Membrane Protein Solubility

Steps:

• Identify regions: outside cell (aqueous), membrane (lipid bilayer), inside cell (aqueous).
• Recall amino acids interact with environments: hydrophilic in water, hydrophobic in lipids.
• Active site outside requires hydrophilic amino acids for solubility and function in watery lumen.
• Membrane-spanning section needs hydrophobic amino acids to embed in fatty tails.
• Cytoplasmic side faces watery interior, so hydrophilic amino acids there.

Why A is correct:

• Matches principle that membrane proteins have hydrophilic domains in aqueous compartments and hydrophobic in the nonpolar membrane core.

Why the others are wrong:

• B: Wrongly places hydrophobic outside, ignoring aqueous exposure.
• C: Identical to B, so same error in external hydrophobicity.
• D: Fails to make membrane section hydrophobic, preventing stable embedding.

Final answer: A