Where are hydrophilic regions of membrane proteins primarily located?

• A. In the interior of the lipid bilayer.
• B. On the cytoplasmic surface only.
• C. Hydrophilic regions face the aqueous environments on both sides of the membrane.
• D. Embedded within the hydrophobic core.

Answer: (C)

Membrane proteins have parts that like water and parts that like lipids. The inside of the lipid bilayer is hydrophobic, so hydrophilic regions aren’t embedded there. Instead, the polar, charged, or water-loving parts form loops or domains that face the water on each side of the membrane—the cytoplasmic side and the extracellular (or periplasmic) side. In transmembrane proteins, the spans crossing the membrane are typically hydrophobic to match the lipid interior, while the hydrophilic portions are exposed to the aqueous environments on either side. So hydrophilic regions face the aqueous environments on both sides of the membrane.