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B2.1.1—Lipid bilayers as basis

Amphipathic phospholipids organize membranes by placing hydrophilic heads toward water and hydrophobic tails inward, forming stable bilayers and vesicles; This foundation explains compartment boundaries, surface orientation, and flexible membrane construction.

Syllabus
First assessment 2025
Objective
B2.1.1
Level
HL

Exam analysis

Chance of appearing6%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper2
Typical marks1–2

Common command terms

  • Identify
  • Explain
  • State
  • Outline
  • Draw
  • Label

Scoring notes

Common mistake
Saying phospholipids are only hydrophobic or only hydrophilic instead of amphipathic.

Recent exam appearances

May 2025Paper2 ["HL"] · TZ23(c)[ 1 ]B2.1.1—Lipid bilayers as basis
May 2024Paper2 ["HL"] · TZ12(a)[ 1 ]B2.1.1—Lipid bilayers as basis
May 2023Paper2 ["HL"] · TZ12(a)[ 2 ]B2.1.1—Lipid bilayers as basis
November 2020Paper2 ["HL"] · TZ02(a)(i)[ 2 ]B2.1.1—Lipid bilayers as basis
November 2019Paper2 ["HL"] · TZ04(a)(i)[ 1 ]B2.1.1—Lipid bilayers as basis
Practice this objective

Coverage 2014–2025 · Updated 15 Jul 2026

Concept essentials

  • Phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails.
  • Bilayers form because tails avoid water while heads remain exposed to aqueous surroundings.
  • Vesicles and many cellular membranes depend on phospholipid bilayer structure.
  • Correct diagrams show heads at the membrane surfaces and tails directed toward the centre.
ConceptIB Biology HL