The Kink Test

AP Biology · Unit 1 · Topic 1.5

The Kink Test

One double bond bends a fatty acid tail, and that bend decides whether a fat is solid or liquid, and how well a membrane flows.

Learning objective 1.5.A Describe the structure and function of lipids.

One double bond bends the tail

Pick a fatty acid. The top of the bench draws its real shape — every double bond puts a kink in the carbon chain. Below it, three copies try to pack side by side, the way tails do inside a fat or a membrane. Then move the temperature and see what melts.

Choose a fatty acid

carbon chain C=C double bond — the kink carboxyl oxygens packed solid flowing liquid
20 °C
Pick a fatty acid to see its shape.
Carbons0
C=C double bonds0
Saturation—
Melts at—
State right now—

Why the kink changes everything

Saturated means no double bonds at all

A saturated fatty acid contains only single bonds between its carbon atoms. The chain runs almost straight, so neighboring tails lie flat against each other along their whole length.

An unsaturated fatty acid contains at least one double bond between carbon atoms, and that bond puts a permanent bend in the chain. Add more double bonds and the tail becomes more unsaturated and more bent.

Press through the five fatty acids and watch the straightness figure in the label fall as double bonds are added.

Packing is what sets the melting point

Straight tails stack tightly. Many small attractions form along their whole length, so a lot of energy is needed to pull them apart — the melting point is high and the fat is solid at room temperature.

Kinked tails cannot lie flat against each other. Gaps open up, fewer attractions form, and far less energy separates them. The melting point drops and the lipid is liquid at room temperature.

What lipids do

Lipids are typically nonpolar and hydrophobic, and their jobs follow from how their subcomponents are put together.

Learning check

Answer these in your own words before you open the answers.

  1. Describe the difference between a saturated and an unsaturated fatty acid in terms of bonds between carbon atoms.
  2. Explain why an unsaturated fat is more likely to be liquid at room temperature than a saturated fat of the same chain length.
  3. A food manufacturer removes double bonds from a liquid vegetable oil. Predict the effect on the product and justify your prediction.
  4. Describe the function of phospholipids and of cholesterol in an animal cell membrane.
Show answers

1. A saturated fatty acid has only single bonds between its carbon atoms. An unsaturated one has at least one double bond between carbon atoms, and that double bond kinks the chain.

2. The kinks stop the tails packing closely together. Fewer attractions form between neighboring molecules, so less energy is needed to separate them and the melting point is lower. Comparing the 18-carbon fatty acids in this tool, adding three double bonds drops the melting point by about 80 °C — from a solid to a liquid at room temperature.

3. The product becomes more solid. Removing double bonds makes the tails straighter, so they pack more closely, more attractions form between them, and the melting point rises above room temperature.

4. Phospholipids group together to form the lipid bilayer of plasma and cell membranes. Cholesterol provides essential structural stability to animal cell membranes.