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Fatty acids can either be saturated or unsaturated. If they are saturated, they stack easily and form solids at room temperature. If they are unsaturated, however, double bonds are introduced, and these double bonds introduce kinks into the structure of the fatty acid molecule. In a different question on this site, it was suggested that the reason why double bonds cause kinks to form is that they interrupt the zig zag shape of the molecule.

However, what was left unexplained is how double bonds interrupt the zig zag shape. If we were dealing with a cis fatty acid, and both hydrogens were located on top, why would this lead to a kink?

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Instead of zig-zag, zig-zag, zig-zag etc. it now goes zig-zag, zig, zig-zag, zig- etc (the bold zig is due to the cis double bond). So the direction of the alkyl chain on one side of the cis double bond is different from that of the alkyl chain on the other. You could partially correct for that "wrong turn" by rotating a single bond by $180^\circ$, but that changes a staggered conformation into an eclipsed one.

Here is a link to a 3D model: http://www.biotopics.co.uk/jsmol/fattyacids.html and a snapshot in case the link goes away:

enter image description here

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  • $\begingroup$ Is there an extra zig because of the hydrogen on the next carbon occupying the region where the next carbon would have needed to go to form the next zag? (and if so, can you add it to your answer?) $\endgroup$
    – arara
    Feb 3, 2019 at 21:13
  • $\begingroup$ A cis-bond itself is a zig-zig (or U-shaped) while a trans-bond is zig-zag (just like a single bond in staggered antiperiplanar conformation). So yes, compared to the trans-bond you are swapping the single bonds to hydrogen and to the carbon on one side of the double bond. $\endgroup$
    – Karsten ♦
    Feb 3, 2019 at 21:23

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