Which of the following transport mechanisms shows saturation of transport but does not show uphill transport?

Options

A. Simple diffusion
B. Active transport
C. Facilitated diffusion
D. Both simple and facilitated diffusion

Answer

To solve the question, "Which of the following transport mechanisms shows saturation of transport but does not show uphill transport?", we will analyze the different transport mechanisms provided in the options: simple diffusion, active transport, facilitated diffusion, and both simple and facilitated diffusion.

### Step-by-Step Solution:

1. **Understanding Transport Mechanisms**:
- **Simple Diffusion**: This is a passive transport mechanism where substances move from an area of higher concentration to an area of lower concentration (downhill transport). It does not require energy (ATP) and does not involve any transport proteins. Importantly, it does not show saturation because the rate of transport is not limited by the availability of transport proteins.

- **Active Transport**: This mechanism involves the movement of substances against their concentration gradient (from low to high concentration), which is known as uphill transport. Active transport requires energy (ATP) to function. While it can show saturation due to the limited number of transport proteins, it is not the correct answer because it involves uphill transport.

2. **Facilitated Diffusion**:
- This is also a passive transport mechanism but involves the use of transport proteins to help move substances across the plasma membrane. Like simple diffusion, facilitated diffusion moves substances from high to low concentration (downhill transport). However, because it relies on specific transport proteins, it can reach a maximum transport rate when all available transporters are occupied, indicating saturation. Importantly, it does not require ATP.

3. **Evaluating the Options**:
- **Simple Diffusion**: Does not show saturation and is downhill transport. (Incorrect)
- **Active Transport**: Shows saturation but involves uphill transport. (Incorrect)
- **Facilitated Diffusion**: Shows saturation and is downhill transport. (Correct)
- **Both Simple and Facilitated Diffusion**: Includes simple diffusion, which does not show saturation. (Incorrect)

### Conclusion:
The transport mechanism that shows saturation of transport but does not show uphill transport is **Facilitated Diffusion**.

### Final Answer:
**Facilitated Diffusion**