Why does a Phospholipid have an unsaturated fatty acid tail?

[Hammad Shahid]

Homework Statement

Hi guys. In the biochemistry chapter, the book shows the structure of a phospholipid (picture attached) with a PO4 3- group and 2 fatty acid chains attached to a glycerol molecule. However, one of the fatty acid chains has a C=C bond while the other is fully saturated. I’m wondering what’s the reason for this.

Homework Equations

The Attempt at a Solution

I looked through the section and it doesn’t give an explanation for it. I also searched it up but I couldn’t find what I was looking for, and the images online had similar structures as well.
Thanks for help.

 

[Borek]

I doubt there is a "reason", looks like just an example made to show several important properties of such molecules.

 

[Hammad Shahid]

I doubt there is a "reason", looks like just an example made to show several important properties of such molecules.

Okay thank you. That is what I thought.
So that means there’s variation in the fatty acid chains? And does that affect the phospholipid in any way (if the chains are linear or not)?

 

[TeethWhitener]

Yes and yes. cis-unsaturated fatty acids pack less efficiently than trans-unsaturated and saturated fatty acids. A higher proportion of cis-acids causes the phospholipid membrane to behave more as a fluid, whereas a higher proportion of saturated/trans-unsaturated acids acts to stiffen the phospholipid membrane. The balance of these affects the membrane's mechanical properties as well as the transport of various species across the membrane. Here's a relevant wiki page:
https://en.wikipedia.org/wiki/Lipid_bilayer_phase_behavior