Questions: Where are the protein channels that are used during facilitated diffusion located?

The answer is the second one: inside the cell membrane.

Explanation for each option:

1. Outside the cell: This is incorrect. Protein channels are not located outside the cell. They are embedded within the cell membrane to facilitate the movement of substances across it.

2. Inside the cell membrane: This is correct. Protein channels are integral membrane proteins that span the cell membrane. They provide a pathway for specific molecules to pass through the membrane via facilitated diffusion.

3. Inside the nucleus: This is incorrect. Protein channels involved in facilitated diffusion are not located inside the nucleus. They are part of the cell membrane, which surrounds the entire cell, not just the nucleus.

In summary, protein channels used during facilitated diffusion are located within the cell membrane.