Not all fatty acids are equal!
The location of a double bond and its geometric configuration have profound effects on the biological activity of fatty acids
Double bonds may be described as either cis or trans in orientation.
The term cis indicates that the hydrogen atoms attached to the carbons connected by a double bond, are on the same side of the carbon chain. This configuration gives a bent V-shaped geometry to the unsaturated fatty acid. In nature, the double bonds in most unsaturated fatty acids have a cis orientation.
The term trans indicates that the hydrogen atoms attached to the carbons connected by a double bond, are on opposite sides of the carbon chain. This configuration does not introduce a bend in the fatty acid like the cis orientation; instead the unsaturated fatty acid more closely resembles a straight chain saturated fatty acid. A trans double bond is most commonly introduced into a fatty acid chain during chemical processing of plant oils or by bacterial biohydrogenation in the first stomach (rumen) of ruminant animals.
For specific examples of cis and trans bonds in fatty acids click here.
Double bonds can be located anywhere along the carbon chain. If more than one double bond is present, they can be positioned in either a non-conjugated or conjugated arrangement.
The term non-conjugated indicates that two double bonds in the fatty acid carbon chain are separated by one or more methylene groups. The majority of double bonds in polyunsaturated fatty acids found in nature are non-conjugated and separated by a single methylene group (CH2).
The term conjugated indicates that two double bonds in the fatty acid carbon chain are separated by only a single bond and not a methylene group as is the case for non-conjugated bonds.
For a specific example of conjugated vs. non-conjugated fatty acids click here.