Triglycerides: Saturated vs Unsaturated Fats
How a triglyceride forms from glycerol and three fatty acids by condensation (ester bonds), and how saturated and unsaturated fatty acids differ. Includes the 4-mark exam question, a worked model answer and the examiner trap on calling triglycerides polymers.
Chapters (8)
Exam question [4 marks]
Describe how a triglyceride is formed, and explain how a saturated fatty acid differs from an unsaturated fatty acid.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Formed by a condensation reaction, which releases water | "condensation", "water is lost / removed / released", "three waters released" | "hydrolysis"; "water is added"; "a polymerisation reaction" |
| 2 | One glycerol joins to three fatty acids | "glycerol + 3 fatty acids", "three fatty acids attached to a glycerol backbone" | "one fatty acid and three glycerols"; "fatty acids join to each other" |
| 3 | Each join is an ester bond | "ester bonds between glycerol and the fatty acids", "three ester bonds" | "glycosidic bond"; "peptide bond"; "hydrogen bond" |
| 4 | Saturated: no carbon-carbon double bonds in the R group; unsaturated: one or more carbon-carbon double bonds in the R group | "C=C", "single bonds only" vs "at least one double bond between carbons"; "hydrocarbon chain" or "tail" for R group; "saturated has the maximum number of hydrogens / unsaturated has fewer hydrogens because of a C=C" (C=C must be stated) | "saturated has no hydrogen / unsaturated has no hydrogen"; "double bond to oxygen"; statement about only one type |
Mark 4 needs both types contrasted, or one clearly defined by the presence or absence of C=C. A triglyceride is not a polymer; do not credit "polymer" or "monomer" for the fatty acids. Extra credit-worthy points that add no marks beyond 4: unsaturated fatty acids are kinked; saturated tails pack more tightly, so saturated fats tend to be solid at room temperature.
Model answer (4/4)
A triglyceride is formed by condensation reactions, which release water. One glycerol molecule joins to three fatty acids, and each join is an ester bond. In a saturated fatty acid the R group has no carbon-carbon double bonds, only single bonds. In an unsaturated fatty acid the R group has at least one carbon-carbon double bond.
Examiner trap
- Saying triglycerides are polymers (they are not)
- Saying unsaturated means 'no hydrogen'
- Writing glycerol + three fatty acids but forgetting three waters released
Key terms
- triglyceride
- glycerol
- fatty acid
- ester bond
- saturated
- unsaturated
- C=C double bond
- R-group
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:30Glycerol and fatty acids
A triglyceride is built from two kinds of molecule. The first is glycerol. It is a short chain of three carbons, and each carbon carries an OH group. The second is a fatty acid. It has a carboxyl group at one end, and a long hydrocarbon tail at the other, called the R group. Glycerol is the backbone. The fatty acids are the tails.
0:54The ester bond
They join by a condensation reaction. An OH on the glycerol meets the OH of the fatty acid's carboxyl group. A molecule of water is released, and a new bond forms between them. That bond is called an ester bond. Condensation means joining two molecules and losing water. The reverse, using water to break the bond, is hydrolysis.
Remember that pair of words from the carbohydrate videos. They come up again here.
1:23Building a triglyceride
Glycerol has three OH groups, so it can join with three fatty acids. Each time, one water molecule is released and one ester bond forms. So one glycerol plus three fatty acids makes one triglyceride, with three ester bonds, and three molecules of water released. Do not forget that three. It is a classic lost mark.
1:47Saturated vs unsaturated
Now the difference between fatty acids lies in that R group tail. In a saturated fatty acid, every carbon in the tail is joined by single bonds. The carbons are saturated with hydrogen, so the tail is straight. In an unsaturated fatty acid, there is at least one double bond between two carbon atoms in the tail.
That double bond bends the chain, so the tail is kinked.
Why does this matter? Look at how the tails sit together. Straight saturated tails pack closely, so saturated fats tend to be solid at room temperature, like butter. Kinked unsaturated tails cannot pack so tightly, so unsaturated fats tend to be liquid oils, like olive oil. And the three fatty acids in one triglyceride do not have to be identical. Glycerol does not mind which tails it gets.
2:40Quick test: spot the unsaturated
The spec says you must recognise these from a diagram. So scan the tail, from the carboxyl group all the way to the far end. If you see a carbon double bonded to a carbon, it is unsaturated. If you see none, it is saturated. Only a double bond between two carbons counts. The double bond to oxygen in the carboxyl group does not.
3:33Examiner trap and recap
The trap. A triglyceride is not a polymer, because it is not a long chain of repeating units. It is one glycerol with three fatty acids. Recap. Glycerol plus three fatty acids, by condensation, makes ester bonds and three waters. A double bond in the tail means unsaturated.
Next time, we swap one fatty acid for a phosphate group and make a phospholipid.
Watch first
This lesson builds on:
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
Keep going
More from 3.1 Biological molecules
- Phospholipids vs Triglycerides: Emulsion Test · AQA 7402 · 3.1.3
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
- Maltose, Sucrose, Lactose: Glycosidic Bonds · AQA 7402 · 3.1.2
- Alpha vs Beta Glucose: Starch vs Cellulose · AQA 7402 · 3.1.2
All 21 3.1 Biological molecules lessons Whole playlist on YouTube
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-005.