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. It is important to note that this process does not require energy.

2. **Identify Key Characteristics**:
- **Carrier Proteins**: Facilitated diffusion requires specific proteins that help transport molecules across the cell membrane. These proteins are known as carrier proteins or transport proteins.
- **Passive Process**: This process does not require ATP (adenosine triphosphate) or any other form of energy. Instead, it relies on the natural movement of molecules from an area of higher concentration to an area of lower concentration.
- **Concentration Gradient**: Facilitated diffusion occurs along the concentration gradient, meaning it moves substances from areas of high concentration to areas of low concentration, not against it.

3. **Analyzing the Options**:
- **Option 1**: Needs a carrier protein - This is true for facilitated diffusion.
- **Option 2**: Is an active process - This is false; facilitated diffusion is a passive process.
- **Option 3**: Occurs against the concentration gradient - This is also false; it occurs along the concentration gradient.
- **Option 4**: Needs ATP - This is false; facilitated diffusion does not require ATP.

4. **Conclusion**: Based on the analysis of the options, the correct answer is that facilitated diffusion needs a carrier protein.

**Final Answer**: Option 1: Needs a carrier protein.

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