Saturated, monounsaturated, and polyunsaturated fatty acids differ in the number of carboncarbon double bonds in their hydrocarbon chains. This affects their shape, packing, melting point, and whether the lipids containing them are usually solid or liquid.
How Their Structures Differ
A saturated fatty acid has no carbon to carbon double bonds: every possible carbon bond is “saturated” with hydrogen. A monounsaturated fatty acid has one carbon to carbon double bond, while a polyunsaturated fatty acid has two or more.
In many naturally occurring unsaturated fatty acids, each double bond produces a bend in the hydrocarbon chain. Bent chains cannot pack together as closely as straight saturated chains, so intermolecular attractions are weaker and less thermal energy is needed to separate the molecules.
| Type of fatty acid | Carbontocarbon double bonds | Typical consequence |
|---|---|---|
| Saturated | None | Straight chains pack closely; higher melting point; often found in solid fats |
| Monounsaturated | One | One bend reduces packing; lower melting point than comparable saturated fatty acids |
| Polyunsaturated | Two or more | Multiple bends reduce packing further; generally the lowest melting point of the three |
For example, triglycerides containing mostly saturated fatty acids are commonly solid at room temperature. These fats are useful for long-term energy storage and thermal insulation in endotherms. Triglycerides rich in unsaturated fatty acids are commonly oils; their lower melting points help them remain liquid, including in many plant tissues.
A common misconception is that “unsaturated” means the molecule contains less fat. It does not: the term refers specifically to the presence of carboncarbon double bonds in the fatty acid chains.
IB Exam Technique
This is SL and HL content in B1.1 Carbohydrates and lipids. For an explain question, link the full sequence: more double bonds → more bends → less close packing → weaker intermolecular attractions → lower melting point. Do not merely state that saturated fats are solid and unsaturated fats are liquid.