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Flashcards in Lecture 13 Deck (30)

What are the components of a drug diffusion study apparatus?

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


What happens in the donor and receptor compartments over time?

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


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

we can apply sink conditions


what are sink conditions?

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.


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

the first law of fick


What happens if you increase the membrane thickness?

the flux will decrease


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

measurement of the actual concentration gradient is very difficult to do


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

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


what is K?

the partition coefficient of the diffusant between the membrane/solvent


what can change K?

temperature etc.


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

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


What is the permeability coefficient?

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


How can permeability also be expressed?

Dm/dt = SPCd or M = SPCdt

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


How can the permeability coefficient be measured?

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


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

there is no change in rate


What is lag time?

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


How can we get rid of the lag time?

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


How do we substract the lag time?

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


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

lag time, diffusion coefficient, and permeability coefficient


what is a permeation study?



What is the basic structure of the mammalian skin?



What is osmosis?

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


What is dialysis

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


What is hemodialysis

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


What is ultrafiltration

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


What are the mechanisms of transport across biological membranes?

passive transport
facilitated transport
active transport


What is passive transport

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


What is active transport?

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


What is facilitated transport?

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.


What are facilitated transport and active transport classified under?

Carrier mediated transfer