Simple diffusion and facilitated diffusion share which of the following characteristics?
**Core Concept**
Simple diffusion and facilitated diffusion are two types of passive transport mechanisms across cell membranes. Simple diffusion involves the movement of molecules from an area of higher concentration to an area of lower concentration, driven by thermal energy. Facilitated diffusion, on the other hand, requires the assistance of carrier proteins or channel proteins to facilitate the movement of molecules across the membrane.
**Why the Correct Answer is Right**
The correct answer is that both simple diffusion and facilitated diffusion are **directionally dependent on concentration gradients**. This means that the movement of molecules is from an area of higher concentration to an area of lower concentration, resulting in net movement of molecules down their concentration gradient. This is a fundamental principle of both simple diffusion and facilitated diffusion.
**Why Each Wrong Option is Incorrect**
**Option A:** Not a characteristic of simple diffusion and facilitated diffusion.
Simple diffusion is a random process, whereas facilitated diffusion is a carrier-mediated process.
**Option B:** Not a characteristic of simple diffusion and facilitated diffusion.
Active transport, on the other hand, requires energy to move molecules against their concentration gradient.
**Option C:** Not a characteristic of simple diffusion and facilitated diffusion.
Osmosis is a type of passive transport that involves the movement of water molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient of solutes.
**Clinical Pearl / High-Yield Fact**
One important difference between simple diffusion and facilitated diffusion is that simple diffusion does not require energy or the assistance of proteins, whereas facilitated diffusion requires the assistance of carrier proteins or channel proteins.
**Correct Answer: C. Osmosis is a type of passive transport that involves the movement of water molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient of solutes.