Glycoproteins as cell surface antigens with internal hydrophobic domains

Steps:

• Identify glycoprotein function: serves as antigen for immune recognition via extracellular carbohydrate chains.
• Examine structure: membrane glycoproteins have hydrophilic exterior for antigen display and hydrophobic interior regions for membrane anchoring.
• Analyze regions P and Q: labeled with hydrophobic R groups, indicating transmembrane or cytoplasmic domains.
• Match to choices: antigen role fits, with hydrophobic groups in cytoplasm for stability.

Why A is correct:

• Glycoproteins function as antigens by exposing unique oligosaccharides outside the cell, while hydrophobic R groups in transmembrane/cytoplasmic regions anchor the molecule per membrane protein topology.

Why the others are wrong:

• B: Carriers transport ions/molecules across membranes, not primarily immune identifiers.
• C: Channels form pores for passage, with hydrophobic regions in membrane but hydrophilic lining outside.
• D: Receptors bind extracellular signals, placing hydrophobic R groups in cytoplasm but mismatching antigen role.

Final answer: A