Explain why the cis-form is the predominant configuration of unsaturated fatty acids?

Answer 1

If the double bond is a cis bond the carbon atoms are on the same side this causes a kink in the chain allowing different and more rigid shapes to be formed. A trans bond has the carbon atoms on opposite sides leaving the structure similar to the saturated form, with just a bit more rigidity.
Trans fatty acids are generally a product of processing (hydrogenation) not normally found naturally making them difficult to digest properly.

Answer 2

Trans forms are often more thermodynamically stable (lower in energy) than cis and might be expected to predominate except that the two are not formed in a random manner. Such compounds are usually formed in bio-chemically important reactions involving enzymes which often only produce one of the possible isomers that could be formed. If the cell has a need/use for the cis isomer only then is likely that it will have evolved enzymes that will produce the cis form exclusively. To produce the trans form might be a waste of resources and energy if it has no need for that isomer or cannot convert it into something useful.

Some chemical reactions will only produce one possible isomer while others may produce a mixture. Without knowing exactly which enzymes produce these molecules, and the mechanism of how they operate, I cannot be more specific.

Answer 3

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