Rigidity question..saturated and unsaturated

[BeatMCAT]

So, I was looking over an example problem and it said that "A cell membrane would be most rigid if both of its fatty acids were..."
and then answer choices. They chose completely saturated and long molecules.

But doesnt the rigidity increase with having double bonds, which would be unsaturated. I thought it would be unsaturated and long molecules.

Let me know if i have wrong idea. Thanks

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[PiBond]

So, I was looking over an example problem and it said that "A cell membrane would be most rigid if both of its fatty acids were..."
and then answer choices. They chose completely saturated and long molecules.

But doesnt the rigidity increase with having double bonds, which would be unsaturated. I thought it would be unsaturated and long molecules.

Let me know if i have wrong idea. Thanks

Double bonds prevent rotation, so yes, an unsaturated fatty acid tail may be more "rigid" per se. However, the double bonds (ie kinks) also prevent close packing between adjacent phospholipids so a membrane with predominantly unsaturated phospholipids would be more fluid than a membrane with saturated tails where there is better packing an an increased amount of van der Waals forces.

Does that make sense?

 

[vandyam]

I'm sure I'm undersimplifying it, but I remember it b/c saturated fats turn solid when cold (aka, sold = more rigid than liquid). Relating it to other things helps me remember these questions easier.

 

[collegestud2013]

the alkene double bonds in unsaturated fats are in the cis diastereomer, which prevents close packing and thus unsaturated fatty acids have a LOWER melting point than saturated FAs, making unsaturated FAs MORE FLUID than saturated FAs at a given temperature.