Final answer:
The correct option is 3. Aquaporins are integral proteins that function as specific water channels allowing water to move rapidly across the cell membrane, essential for maintaining water balance.
Step-by-step explanation:
Aquaporins are integral proteins involved in facilitated transport across the cell membrane. They function as specific water channels that allow for the rapid movement of water molecules across the cell membrane, thereby playing a crucial role in water balance within the cell and throughout the body.
Aquaporins are not β-barrel proteins in the outer membrane of bacteria, ABC proteins, or symporters; they are specialized channel proteins designed to facilitate the movement of water, and water only, through their hydrophilic channels. These proteins are present in many different cell types, including kidney cells, which play a vital role in maintaining water balance in vertebrates.