Saturated and unsaturated lipids

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Saturated and unsaturated lipidsSpec 1.5(ii)

In short

Saturated lipids contain fatty acids with no carbon-to-carbon double bonds, so their straight chains pack closely and they are usually solid at room temperature. Unsaturated lipids contain one or more C=C double bonds, which bend the chain, so they pack less closely and are usually liquid oils. Diets high in saturated fat are linked with raised LDL cholesterol.

Saturated and unsaturated lipids compared
Feature	Saturated	Unsaturated
C=C double bonds in the hydrocarbon chain	None: every carbon holds the maximum number of hydrogen atoms	One (monounsaturated) or more than one (polyunsaturated)
Shape of the chain	Straight	Bent (kinked) at each double bond
Packing of molecules	Pack closely together	Pack less closely
Melting point	Higher, so usually solid at room temperature	Lower, so usually liquid at room temperature
Typical sources	Butter, lard, animal fats	Olive oil, sunflower oil, oily fish

A saturated fatty acid with a straight chain and no C=C bonds; a monounsaturated fatty acid with one cis C=C bond causing a bend; a polyunsaturated fatty acid with more bends; straight chains packing tightly as a solid fat and bent chains packing loosely as a liquid oil. — A C=C double bond puts a kink in the chain, so unsaturated chains pack less closely and melt at a lower temperature.

Because straight chains pack closely, there are more intermolecular attractions between them, so more energy is needed to separate the molecules. This explains why saturated fats have a higher melting point than unsaturated oils.

Health link: diets high in saturated fat tend to raise the concentration of low-density lipoprotein (LDL) cholesterol in the blood, which raises the risk of cardiovascular disease (see the section on dietary risk factors).

Common mistake:

Unsaturated means at least one C=C double bond in the hydrocarbon chain. Do not write 'unsaturated fats have no hydrogen' or confuse the term with the number of ester bonds.

Quick check

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A molecule with a slightly positive end and a slightly negative end because its electrons are shared unevenly.
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α-1,4 glycosidic bonds (main chain) and α-1,6 glycosidic bonds (branch points).
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Glucose and fructose.
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Three, one for each ester bond formed.
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Yellow-brown to blue-black.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

Why is water a good solvent?

Water is a good solvent because its molecules are dipoles, with a slightly negative oxygen end and slightly positive hydrogen ends. These charged ends are attracted to ions and polar molecules, surround them and keep them apart, so they dissolve. This lets blood plasma and sap carry substances.

What is the difference between starch and glycogen?

Both are polysaccharides made of α-glucose, but starch is the energy store in plants and glycogen is the store in animals. Glycogen is more highly branched, with α-1,6 branches every 8 to 12 units, so it has more free ends and releases glucose faster.

What is a condensation reaction?

A condensation reaction joins two molecules together and releases a molecule of water. When monosaccharides join, a glycosidic bond forms. When glycerol and fatty acids join, ester bonds form. Hydrolysis is the reverse, as water is added to break the bond.

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