Physiology of Cell Membrane Transport – Study Notes (BIOL 2401, Chapter 3)

Physiology of Cell Membrane Transport

Introduction to Membrane Transport

The plasma membrane is a selectively permeable barrier composed primarily of phospholipids and membrane proteins. This structure regulates the passage of substances into and out of the cell, maintaining cellular homeostasis and enabling communication with the environment.

• Phospholipid bilayer: Forms the basic structural framework of the membrane, providing fluidity and selective permeability.

• Membrane proteins: Serve as transporters, channels, receptors, and enzymes, facilitating the movement of specific molecules.

• Selective permeability: Only certain substances can cross the membrane freely; others require specialized transport mechanisms.

• Example: Oxygen and carbon dioxide diffuse freely, while ions and glucose require protein-mediated transport.

Types of Membrane Transport

Overview of Transport Mechanisms

Transport across the cell membrane occurs via two main mechanisms: passive transport and active transport. The distinction is based on energy requirements and direction relative to the concentration gradient.

• Passive transport: Movement of substances down their concentration gradient without direct energy input from the cell.

• Active transport: Movement of substances against their concentration gradient, requiring energy (usually from ATP).

• Diffusion energy: In passive transport, the energy comes from the natural tendency of molecules to move from areas of high to low concentration.

• Example: Sodium-potassium pump uses ATP to move Na+ and K+ ions against their gradients.

Passive Transport Mechanisms

Types of Passive Transport

All passive transport mechanisms rely on diffusion, but differ in how substances cross the membrane. The main types are:

• Simple diffusion: Direct movement of molecules across the lipid bilayer without the need for membrane proteins. Typically applies to small, nonpolar molecules (e.g., O2, CO2).

• Facilitated diffusion: Movement of molecules via membrane proteins. This includes:

  • Channel-mediated diffusion: Uses protein channels to allow specific ions or water molecules to pass through the membrane.

  • Carrier-mediated diffusion: Involves carrier proteins that bind and transport specific substances across the membrane.

• Key distinction: Simple diffusion does not require proteins; facilitated diffusion does.

• Example: Glucose enters cells via carrier-mediated facilitated diffusion.

Principles of Transport and Diffusion

Physical Basis of Diffusion

Diffusion is driven by the random thermal motion of molecules. Molecules move from regions of higher concentration to regions of lower concentration until equilibrium is reached.

• Concentration gradient: The difference in concentration between two regions; drives diffusion.

• Direction: Always from high concentration to low concentration (down the concentration gradient).

• Equilibrium: Achieved when the net movement (flux) of molecules is equal in both directions.

• Example: Oxygen diffuses from alveoli (high concentration) into blood (low concentration).

Additional info: Academic Context

• Thermodynamics: Diffusion is a spontaneous process that increases entropy (disorder) in a system. No energy input is required for passive diffusion.

• Fick's Law of Diffusion: The rate of diffusion (flux) can be described by Fick's equation: $\text{Flux} = A \times D \times (C_1 - C_2)$ where:

  • $A$ = area available for diffusion

  • $D$ = diffusion constant

  • $(C_1 - C_2)$ = concentration gradient across the membrane

• Factors affecting diffusion rate:

  • Temperature (higher = faster diffusion)

  • Molecule size (smaller = faster diffusion)

  • Concentration gradient (greater = faster diffusion)

  • Membrane surface area (larger = faster diffusion)

  • Viscosity of medium (higher = slower diffusion)

  • Charge of molecule (charged molecules may require channels)