LECTURE 4 FACILITATED DIFFUSION PHYSIOLOGY 1 LECTURE 4 FACILITATED DIFFUSION

Facilitated Diffusion Definition Facilitated diffusion is the process by which an integral membrane protein creates a hole in the biological membrane through which a specific molecule can diffuse down it’s concentration gradient.

Facilitated Diffusion Basic Concept Integral membrane proteins are stimulated to change their structural conformation such that an opening in the membrane is created. The transport molecule can then diffuse across the membrane following it’s concentration gradient.

Facilitated Diffusion Diffusion Through a Biological Membrane The oil water partition coefficient (K) Kx = [x]o / [x]w Significance - In general all molecules are at least to some degree soluble in lipids. Therefore, all molecules can, given sufficient time diffuse across the membrane. The problem is that for most biologically useful molecules the cell does not have the time to wait for simple diffusion to supply it’s needs.

Facilitated Diffusion Transport - Simple Diffusion Simple Diffusion - Diffusion through the lipid bilayer is dependent primarily on the concentration gradient. Increase in concentration gives a linear increase in flux.

Facilitated Diffusion Transport - Carrier Mediated 2. Active Transport Carrier mediated transport requires the use of and the cycling of integral membrane proteins. So the flux rate depends on the number of transporters and their cycling rate.

Facilitated Diffusion Transport - Carrier Mediated Common Properties 1. Species specific 2. Saturation kinetics - Vmax 3. Competition for transport a. Competitive inhibition b. Noncompetitive inhibition 4. Obligative coupling of fluxes 5. Higher flux than predicted by Fick’s Law

Facilitated Diffusion Transport - Carrier Mediated Common Properties Species Specific - Normally a transport protein will only transport a specific molecule or type of molecule

Facilitated Diffusion Transport - Carrier Mediated Common Properties Saturation Kinetics - In a cell membrane there will exist at any one time a specific number of transport proteins for any specific transport molecule. Since, there also exist a specific cycling time for a transport protein to move a transport molecule, the system must become saturated and a maximum flux rate (Vmax) reached for increasing concentrations

Facilitated Diffusion Transport - Carrier Mediated Common Properties Competition for Transport - The presence of an inhibitor in the transport system means that the system will not operate as efficiently. Competitive Noncompetitive

Facilitated Diffusion Transport - Carrier Mediated Common Properties Competition for Transport - Competitive Inhibitor - A competitive inhibitor competes with the transport molecule for the binding site on the transport protein - Increasing concentrations of the transport molecule imply that the chance that a transport molecule will bind becomes increasingly greater no effective change in Vmax.

Facilitated Diffusion Transport - Carrier Mediated Common Properties Competition for Transport - Noncompetitive Inhibitor - A noncompetitive inhibitor binds to an allosteric site on the transport protein - This means that a number of transport proteins have effectively been removed from the system, therefore, Vmax is decreased.

Facilitated Diffusion Transport - Carrier Mediated Common Properties Obligative Coupling of Fluxes If the transport protein is moving two or more transport molecules at the same time the same ratio of transport molecules is maintained Example - Na+K+ATPase moves 3 Na+ out and 2 K+ in

Facilitated Diffusion Transport - Carrier Mediated Common Properties Higher Flux Rate than Predicted by Fick’s Law This is true only for physiologically normal concentrations of the transport molecules. At higher concentrations simple diffusion may in fact have a higher flux rate.

Facilitated Diffusion Molecular Mechanisms