Facilitated diffusion

Options

A. needs a carrier protein
B. is an active process
C. occurs against the concentration gradient
D. needs ATP.

Answer

**Step-by-Step Solution:**

1. **Understanding Facilitated Diffusion:**
- Facilitated diffusion is a type of passive transport that allows substances to cross membranes with the assistance of special proteins.

2. **Identifying Key Characteristics:**
- It occurs down the concentration gradient, meaning it moves from an area of higher concentration to an area of lower concentration.
- It does not require energy (ATP) because it is a passive process.

3. **Analyzing the Options:**
- **Option A:** Needs a carrier protein.
- This is true. Facilitated diffusion requires specific proteins to help transport molecules across the membrane.
- **Option B:** Is an active process.
- This is false. Active processes require energy, which facilitated diffusion does not.
- **Option C:** Occurs against the concentration gradient.
- This is also false. Facilitated diffusion moves with the concentration gradient, not against it.
- **Option D:** Needs ATP.
- This is false. Facilitated diffusion does not require ATP or energy.

4. **Conclusion:**
- The only correct statement regarding facilitated diffusion is that it needs a carrier protein. Therefore, the correct answer is **Option A**.