Facilitated diffusion involve

Options

A. Downhill transport
B. Without expenditure of metabolic energy
C. Involement of proteins
D. All of these

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 does not require energy input from the cell.

2. **Analyzing the Options**:
- **Option 1: Downhill Transport**:
- Facilitated diffusion occurs along the concentration gradient, meaning substances move from an area of higher concentration to an area of lower concentration. This is referred to as "downhill" transport.
- **Option 2: Without Expenditure of Metabolic Energy**:
- Since facilitated diffusion is a passive process, it does not require ATP or any other form of metabolic energy.
- **Option 3: Involvement of Proteins**:
- Facilitated diffusion specifically involves transmembrane proteins that help transport molecules across the cell membrane.

3. **Conclusion**:
- Since all three options (1, 2, and 3) are correct, the best answer is **Option 4: All of these**.

**Final Answer**: Option 4: All of these.

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