Fatty acids with or without C=C double bonds
Solid butter vs. liquid olive oil
In a saturated fatty acid, all carbons of the hydrocarbon chain are linked by single bonds and carry as many hydrogen atoms as possible. The chain is straight, so the molecules pack tightly. Unsaturated fatty acids contain at least one carbon–carbon double bond, and in the usual cis form, each double bond puts a kink in the chain. Kinked chains pack loosely, which lowers the melting point. That is why fats rich in saturated fatty acids, such as butter, are solid at room temperature, while plant oils rich in unsaturated fatty acids, such as olive oil, are liquid. In membranes, unsaturated fatty acids keep the lipid bilayer fluid.