Question

Which one of the following statement holds true for Passive Diffusion?

• A. Greater the area and greater the thickness, faster is the diffusion
• B. Rate of drug transfer is directly proportional to the concentration gradient between GI fluids and the blood compartment
• C. Rate of transfer of ionised drug species is 3‐4 times the rate for unionised drug species
• D. Greater the membrane/water partition coefficient of drug, slower is the absorption

Answer 1

The Correct Option is B

To evaluate which statement holds true for passive diffusion, let's break down each option with respect to known principles of passive diffusion in physical pharmacy.

1. Greater the area and greater the thickness, faster is the diffusion: This statement is incorrect. According to Fick’s law of diffusion, the rate of diffusion is directly proportional to the surface area but inversely proportional to the thickness of the membrane. Therefore, greater the area results in faster diffusion, while greater thickness leads to slower diffusion.
2. Rate of drug transfer is directly proportional to the concentration gradient between GI fluids and the blood compartment: This statement is correct. Passive diffusion is driven by the concentration gradient. According to Fick’s first law of diffusion, the rate of transfer of drug molecules across a biological membrane is directly proportional to the concentration difference across that membrane.
3. Rate of transfer of ionised drug species is 3‐4 times the rate for unionised drug species: This statement is incorrect. The rate of diffusion is higher for unionized drug species compared to ionized ones because unionized molecules are more lipid-soluble and can cross cell membranes more easily.
4. Greater the membrane/water partition coefficient of drug, slower is the absorption: This statement is incorrect. A higher membrane/water partition coefficient indicates that the drug is more lipophilic and therefore more readily absorbed across biological membranes.

Based on the principles of passive diffusion, the correct answer is that the rate of drug transfer is directly proportional to the concentration gradient between gastrointestinal (GI) fluids and the blood compartment.