In the context of the lipid bilayer, what is a primary function of carbohydrates attached to membrane lipids and proteins?

• A. They function primarily as transmembrane channels that allow polar solutes to diffuse through the membrane.
• B. They form the hydrophobic core of the membrane and create the permeability barrier to ions.
• C. They provide the ATP needed for active transport across the membrane by serving as the main energy source in the bilayer.
• D. They act as cell-surface markers that mediate cell recognition and communication (e.g., in glycoproteins and glycolipids).

Answer: (D)