Membrane Properties - Passive Transport Flashcards
What is flux?
the rate of movement of molecules across a boundary
Net flux
difference between two oppositely oriented unidirectional fluxes
Jabnet = Jab - Jba
Flux is proportional to what?
the concentration of the molecules
When is net flux equal to 0?
when the concentration on side A is equal to the concentration on side B
For a nonelectrolyte, the driving force is what?
chemical gradient
deltaC/deltax
deltaC = difference in concentration between regions of interest
deltax = distance separating these regions
Jnet = ?
Fick’s Law
Jnet = D * deltaC/deltax
Jnet (moles/(time*area))
D = diffusion coefficient, unique for each solute and conditions (cm^2/sec)
What are two important aspects of Fick’s Law?
- simple diffusion is directly proportional to the magnitude of the concentration gradient (deltaC/deltax)
- the time required for a molecule to diffuse between two points is proportional to the square of the distance separating the points
If the gradient doubles (deltaC/deltax), what happens to the net movement of molecules (Jnet)?
it also doubles
*IMPORTANT DIAGNOSTIC OF PASSIVE DIFFUSION
Cellular processes can typically rely on diffusion to:
- provide substrates
- remove wastes
- beyond the scale of single cells, things change
- in brain or muscle, diffusion works well because cells normally lie close to capillary as a source/sink for nutrients/waste
- for nerve cells with long axonal or dendritic processes, the greater distance isn’t so great for diffusion
Axonal transport
- necessary in nerve cells because of long axonal or dendritic processes
- this is a form of convective transport
Influence of distance on diffusion is important.
Ex: consequence of stroke is a reduction or cessation of blood flow resulting in an ischemic condition in areas downstream of the obstruction. Why is this bad?
cells in these downstream areas can be several millimeters to several centimeters from a capillary source of nutrients and O2. Diffusion over such distances can take from minutes to many hours, too slow to subserve metabolic demand.
Lipids do not interact effectively with what type of molecule, polar or nonpolar?
polar (ie water, or other substances containing either polar covalent bonds (like C-O bonds), or charged moities (like dissociated carboxyl groups, -COO(-), or portonated amino groups, -NH3+))
*basically, molecules that dissolve effectively in water
Diffusive flux of a substance across a membrane:
J = ?
J = P*deltaC
P = permeability coefficient
Lipophilic molecules (hydrophobic, nonpolar) diffuse ____ readily across membranes than do polar (hydrophilic) molecules
more
deltaPI = RTdeltaC_s
deltaPI = osmotic pressure between the two solutions deltaC_s = difference in concentration of osmotically active solutes across the semipermeable membrane R = gas constant T = temperature in degrees Kelvin
