Lipids: hydrophobic properties, triglycerides and phospholipids

Molecules (Form and function) · Carbohydrates and lipids · note 6 of 8

Spec B1.1.8, B1.1.9Download all Carbohydrates and lipids notes (PDF)Download all notes (PDF)13 pages

Lipids: hydrophobic properties, triglycerides and phospholipidsSpec B1.1.8, B1.1.9

In short

Lipids are substances in living organisms that dissolve in non-polar solvents but are only sparingly soluble in aqueous solvents. They include fats, oils, waxes and steroids. A triglyceride forms by condensation of one glycerol with three fatty acids; a phospholipid forms when glycerol links to two fatty acids and one phosphate group.

Lipids: Substances in living organisms that dissolve in non-polar solvents but are only sparingly soluble in aqueous solvents. They include fats, oils, waxes and steroids.

Lipids are mostly hydrocarbon. C–H bonds share electrons almost equally, so they are non-polar and cannot form hydrogen bonds with water. This makes lipids hydrophobic: in water, lipid molecules cluster together instead of dissolving.

Formation by condensation

Glycerol has three –OH groups. Each can react with the carboxyl (–COOH) group of a fatty acid in a condensation reaction, forming an ester bond and releasing water.

Triglycerides and phospholipids
Feature	Triglyceride	Phospholipid
Made from	One glycerol + three fatty acids	One glycerol + two fatty acids + one phosphate group
Water released	Three molecules, one per ester bond	One per bond formed (three in all)
Polarity	Entirely hydrophobic	Hydrophilic phosphate head, hydrophobic tails
Main role	Energy storage and insulation	Membranes

Condensation of glycerol with three fatty acids forms a triglyceride with three ester bonds; a phospholipid has two fatty acids and a phosphate group.

Common mistake:

A triglyceride is not a polymer: glycerol and fatty acids are not repeating identical monomers. It is still made by condensation and broken down by hydrolysis.

Written and checked against the IB Biology SL specification · Updated October 2026

Frequently asked questions

What is the difference between alpha and beta glucose?

Alpha and beta glucose differ only at carbon 1: in alpha-glucose the –OH group is below the ring and in beta-glucose it is above. This small difference matters. Alpha-glucose forms coiled, branched starch and glycogen for energy storage, while beta-glucose forms straight cellulose chains that bundle into strong fibres.

Why are starch and glycogen good energy stores?

Starch and glycogen are good energy stores because coiling and branching make them compact, and their large size makes them relatively insoluble, so they have little osmotic effect. Glucose can easily be added by condensation or removed by hydrolysis, and glycogen's many branch ends let it release glucose quickly when needed.

Why are lipids better than carbohydrates for long-term energy storage?

Triglycerides release about twice as much energy per gram as carbohydrates when oxidised. They are insoluble, so they have no osmotic effect, and they are stored without water, whereas glycogen is stored bound to water. A fat store therefore adds much less mass for the same energy, which suits long-term storage.

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