormones or proteins through receptor interactions.

Cell Membrane Function

• The cell membrane acts as a selective barrier, regulating the internal environment of the cell by controlling what enters and leaves, which is crucial for cell survival and function.