Pharmacology Overview Quiz

Study Notes

The driving force for passive absorption is the concentration gradient across a membrane.
The drug moves from a region of high concentration to one of lower concentration.
The majority of drugs gain access to the body via this mechanism.
Lipid-soluble drugs readily cross biological membranes due to their solubility in lipid bilayers.

Other agents can enter the cell through specialized transmembrane carrier proteins.
These proteins undergo conformational changes allowing the passage of drugs or endogenous molecules into the interior of cells.
The process moves molecules from an area of higher concentration to one of lower concentration.
The process does not require energy and can be saturated.

This mode of drug entry involves specific carrier proteins across the membrane.
Some drugs closely resembling naturally occurring metabolites are actively transported across cell membranes by these specific carrier proteins.
Active transport is energy-dependent and driven by the hydrolysis of adenosine triphosphate (ATP).
Active transport moves drugs against the concentration gradient (from low to high drug concentration).
The process shows saturation kinetics.

This type of drug delivery transports drugs of exceptionally large size across cell membranes.
Endocytosis involves engulfment of a drug molecule by the cell membrane and transport into the cell by pinching off the drug-filled vesicle.
For example, vitamin B is transported across the gut wall by endocytosis.

Most drugs are either weak acids or weak bases.
Acidic drugs (HA) release H+ ions, causing a charged anion (A).
Basic drugs are usually charged; loss of a proton produces the uncharged base (B).
An uncharged drug passes through membranes more readily than a charged drug.
For weak acids, the uncharged HA permeates through membranes; the charged A does not.
For weak bases, the uncharged B penetrates through cell membranes; the charged BH+ does not.
The effective concentration of the permeable form of each drug at its absorption site is determined by the relative concentrations of charged and uncharged forms.
The ratio between the two forms depends on the pH at the site of absorption and the pKa. (Henderson-Hasselbalch equation)