Majority of drugs are transported across the membrane by
**Core Concept**
The majority of drugs are transported across cell membranes via passive diffusion, facilitated diffusion, or active transport mechanisms. Passive diffusion and facilitated diffusion are essential for the movement of lipophilic and hydrophilic substances, respectively. Active transport, however, involves the movement of substances against their concentration gradient.
**Why the Correct Answer is Right**
Passive diffusion is the primary mechanism for the movement of drugs across cell membranes. This process relies on the concentration gradient of the drug, allowing it to move from an area of high concentration to an area of low concentration. Facilitated diffusion, on the other hand, involves the assistance of carrier proteins or channel proteins to facilitate the movement of substances across the membrane. Active transport, which requires energy, is less common for drug transport but plays a crucial role in the movement of essential nutrients and waste products.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the primary mechanism of drug transport across cell membranes.
**Option B:** This option is incorrect because it does not accurately represent the majority of drug transport mechanisms.
**Option C:** This option is incorrect because while active transport is an essential mechanism, it is not the majority mechanism for drug transport.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the lipophilicity of a drug affects its ability to cross cell membranes. Drugs with high lipophilicity can diffuse more easily across the membrane, while hydrophilic drugs require facilitated diffusion or active transport.
**Correct Answer:** D.