cell organisation and communication

Membrane Transport

• Movement of molecules across a membrane: Membranes are selectively permeable, meaning they have protein channels embedded to control what passes through.
• Diffusion: Movement from high to low concentration. Requires channels for movement.
• Osmosis: Water moves across a selectively permeable membrane towards the region of higher solute concentration.
• Water Balance of Cells:
  • Isotonic: Solute concentration same as inside the cell; no net water movement.
  • Hypertonic: Solute concentration higher than inside the cell; cell loses water.
  • Hypotonic: Solute concentration lower than inside the cell; cell gains water.
• Active Transport: Moves substances against concentration gradients, using energy (ATP). Example: Sodium-Potassium pump.
• Sodium-Potassium Pump Mechanism:
  • Initially open to the intracellular side; binds 3 sodium ions.
  • ATP transfers a phosphate, changing the protein's shape to open to the extracellular side.
  • Releases sodium ions.
  • Opens to bind 2 potassium ions.
  • Releases phosphate; opens back to intracellular, and the cycle repeats.
  • Moves against the concentration gradient.
• Channel Proteins: Allow specific substances to pass through the membrane. Form hydrophilic pores for ion passage.

Specialised Membrane Proteins

• Channel Proteins: Allow substances to pass through the cell membrane passively (without energy).
  • Aquaporins: Facilitate rapid water movement, especially in erythrocytes and kidney cells. Water's polarity affects its movement.
  • Ion Channels: Allow specific ions (K+ and Na+) to diffuse through. Maintain cell's resting membrane potential. Often 'gated' (respond to stimuli).
• Erythrocyte Plasma Membrane: Maintains membrane potential via active transport of K+ inward and Na+ outward. Facilitates rapid water and ion exchange.

Types of Transport

• Passive Transport: Movement without energy expenditure.
  • Diffusion: Movement down a concentration gradient.
  • Facilitated Diffusion: Movement with help of transport proteins (channels).
  • Osmosis: Movement of water.
• Active Transport: Movement with energy expenditure.
• Bulk Transport: Movement of large particles.
  • Endocytosis: Cell engulfs material.
    • Phagocytosis: Engulfing solid particles (e.g., bacteria).
    • Pinocytosis: Engulfing liquid or very small particles.
    • Receptor-mediated Endocytosis: Specific molecule binding to receptor proteins.
  • Exocytosis: Vesicles release contents outside the cell.

Explore the critical concepts of membrane transport, including diffusion, osmosis, and active transport mechanisms. This quiz will help you understand how cells manage water balance and the role of the sodium-potassium pump in maintaining cellular function.