B1.1.8—Hydrophobic properties of lipids

Lipids are mostly hydrophobic molecules with low oxygen-to-carbon ratios, making them insoluble in water and dependent on transport complexes in blood.

Exam analysis

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Latest appearanceMay 2018

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Scoring notes

Common mistake: Calling oils insoluble without linking this to non-polar hydrocarbon chains and lack of hydrogen bonding.

Recent exam appearances

May 2018Paper1 ["HL"] · TZ16[ 1 ]B1.1.8—Hydrophobic properties of lipids

May 2017Paper1 ["HL"] · TZ25[ 1 ]B1.1.8—Hydrophobic properties of lipids

Coverage 2017–2018 · Updated 15 Jul 2026

Long hydrocarbon regions contain mostly non-polar C–C and C–H bonds. They cannot replace the hydrogen-bonding interactions that water molecules make with one another, so water excludes them and lipid molecules aggregate.

Observation	Structural explanation
poor solubility in water	large non-polar region has little attraction to water
solubility in non-polar solvents	similar intermolecular interactions are compatible
fat droplets coalesce	hydrophobic regions minimise contact with water

Fats, oils, waxes, phospholipids and steroids are grouped as lipids by physical behaviour, not by one repeating monomer. Lipids are therefore not true polymers.

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

[ 1 ]

Which substance must be transported in the blood by lipoprotein complexes?

Concept essentials

Lipids are generally hydrophobic because large parts of their molecules are non-polar.
Oils do not form hydrogen bonds with water, while glucose can.
Steroids are lipids partly because they have a low oxygen-to-carbon ratio.
Cholesterol and fats are transported in blood within lipoprotein complexes.