Cell Membranes and Diffusion

Question 1

how do local anesthetics work?

Answer 1

The anesthetic crosses the cell membrane and then binds to the sodium channel, closing the channel that allows sodium to flow into the cell, blocking nerve conduction by reducing the influx of sodium aka depolarization

Question 2

how do hydrophilic solutes/ions and hydrophobic solutes cross a cell membrane?

Answer 2

hydrophilic/ions: cannot diffuse, must use integral proteins (channels/ pores)
hydrophobic: dissolve across / diffuse

Question 3

Is potassium higher inside or outside the cell? Chloride? Sodium? Calcium?

Answer 3

K+: much higher inside
Cl-: higher outside
Na+: higher outside
Ca2+: higher outside

Question 4

is the ECF or ICF more alkaline?

Answer 4

ECF slightly alkaline

Question 5

what are the intracellular and extracellular concentrations of Na+, K+, Ca2+, Cl-, H+, pH and osmolarity?

Answer 5

Na+: 10mM, 142mM
K+: 135mM, 4mM
Ca2+: 100nM, 1mM
Cl-: 5nM, 105mM
H+: 100-63nM, 40nM
pH: 7-7.2, 7.4
osmolarity: 290 mm/L, 290mm/L

Question 6

the efficacy of many drugs depends on what?

Answer 6

lipid solublity

Question 7

what is the normal thickness of the plasma membrane?

Answer 7

7.5-10 nm, mechanically weak

Question 8

what is the fluid mosaic model?

Answer 8

the distribution of proteins in a phospholipid bilayer

Question 9

what is a common phospholipid that forms a lipid bilayer?

Answer 9

phosphatidylcholine: glycerol backbone, two fatty acids, a phosphate group and a choline

Question 10

what is the role of cholesterol in membrane lipids?

Answer 10

composes around 19-50% of all membranes and helps to stabilizes the membrane and reduces mobility

Question 11

how are integral proteins of a membrane regulated?

Answer 11

by voltage or binding of certain small molecules

Question 12

what are aquaporins?

Answer 12

hydrophilic pathways through the membrane that are not regulated like channels

Question 13

what is diffusion?

Answer 13

process whereby particles in a gas or liquid tend to intermingle due to their spontaneous motion caused by thermal agitation

Question 14

net diffusion is always?

Answer 14

from high to low concentration

Question 15

the underlying molecular movement of diffusion is ?

Answer 15

random and related to heat

Question 16

what is the driving force of diffusion?

Answer 16

concentration difference

Question 17

what effects the rate of diffusion across a membrane ?

Answer 17

physical characteristic of the particle

Question 18

what is a not partitioned membrane?

Answer 18

substance dissolves equally well in both membrane and aqueous solution

Question 19

net flux is directly proportional to what?

Answer 19

concentration difference

Question 20

what is Fick’s First Law of Diffusion (without partitioning)?

Answer 20

J = D (∆C / ∆x)
- J = flux
- ∆C = concentration difference
- ∆x = membrane thickness
- D = diffusion coefficient

Fick’s Law hold true for diffusion through membranes and also diffusion in free solution

Question 21

why do we use Fick’s Law?

Answer 21

Fick’s law is a way to quantitate diffusion through membranes and also diffusion in free solution

Question 22

what is a partitioned membrane??

Answer 22

the solubility of the substance in the membrane is different from its solubility in the aqueous solution on either side of the membrane

Question 23

what is Fick’s Law of Diffusion (with partitioning)?

Answer 23

J = Kp * D *∆C / ∆x OR
J = P(Cin - Cout)
- Kp (partition coefficient)= solubility in oil / solubility in water
- P = permeability

This expression shows that if a solute prefers the oil phase, i.e., its Kp is >1, then it will cross membranes more quickly. If Kp is < 1 it means the solute prefers to be in the water.

Question 24

which solutes cross the membrane by diffusion more easily with Fick’s Law of Diffusion (with partitioning)?

Answer 24

hydrophobic (lipophilic) solutes

Question 25

Explain the Einstein approximation for diffusion distance as a function of time and how this non-linear relationship relates to the speed of diffusion over long and short distances

Answer 25

The distance a particle moves is proportional to the square root of the time. Diffusion is a fast process over short distances: Diffusion is a slow process over long distances