Although the overall structure of unsaturated fatty acids is similar to saturated fatty acids, there is one important difference; the long aliphatic tail contains one or more alkene functional groups.
Part of the chain with the structure -CH2-CH2- becomes -CH=CH-; two carbon atoms are attached to each other with a double bond instead of a single bond.
There are several ways of illustrating where these double bonds are located in the fatty acid molecule.
Delta-9,12 : Counting from the COOH group, the two double bonds join the ninth-tenth and twelfth-thirteenth carbon atoms.
Omega-3 : The first double bond starts at the third carbon atom. This time the counting begins at the end of the chain furthest from the -COOH group.
Molecular sequences can also be drawn out in long form, thus;
A typical Omega-3 fatty acid is alpha linolenic and is drawn
Whereas an Omega-6 fatty acid such as linoleic acid would be drawn