Question

The process in which the drug diffuses across the membrane in the direction of its concentration gradient and the membrane plays no active role in the process, is called

• A. Active transport
• B. Facilitated diffusion
• C. Passive diffusion
• D. Specialized transport

Hint:

To easily differentiate transport mechanisms, ask two questions:
1. Does it require energy/carriers? If no, it is passive.
2. Does the membrane participate? If no, it is passive diffusion.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
For a drug to produce its therapeutic effect, it must cross biological membranes to reach its site of action.
Biological membranes are selectively permeable lipid bilayers containing embedded proteins.
Transport across these membranes occurs via several distinct physical and biological processes, which can be broadly classified based on energy requirements and the involvement of membrane carriers.

Step 2: Detailed Explanation:
Let us analyze the key transport mechanisms across biological membranes:
1. Passive Diffusion (C):
This is the most common mechanism by which the majority of drugs are absorbed and distributed.
In passive diffusion, the drug molecules move across the lipid bilayer down their concentration gradient (from an area of high concentration to an area of low concentration).
This process is purely physical, governed by Fick's Law of Diffusion, and does not require cellular energy (ATP).
Crucially, the membrane itself acts merely as a physical barrier and plays no active biological role (it has no carrier proteins or energy-driven pumps involved).
Therefore, this description perfectly matches passive diffusion.
2. Active Transport (A):
This process moves drug molecules against their concentration gradient.
It requires specific carrier proteins and consumes cellular energy in the form of ATP.
Here, the membrane plays a highly active role.
3. Facilitated Diffusion (B):
This is a carrier-mediated transport process that does not require energy, meaning the drug still moves along its concentration gradient.
However, because it depends on specific trans-membrane carrier proteins, the membrane plays an active role in facilitating transport.
4. Specialized Transport (D):
This is an umbrella term that includes carrier-mediated transport, active transport, facilitated diffusion, and pinocytosis, all of which involve active cellular structures.

Step 3: Final Answer:
The transport mechanism described is passive diffusion, where the drug moves along its concentration gradient without active membrane participation.
Therefore, the correct option is (C).