Neutral molecules are transpoed by:
**Core Concept**
Passive diffusion and facilitated diffusion are two primary mechanisms for the transport of neutral molecules across cell membranes. Neutral molecules, lacking a charge, rely on these mechanisms to cross the lipid bilayer.
**Why the Correct Answer is Right**
The correct answer is **passive diffusion**, which involves the movement of molecules from an area of higher concentration to an area of lower concentration, without the need for energy input. This process relies on the random motion of molecules and is driven by the concentration gradient. Facilitated diffusion, on the other hand, involves the assistance of carrier proteins or channel proteins to facilitate the movement of molecules across the membrane.
**Why Each Wrong Option is Incorrect**
**Option A:** Active transport is incorrect because it involves the movement of molecules against their concentration gradient, requiring energy input in the form of ATP.
**Option B:** Osmosis is incorrect because it refers specifically to the movement of water molecules across a semipermeable membrane, driven by the concentration of solutes.
**Option C:** Phagocytosis is incorrect because it is a type of endocytosis involving the engulfment of large particles or microorganisms by the cell membrane, and is not related to the transport of neutral molecules.
**Clinical Pearl / High-Yield Fact**
Passive diffusion is a critical mechanism for the transport of oxygen and carbon dioxide across the alveolar-capillary membrane in the lungs, allowing for the exchange of these gases between the atmosphere and the bloodstream.
**Correct Answer:** A. Passive diffusion.