# Explain how the structure of a phospholipid bilayer enables it to act as a barrier to the movement of ions and polar molecules across the plasma membrane.

> IB DP Biology Higher Level (2023 syllabus) — B2.1 Membranes and membrane transport · Explain · 4 marks

## Mark scheme (4 marks)

1. Phospholipids are amphipathic, with hydrophilic phosphate heads facing outward (toward aqueous environments) and hydrophobic fatty acid tails facing inward.
2. The hydrophobic core of the bilayer (formed by the non-polar fatty acid tails) repels / is impermeable to charged or polar species.
3. Ions carry a net charge and are surrounded by a hydration shell of water molecules, making their passage into the non-polar core energetically unfavourable / requiring a very large energy input.
4. Therefore ions and polar molecules require channel proteins or carrier proteins (or pumps) to cross the membrane, demonstrating how bilayer structure underpins selective/controlled transport.

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