A level-readyAll subjects · Biology · WBI11 (Unit 1)
Lipids: triglycerides, saturated and unsaturated
Triglycerides are the main fats and oils in our diet. How they are built, and whether their fatty acids are saturated, matters for energy storage and for heart health.
Building a triglyceride
A triglyceride is made from one glycerol molecule and three fatty acids. Each fatty acid joins to glycerol in a condensation reaction that forms an ester bond and releases one water molecule.
So making one triglyceride forms 3 ester bonds and releases 3 water molecules. Hydrolysis (adding water) breaks the ester bonds again.
| Part | Detail |
|---|---|
| Glycerol | 3-carbon alcohol with three -OH groups |
| Fatty acid | Hydrocarbon chain with a carboxyl group (-COOH) at one end |
| Ester bond | Formed between -OH of glycerol and -COOH of a fatty acid |
Saturated and unsaturated
- Saturated fatty acid: no carbon-carbon double bonds (C=C); the chain holds the maximum number of hydrogen atoms. Chains are straight and pack closely, so saturated fats (mostly animal fats) tend to be solid at room temperature.
- Unsaturated fatty acid: one (monounsaturated) or more (polyunsaturated) C=C double bonds. Each double bond puts a kink in the chain, so molecules cannot pack closely and these lipids (mostly plant oils) tend to be liquid at room temperature.
Why lipids store energy well
Triglycerides are insoluble, so they do not affect water potential. They contain many C-H bonds and release about twice as much energy per gram as carbohydrate when respired. Diets high in saturated fat are linked to higher blood LDL cholesterol, which you meet in the CVD lessons.
Worked example
How many water molecules are released when 5 triglyceride molecules are made?
- Each triglyceride forms 3 ester bonds.
- Each ester bond releases 1 water molecule.
- 5 x 3 = 15
Answer: 15 water molecules.
Check yourself
- Which bond joins a fatty acid to glycerol?
- Glycosidic bond
- Ester bond
- Peptide bond
- Phosphodiester bond
Show answer
Ester bond. Glycerol's -OH reacts with the fatty acid's -COOH to form an ester bond.
- How many water molecules are released when one triglyceride is formed?
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3. Three fatty acids each form one ester bond with glycerol, releasing one water each.
- What makes a fatty acid saturated?
- It has at least one carbon-carbon double bond
- It has no carbon-carbon double bonds
- It contains nitrogen
- It is joined to a phosphate group
Show answer
It has no carbon-carbon double bonds. Saturated chains hold the maximum hydrogen: no C=C bonds.
- Why are oils rich in unsaturated fatty acids usually liquid at room temperature?
- They have more ester bonds
- They contain more glycerol
- C=C double bonds kink the chains so they cannot pack closely
- Their chains are completely straight
Show answer
C=C double bonds kink the chains so they cannot pack closely. Kinks stop the molecules packing tightly, lowering the melting point.
- What reaction breaks a triglyceride into glycerol and fatty acids?
- Hydrolysis
- Condensation
- Oxidation
- Phosphorylation
Show answer
Hydrolysis. Adding water breaks the ester bonds: hydrolysis.
- Which is a reason triglycerides are good energy stores?
- They dissolve easily in cytoplasm
- They contain less energy per gram than glucose
- They are insoluble, so they do not lower the water potential of cells
- They are made of amino acids
Show answer
They are insoluble, so they do not lower the water potential of cells. Being insoluble, they have no osmotic effect, and they are energy-rich.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.