Lecture 13

Question 1

What are the components of a drug diffusion study apparatus?

Answer 1

donor compartment, 
receptor compartment
membrane separating the compartments
stirrer bars (on in each compartment)
tube where substance enters, and tube where substances leaves

Question 2

What happens in the donor and receptor compartments over time?

Answer 2

the concentration of the sample will decrease in the donor compartment and increase in the receptor compartment by diffusion through the membrane until equilbrium is reached

Question 3

how can the mass transfer process be maintained / hve the time extended to reach equilibrium?

Answer 3

we can apply sink conditions

Question 4

what are sink conditions?

Answer 4

in many diffusion experiments this consist of continually removing and replacing the receptor compartment with fresh solvent to maintain its concentration at a low level.

Question 5

What can steady diffusion across a think film be described by?

Answer 5

the first law of fick

Question 6

What happens if you increase the membrane thickness?

Answer 6

the flux will decrease

Question 7

Why are the receptor and donor concentrations measured instead of the actual concentration gradient?

Answer 7

measurement of the actual concentration gradient is very difficult to do

Question 8

What doesn’t C1 = donor concentration and C2 = receptor concentration?

Answer 8

the concentrations are related to each other by the equation K = C1/Cd = C2/Cr

Question 9

what is K?

Answer 9

the partition coefficient of the diffusant between the membrane/solvent

Question 10

what can change K?

Answer 10

temperature etc.

Question 11

under sink conditions, what does the receptor concentration equal approximately?

Answer 11

0 as you are constantly removing the substance to maintain a low level

Question 12

What is the permeability coefficient?

Answer 12

D K/h
Symbol: P
Units: cm/sec
related to flux by the equation J = PCd

Question 13

How can permeability also be expressed?

Answer 13

Dm/dt = SPCd or M = SPCdt

where M is the amount of permeant in the receptor
S is the total area of the membrane

Question 14

How can the permeability coefficient be measured?

Answer 14

we can graph the amount of permeant in the receptor vs. time
If S and Cd are known, the straight line of this graph can be used to calculate P.

P = slope/Cd x S

Question 15

Why is the graph of amount of permeant in receptor vs. time a reaction of zero order?

Answer 15

there is no change in rate

Question 16

What is lag time?

Answer 16

theoretically we should maintain a steady state under sink conditions however in reality there is a lag time

Question 17

How can we get rid of the lag time?

Answer 17

as we are not interested in the lag time we can rewrite the time component of M = SPCdt to substract the lag time

Question 18

How do we substract the lag time?

Answer 18

M = SPCd (t-tl)

where tl is the time required for a penetrant to establish a uniform concentration gradient within the membrane separating the donor from the receptor compartments

Question 19

What can we calculate by plotting mass of penetrant vs. time?

Answer 19

lag time, diffusion coefficient, and permeability coefficient

Question 20

what is a permeation study?

Answer 20

__

Question 21

What is the basic structure of the mammalian skin?

Answer 21

epidermis
dermis
hypodermis

Question 22

What is osmosis?

Answer 22

Is the diffusion of a solvent through a semipermeable membrane (NOT solute)
Follows the laws of diffusion, where the concentration gradient is the driving force
Passive process

Question 23

What is dialysis

Answer 23

This is the separation process based on unequal rates of passage of solutes and the solvent through a semipermeable membrane

Question 24

What is hemodialysis

Answer 24

process through which blood is filtered specifically to exclude certain small molecules like K+, urea while preserving larger molecules (proteins)

Question 25

What is ultrafiltration

Answer 25

Process used to separate macromolecules using a membrane and involves applying hydraulic pressure
We use molecular weight cut-off membranes which are specific pore sized to allow substances of molecular weight lower than the pores to go through

Question 26

What are the mechanisms of transport across biological membranes?

Answer 26

passive transport
facilitated transport
active transport

Question 27

What is passive transport

Answer 27

simple diffusion through driven by concentration gradient on the two sides of the membrane

Usually for lipid soluble compounds like vitamins, small molecules such as water and CO2

They move from an area of high concenration to an area of low concentration either by diffusing directly across the membrane or through a protein channel which is usually embedded in the membrane

Question 28

What is active transport?

Answer 28

where a carrier or membrane transporter is used to bind a drug and transport it across the membrane
Energy is required as the driving force as it can proceed against the concentration gradient

Question 29

What is facilitated transport?

Answer 29

This process does not require energy.
However it does need a specific carrier.
Usually for very large drugs.
It again uses the concentration gradient as the driving force.

Question 30

What are facilitated transport and active transport classified under?

Answer 30

Carrier mediated transfer