True about facilitated diffusion-
**Core Concept**
Facilitated diffusion is a type of passive transport that involves the movement of molecules across a cell membrane with the assistance of carrier proteins or channel proteins. This process allows for the movement of molecules down their concentration gradient without the use of energy.
**Why the Correct Answer is Right**
Facilitated diffusion relies on the presence of carrier proteins or channel proteins embedded in the cell membrane. These proteins provide a pathway for molecules to cross the membrane, increasing the rate of diffusion and allowing for the movement of molecules that are too large or polar to pass through the lipid bilayer on their own. For example, glucose molecules are transported into cells via facilitated diffusion using glucose transport proteins, which are embedded in the cell membrane and facilitate the movement of glucose down its concentration gradient.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because facilitated diffusion is a type of passive transport, not active transport, which requires energy to move molecules against their concentration gradient.
**Option B:** This option is incorrect because facilitated diffusion involves the movement of molecules down their concentration gradient, not against it, and does not require energy.
**Option C:** This option is incorrect because facilitated diffusion involves the movement of molecules across the cell membrane using carrier proteins or channel proteins, not by simply dissolving in the lipid bilayer.
**Clinical Pearl / High-Yield Fact**
It's worth noting that facilitated diffusion is an important mechanism for the uptake of glucose and other nutrients into cells, and is impaired in certain diseases such as diabetes mellitus, where glucose transport proteins are reduced or dysfunctional.
**Correct Answer: B. Facilitated diffusion involves the movement of molecules down their concentration gradient without the use of energy.**