Which of the following is not true of the carrier molecules involved in facilitated diffusion?

Options

A. They increase the speed of transport across a membrane
B. They concentrate solute molecules on one side of the membrane
C. They have specific binding sites for molecules they transport
D. Transport protein may undergo conformational change upon binding of solutes.

Answer

### Step-by-Step Solution:

1. **Understanding Facilitated Diffusion**: Facilitated diffusion is a process where molecules move across a cell membrane with the help of carrier proteins. This process does not require energy and relies on the concentration gradient.

2. **Analyzing the Options**:
- **Option 1**: "They increase the speed of transport across a membrane."
- This statement is incorrect. Carrier proteins do not increase the speed of transport; the rate of diffusion is primarily determined by the concentration gradient.

- **Option 2**: "They concentrate solute molecules on one side of the membrane."
- This statement is correct. When molecules are transported from one side of the membrane to the other, they can create a higher concentration on one side.

- **Option 3**: "They have specific binding sites for molecules they transport."
- This statement is also correct. Carrier proteins have specific binding sites that allow them to recognize and bind to particular molecules.

- **Option 4**: "Transport proteins undergo conformational changes upon binding of the solute."
- This statement is correct as well. When a solute binds to the carrier protein, it often causes a change in the protein's shape, allowing the solute to be transported across the membrane.

3. **Identifying the Incorrect Statement**: The first option is the only statement that is not true regarding carrier molecules involved in facilitated diffusion.

### Final Answer:
The statement that is not true of the carrier molecules involved in facilitated diffusion is: **"They increase the speed of transport across a membrane."**

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