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4.07 Overview: Mechanisms of Transport Across Membranes

Posted28/10/2024
Updated21/12/2024
ByBMF
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Mechanisms of Transport Across Membranes

  • Transport across cell membranes is essential for cellular function, allowing cells to obtain nutrients, expel waste, and maintain internal balance.
  • The main mechanisms are divided into passive (no energy required) and active (energy-dependent) transport.

1. Passive Transport

  • Definition: Movement of molecules across the membrane without energy input, typically down the concentration gradient (from high to low concentration).
  • Types:
    • Simple Diffusion:
      • Description: Molecules move directly through the lipid bilayer.
      • Examples: Small, nonpolar molecules like oxygen (O₂), carbon dioxide (CO₂), and lipids.
    • Facilitated Diffusion:
      • Description: Transport of molecules via specific channel or carrier proteins in the membrane.
      • Examples: Ions (e.g., Na⁺, K⁺) through ion channels; glucose via glucose transporters.
    • Osmosis:
      • Description: The passive movement of water across a semi-permeable membrane, typically through aquaporins.
      • Examples: Water balance in kidney cells, where aquaporins regulate water reabsorption.

2. Active Transport

  • Definition: Movement of molecules against their concentration gradient, requiring energy, usually in the form of ATP.
  • Types:
    • Primary Active Transport:
      • Description: Direct use of ATP to pump molecules across the membrane.
      • Example: Sodium-Potassium Pump (Na⁺/K⁺ ATPase), which moves sodium out and potassium into the cell, essential for maintaining cellular ion balance and membrane potential.
    • Secondary Active Transport (Cotransport):
      • Description: Uses the energy from an ion gradient established by primary active transport to move other molecules.
      • Types:
        • Symport (Cotransport): Both ions and molecules move in the same direction.
        • Antiport (Countertransport): Ions and molecules move in opposite directions.
      • Examples: Sodium-glucose cotransporter in kidney cells (symport); sodium-calcium exchanger (antiport).

3. Bulk Transport

  • Definition: Transport of large molecules or groups of molecules via vesicles, involving the membrane itself. Bulk transport is energy-dependent.
  • Types:
    • Endocytosis:
      • Description: Process where the cell membrane engulfs extracellular material, bringing it into the cell.
      • Subtypes:
        • Phagocytosis (“cell eating”): Engulfs large particles, such as bacteria or debris.
        • Pinocytosis (“cell drinking”): Engulfs extracellular fluid and dissolved substances.
        • Receptor-Mediated Endocytosis: Specific uptake of molecules after they bind to cell surface receptors.
      • Examples: White blood cells engulfing pathogens (phagocytosis), uptake of cholesterol via LDL receptors (receptor-mediated endocytosis).
    • Exocytosis:
      • Description: The process where vesicles fuse with the plasma membrane to release their contents outside the cell.
      • Examples: Release of neurotransmitters in nerve cells, secretion of hormones like insulin.

Three different kinds of endocytosis. Phagocytosis is ingestion of large particles. Pinocytosis is continuous small-scale endocytosis. Receptor-mediated endocytosis involves a signal to specifically bring in certain molecules.

Summary Table of Transport Mechanisms

MechanismEnergy RequiredMovement DirectionExamples
Simple DiffusionNoHigh to LowOxygen, carbon dioxide
Facilitated DiffusionNoHigh to LowIons via ion channels, glucose via transporters
OsmosisNoHigh to LowWater via aquaporins
Primary Active TransportYes (ATP)Low to HighSodium-potassium pump
Secondary Active TransportYes (gradient energy)Low to HighSodium-glucose cotransporter, sodium-calcium exchanger
EndocytosisYesInwardPhagocytosis of bacteria, receptor-mediated uptake
ExocytosisYesOutwardSecretion of hormones, release of neurotransmitters

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