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B1.1.10—Fatty acids

Fatty acids vary in chain length, double-bond number, and cis-trans arrangement, affecting molecular shape, saturation, melting point, and physical properties.

Syllabus
First assessment 2025
Objective
B1.1.10
Level
SL

Exam analysis

Chance of appearing11%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1–2

Common command terms

  • Outline
  • State
  • Draw
  • Distinguish
  • Compare
  • Identify

Scoring notes

Common mistake
Calling saturated fatty acids unsaturated because they contain a carboxyl group.

Recent exam appearances

May 2025Paper2 ["SL"] · TZ32(c)[ 1 ]B1.1.10—Fatty acids
November 2024Paper1 ["SL"] · TZ28[ 1 ]B1.1.10—Fatty acids
May 2023Paper1 ["SL"] · TZ27[ 1 ]B1.1.10—Fatty acids
May 2021Paper1 ["SL"] · TZ110[ 1 ]B1.1.10—Fatty acids
November 2017Paper2 ["SL"] · TZ05(c)[ 2 ]B1.1.10—Fatty acids

Coverage 2011–2025 · Updated 15 Jul 2026

Double bonds loosen fatty-acid packing

Fatty acid C=C double bonds Typical tail shape
saturated 0 relatively straight
monounsaturated 1 one cis kink
polyunsaturated 2 or more several kinks
Straight saturated and kinked unsaturated fatty-acid structures with space-filling models and tightly versus loosely packed triglycerides.

More cis double bonds prevent neighbouring tails from approaching closely. Intermolecular attractions are weaker, so less thermal energy is needed to separate the molecules and the melting point falls.

At the same temperature, tightly packed saturated lipids are more likely to be solid; more unsaturated lipids are more likely to remain fluid.

Lipid summary: predict behaviour from molecular regions

Structure seen Property predicted Main biological outcome
3 fatty-acid tails on glycerol hydrophobic, highly reduced dense long-term energy store
more cis C=C bonds in tails poorer packing, lower melting point greater fluidity at a given temperature
phosphate head + 2 tails amphipathic self-assembled bilayer barrier
4 fused carbon rings, mostly non-polar lipid-soluble diffusion through bilayer core

Ester condensation joins fatty acids to glycerol. The components retained in the product determine whether it behaves as a hydrophobic store or an amphipathic membrane molecule.

Across carbohydrates and lipids, use the same reasoning chain:

atoms and functional groups → bonds → molecular shape and polarity → interactions with water and other molecules → biological function

Fatty acids

[ 4 ]

Distinguish between the structures of the different types of fatty acids in food.

Drawing canvas

Concept essentials

  • Saturated fatty acids have no carbon-carbon double bonds in the hydrocarbon chain.
  • Unsaturated fatty acids contain at least one carbon-carbon double bond.
  • Monounsaturated fatty acids have one double bond, while polyunsaturated fatty acids have multiple double bonds.
  • Cis double bonds bend fatty acid chains, whereas trans double bonds keep them straighter.