Excitable Cells - 3

Excitable Cells - 3

1. Diffusion

   • Movement down the concentration gradient causes molecules to move spontaneously
   • Concentration gradient reduces until equilibrium is reached
     • Entropy increases (disorder) – 2nd law of thermodynamics
   • If a membrane is present, ions cannot cross the bilayer and can cause major differences in concentrations either side of the membrane hence producing a gradient
     • Hole in membrane allows ion movement from high→low concentration until equilibrium is reached
     • Molecules which form holes in membranes are said to facilitate diffusion such as

2. Fick's Law

   • The number of molecules moving across an interface is proportional to the: Area of interface (A) & the concentration gradient
   • Molecules travel varying distances depending on whether they move in 1D, 2D or 3D dimensions (Molecule will move faster in 3D than 1D if diffusion coeficient was the same)
     • Molecules diffuse further in 3D - less likely to collide with other molecules
     • Catalysts work by increasing the collision frequency
     • Signalling molecules have longer ranges if they are not bound to membranes

3. Electrochemical gardient

   • Ion diffusion links with electric fields and ions moving under influence of an electric field are electrophoretic movements
   • Total /electrochemical gradient: Gradient caused by diffusion - Gradient caused by electrophoretic movement
   • Higher conc. of Na+ ions outside than inside the cell. Membrane has a pore
     • If anode is placed outside, Na+ ions don’t move inside depending on the electric field strength
       • As the Na+ conc. outside was greater, inside is 0mV and outside is -60mV
         • Nature of ion depends on its charge and the electrical field. E.g. if anode is placed inside a cell, Na+ ions move in whereas Cl- ions move out
   • The direction ions move is determined by electrochemical gradient

4. Rate of diffusion depends on:

   • Size of the electrochemical gradient → Large or Small
   • Nature of the ion → Charges. Links to Nernst equation
   • Number of open ion channels → Many/few/none
   • Properties of the ion channel e.g. Whether ion channel pore mimics the hydration shell of specific ion