The Cell Membrane: Composition, Structure, Functions, and Models

Types of Solution and Transport Processes

Types of Solutions

• Isotonic: Concentrations of water and solute are the same inside and outside the cell.

• Hypertonic: A solution with a higher concentration of solute (and therefore lower water). Water moves out of the cell, causing it to shrivel or undergo crenation.

• Hypotonic: A solution with a lower concentration of solute (and therefore higher water). Water moves into the cell, causing it to expand and potentially lyse (burst).

Transport Processes

1. Passive Transport: Movement of substances without the need for energy, typically down their concentration gradient.

   • Diffusion: Movement of particles from high to low concentration (e.g., $O_2$ diffusing across the membrane for cellular respiration).

   • Facilitated Diffusion: Carrier proteins help larger molecules pass through the membrane from high to low concentration.

   • Osmosis: Diffusion of water through a selectively permeable membrane, often through aquaporins.

2. Active Transport: Movement of materials against the concentration gradient (from low to high concentration), which requires energy (ATP).

   • Sodium-Potassium Pump: Exchanges sodium ions for potassium ions, moving $3 ext{Na}^+$ out and $2 ext{K}^+$ into the cell.

3. Bulk Transport: Involves the movement of large quantities of material in vesicles.

   • Endocytosis: The process where the cell takes in large molecules by engulfing them.

     • Phagocytosis: "Cell eating"; the cell engulfs solid particles.

     • Pinocytosis: "Cell drinking"; the cell engulfs liquids.

     • Receptor-mediated Endocytosis: Specific process where ligands bind to receptors to facilitate bulk intake.

   • Exocytosis: Removal of materials by discharging them from vesicles through the membrane.