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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
SL

Exam analysis

Chance of appearing6%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper2
Typical marks1–4

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

November 2024Paper2 ["SL"] · TZ16(a)[ 4 ]B2.1.1—Lipid bilayers as basis
May 2024Paper2 ["SL"] · TZ12(b)[ 3 ]B2.1.1—Lipid bilayers as basis
May 2021Paper1 ["SL"] · TZ23[ 1 ]B2.1.1—Lipid bilayers as basis
November 2020Paper2 ["SL"] · TZ03(a)[ 2 ]B2.1.1—Lipid bilayers as basis
May 2019Paper3 ["SL"] · TZ11(c)[ 2 ]B2.1.1—Lipid bilayers as basis

Coverage 2015–2024 · Updated 15 Jul 2026

Amphipathic Lipids Close into a Bilayer

A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty-acid tails. In water, these interactions drive spontaneous bilayer formation.

Bilayer formation from phospholipids in water, starting from scattered molecules and ending with a closed bilayer vesicle.

heads face water + tails hide from water → continuous bilayer → exposed edges close → sealed membrane or vesicle

SL Summary: Choose a Membrane Route

Particle property → gradient → route → energy.
Non-polar: bilayer. Water: osmosis/aquaporin. Ion or polar solute downhill: channel/carrier. Uphill: ATP-coupled pump.

The bilayer creates the barrier; proteins make it selective; carbohydrates create an outward interface; component mobility makes the membrane adaptable.

Lipid bilayers as basis

[ 8 ]

Cell membranes separate aqueous environments in cells. Explain how the properties of phospholipids help to maintain the structure of cell membranes.

Drawing canvas

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.