Lecture 4: Solute Transport Across Cell Membranes Part 1

What is simple diffusion?

the movement of an uncharged, lipid-soluble substance through a lipid bilayer (such as gases and steroid hormones)

What is Fick’s Law?

J_X = P_X ([X]_O - [X]_i)

What is J_X?

rate of flux

What is P_X?

permeability coefficient of X

What is ([X]_O - [X]_i)?

concentration gradient

What do each of the variables in the following equation represent: P_X = (D x B) / a

• D = diffusion coefficient; how readily X moves in the membrane

• B = partition coefficient; how well X can dissolve in lipid

• a = thickness of the membrane

What is flux a factor of?

• solute concentrations

• membrane thickness

• how well X moves in the membrane and dissolves in lipid

A substance can passively move across a membrane if there are what?

• favorable driving force

• open pathway

What is the driving force for charged solutes?

an electrochemical gradient

What is an electrochemical gradient?

a gradient that depends on the difference in voltage between two compartments and the difference in concentration

True or False: Passive transport always proceeds downhill.

true

What is meant by net flow?

the electrical gradient counterbalances the chemical gradient

What is equilibrium?

there is no net transport of substances across the membrane; equal and opposite movements of the substance

True or False: Equilibrium is a particular steady state where there’s no net driving force, and no net movement or flux.

true

In an electrical gradient, ions will flow from what to what?

from positive to negative

State the electrochemical potential energy difference equation.

delta mu(x) = RTln([X]_i / [X]_O) + z_xF(V_i - V_O)

What is the chemical potential energy difference?

RTln([X]_i / [X]_O)

What is the electrical potential energy difference?

z_xF(V_i - V_O)

What is z_x?

valence

For positively charge ions, positive, negative, and neutral electrochemical potential energy difference mean what?

• + = net flow of X would be from i to o

• - = net flow of X would be from o to i

• 0 = no net flow of X

What is V_m?

membrane potential; (V_i - V_o)

True or False: V_m always refers to the electrical difference as seen from the intracellular side.

true

What is the Nernst equation?

V_m = E_x = - (RT)/(z_xF) ln ([X_i]/[X_o])

What is the equilibrium potential (E_X)?

the value at which the membrane voltage must be for X to be in equilibrium

What is simplified equation for equilibrium potential?

E_x = - (60 mV) / Z_x log ([X]_i / [X]_o)

True or False: The net driving force in volts = (V_m - E_x).

true

True or False: Ionic flux will always go in the direction to move V_m towards E_x.

true

True or False: When E_x is less negative relative to V_m, cations will enter the cell and anions will leave.

true

True or False: When E_x is more negative relative to V_m, cations will leave the cell and anions will enter.

true