Cell Transmembrane Transport

Question 0

Transmembrane Flux of Water

Answer 0

Number of water molecules that pass through the membrane per unit time and per unit area
10% of transmembrane flux of water occurs passively between the lipids 
90% occurs through aquaporins - selectice membrane proteins
		facilitated diffusion (not passive diffusion)

Question 1

Lipophillic Molecules

Answer 1

Membrane lipid molecules can diffuse freely in he plane of each membrane bi-layer leaflet
Respiratory gases, organic molecules, anesthetics

Some are lipophobic: urea, glycerol

Question 2

Pores vs. Channels

Answer 2

Pore - protein pathway that is always open (not gated)

Channel - open or closed by small reversible step movements of specific amino acids with the channel’s polypeptide chain

Question 3

Potential Energy Gradients

Answer 3

Contribute to passive transmembrane transport (or diffusion)
Chemical - force derived from chemical concentration grad
Electrical - force derived from electrical potential energy difference

Question 4

Fick’s 1st Law of Diffusion

Answer 4

Qxnet(chemical) = (PxA[Xo])-(PxA[X]i) = PxA(Xo-Xi) = PxAdX

Q = mass flow rate
Px = permeability constant (ease of movement into and across membrane)

Question 5

Flux

Answer 5

Transmembrane movement of a solute (J). The amount (mass) of a solute X moved per unit time and per unit cross secitonal area
Qx=PxAd[X]
Jx= Qx/A

Directly proportional to:

i. magnitude of electrical potential energy gradient
ii. ease of transmembrane movement (permeability) of ion
iii. concentration of ion

Jx=Pxd[X]

Question 6

Electrical gradient

Answer 6

Direction of electrical gradient is always from positive to negative

Question 7

Passive transmembrane flux of ion due to externally applied electrical potential energy gradient

Answer 7

Jx=zPx[X]dV
z= valence of the ion
Px = permeability (Px of ion is not only dependent upon the size and geometric shape of the ion, but also upon the interaction of its electrical charge with the electrical field)

Question 8

Total Passiv Net Flux

Answer 8

Jx(total)= Jx(net chemical) + Jx(net electrical)

Question 9

Px - Permeability constant

Answer 9

Px = (DB)/l

D = Diffusion Coefficient - ease of movement along pathway within the cell membrane length^2/time
B = partition coefficient - ease of entry of diffusing molecule into membrane phase (solubility within membrane phase)
B = [Xi]’/[Xi]=[Xo]’/[Xo] = ratio of [X] in intramembrane phase [Xi]’, just adjacent to the concentration in the intracellular phase [Xi], no units

L = length of pathway - membrane thickness

Question 10

Partition Coefficient B

Answer 10

B reflects the difference in force of attraction for the diffusing molecule or ion exerted by the molecules within the membrane phase versus force of attraction in the extra or intracellular aqueous phase
B > unity
increased intramembrane concentration gradient due to an increased intramembrane solubility relative to external aqueous phase, example - lipophilic substances
B < unity
decreased intramembrane concentration gradient due to a decreased intramembrane solubility relative to the external aqueous phase, exampke - lipohobic sustances
B = unity
intramembrane concentration gradient equal to the transmembrane gradient