Body Compartments and Drug Movement B

Question 1

What factors affect diffusion across a lipid membrane?

Answer 1

• Temperature
• Surface Area
• Concentration Gradient
• Particle Size
• Medium

Question 2

What is the permeability coefficient affected by?

Answer 2

• Lipid Solubility = Partition coefficient between membrane and aqueous environment.
• Diffusivity = Mobility of the molecules within the lipid (diffusion coefficient = depends on particle size).

Question 3

What is the effect of having a large transmembrane concentration gradient?

Answer 3

Drives MORE rapid diffusion across the membrane.

Question 4

What does the diffusion of drugs across membranes depend on?

Answer 4

Depends on degree of ionisation.

Question 5

What is pKA?

Answer 5

Acid - Base dissociation constant.

Question 6

What does pKA describe?

Answer 6

Ionic form a molecule will take at a range of pH values.

Question 7

What is the pKA value of a drug?

Answer 7

Hydrogen ion concentration at which 50% of the drug is ionised and 50% is un - ionised.

Question 8

Describe the effect of ionisation on diffusion.

Answer 8

Ionised Drug = Lipid Insoluble therefore DOES NOT diffuse across the membrane.
Unionised Drug = Lipid Soluble therefore DOES diffuse across the membrane.

Question 9

Which drug reaches an equal concentration in each compartment?

Answer 9

Unionised drug reached an equal compartment whereas the ionised drug doesn’t.
As ionisation is increased the drug is ‘trapped’.

Question 10

What affects the ionisation of a drug?

Answer 10

Different pH’s can affect the ionisation of the drug.

Question 11

What is the effect of changing pH on ionisation of acidic drugs?

Answer 11

Ionisation is greatest at an alkali pH.

Question 12

What is the effect of changing pH on ionisation of basic drugs?

Answer 12

Ionisation is greatest at an acidic pH.

Question 13

What proportion of aspirin will be ionised at pH 3.5?

Answer 13

pKA of 3.5