Final answer:
Fatty acid molecules are polyunsaturated if there are three or more double bonds between the carbon atoms. Unsaturated fatty acids have double bonds between carbon atoms in the chain, and they can be classified as alkenes.
Step-by-step explanation:
Fatty acid molecules are said to be polyunsaturated, if there are three or more double bonds between the carbon atoms. When the hydrocarbon chain contains a double bond, the fatty acid is unsaturated.
Unsaturated fatty acids, such as oleic acid, have double bonds between carbon atoms in the chain. Alkenes are organic compounds that contain one or more double or triple bonds between carbon atoms, and unsaturated fatty acids can be classified as alkenes.