Hydrophilic phosphate heads in phospholipid bilayer

Steps:

• Recall cell membrane structure: phospholipid bilayer with polar heads and nonpolar tails.
• Identify hydrophilic parts: polar phosphate heads interact with water.
• Label standard diagram: parts 1 and 2 as outer/inner heads, part 3 as associated polar groups, part 4 as hydrophobic tails.
• Select options including all hydrophilic regions: 1, 2, and 3.

Why A is correct:

• Hydrophilic regions are polar heads (parts 1, 2, 3) that attract water via hydrogen bonding, per amphipathic molecule definition.

Why the others are wrong:

• B excludes part 1, missing an outer hydrophilic head.
• C includes part 4 (hydrophobic tail) and excludes heads 1 and 2.
• D selects only part 4, a nonpolar tail repelled by water.

Final answer: A