Saturated tails pack tighter than kinked ones
- The idea
- Predict
- See it
- Why
- Check
The idea
“Unsaturated tails pack tighter, since double bonds grip neighbors. Straight tails just slide past each other.”
A double bond is stiff, so it seems like it should make a fat firmer. What it holds stiff is a bend, and the bend keeps tails apart.
The short version
The mistake: Unsaturated tails pack tighter, since double bonds grip neighbors. Straight tails just slide past each other.
What's actually true: Straight saturated tails pack tightly and melt higher. Kinked unsaturated tails can't, so they stay fluid when cooler.
- A saturated tail has only single C-C bonds and lies straight. Neighbors fit close, held by weak van der Waals attractions.
- An unsaturated tail has at least one C=C double bond. A cis double bond can't rotate, so its bend stays and holds neighbors apart.
- Organisms use this to tune their membranes. More unsaturated tails keep a membrane fluid in the cold.
On the exam: Straight tails pack tight and melt high. Kinks keep tails apart and keep membranes fluid.