Which of the following best describes integral membrane proteins?

• A. They are covalently attached to lipid groups and found on the periphery of the bilayer.
• B. They are non-covalently integrated into the phospholipid bilayer and usually span the entire bilayer.
• C. They are loosely attached to the surface of the membrane.
• D. They are only found on the extracellular side of the membrane.

Answer: (B)