Phospholipid properties define core membrane functions

Steps:

• Phospholipids are amphipathic, with hydrophilic heads and hydrophobic tails, forming a bilayer.
• The bilayer creates a fluid structure due to weak hydrophobic interactions and unsaturated fatty acids.
• This enables selective permeability, allowing lipid-soluble substances to pass while blocking water-soluble ones.
• Roles 1 (barrier to polar molecules), 2 (fluidity for protein movement), and 3 (structural integrity) all arise from these properties.

Why A is correct:

• Phospholipids' amphipathic nature directly enables bilayer formation, which underpins all three roles per the fluid mosaic model.

Why the others are wrong:

• B excludes role 3, but structural support is inherent to the bilayer.
• C excludes role 2, but fluidity results from phospholipid packing.
• D excludes role 1, but selective permeability stems from the hydrophobic core.

Final answer: A