Membrane Transport


1. Structure of the Cell Membrane

  • Phospholipid Bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

  • Proteins: Integral and peripheral proteins facilitate transport and communication.

  • Cholesterol: Helps maintain membrane fluidity.

  • Carbohydrates: Often attached to proteins or lipids, involved in cell recognition.


2. Types of Transport

A. Passive Transport

  • Definition: Movement of substances without energy input, down their concentration gradient.

  • Types:

    1. Simple Diffusion: Movement of small nonpolar molecules (e.g., O₂, CO₂) through the bilayer.

    2. Facilitated Diffusion: Requires transport proteins (channels or carriers) for larger or polar molecules (e.g., glucose).

    3. Osmosis: Diffusion of water through a selectively permeable membrane.

  • Tonicity:

    1. Isotonic: Equal solute concentration inside and outside.

    2. Hypertonic: Higher solute concentration outside (cell shrinks).

    3. Hypotonic: Lower solute concentration outside (cell swells).

B. Active Transport

  • Definition: Movement of substances against their concentration gradient, requiring energy (ATP).

  • Types:

    1. Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

    2. Secondary Active Transport: Uses energy from the electrochemical gradient established by primary active transport.

      • Symport: Two substances move in the same direction.

      • Antiport: Two substances move in opposite directions.

C. Bulk Transport

  • Endocytosis: Process of taking substances into the cell.

    • Phagocytosis: Uptake of large particles or cells.

    • Pinocytosis: Uptake of liquids or small particles.

    • Receptor-Mediated Endocytosis: Specific uptake facilitated by receptor binding.

  • Exocytosis: Process of releasing substances from the cell via vesicles fusing with the membrane.


3. Membrane Potential

  • Definition: The voltage difference across a membrane, resulting from the distribution of ions.

  • Resting Membrane Potential: Typically around -70 mV, primarily due to potassium ion distribution.

  • Action Potential: Rapid depolarization followed by repolarization in neurons, crucial for nerve signal transmission.


4. Factors Influencing Membrane Transport

  • Concentration Gradient: The difference in concentration of a substance across the membrane.

  • Temperature: Higher temperatures increase molecular movement, enhancing diffusion rates.

  • Membrane Surface Area: Larger surface area increases the rate of transport.

  • Lipid Solubility: Nonpolar molecules diffuse more easily than polar ones.

  • Channel/Carrier Protein Availability: The presence and activity of transport proteins affect rates of facilitated diffusion and active transport.


5. Clinical Applications

  • Drug Delivery: Many medications utilize specific transport mechanisms for effective delivery.

  • Disease Implications: Mutations in transport proteins can lead to conditions like cystic fibrosis or diabetes.

  • Targeting Transport Mechanisms: Developing drugs that affect transport proteins can be key in treating various diseases.