What you need to know
- 1.5(i)triglycerideA lipid made of one glycerol and three fatty acids joined by ester bonds. It is an energy store. synthesis: glycerol, three fatty acids, condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. and ester bonds.
- 1.5(ii)Differences between saturated and unsaturated lipids.
By the end, you should be able to…
- Identify the components of a triglyceride.
- Describe formation of three ester bonds.
- Compare saturated, monounsaturated and polyunsaturated fatty acids.
Build your understanding
A triglyceride has four starting molecules
A triglycerideA lipid made of one glycerol and three fatty acids joined by ester bonds. It is an energy store. is made from one glycerol molecule and three fatty acid molecules. Glycerol has three hydroxyl (–OH) groups. Each fatty acid has a carboxyl (–COOH) group attached to a hydrocarbon chain.
A condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. reaction between a hydroxyl group of glycerol and the carboxyl group of a fatty acid forms an ester bondThe bond formed by condensation between glycerol and a fatty acid.. Repeating this at all three hydroxyl groups produces a triglyceride with three ester bonds and releases three water molecules.
Read the carbon chain
A saturated fatty acidA fatty acid with no C=C double bonds in its hydrocarbon chain, so the chain is straight. has no carbon–carbon double bonds in its hydrocarbon chain. A monounsaturated fatty acid has one C=C bond; a polyunsaturated fatty acid has more than one.
The carbonyl C=O bond in a fatty acid does not make it unsaturated in this classificationSorting organisms into groups based on shared features and evolutionary relationships.. Look specifically for carbon–carbon double bonds. A triglyceride may contain different fatty acid chains, so inspect all three when given a diagram.
Structure and physical behaviour
Cis double bonds introduce bends into fatty acid chains. These generally make neighbouring chains pack less closely. Compared with similar saturated chains, this reduces the intermolecular attractions and tends to lower the melting point.
Many fats rich in saturated chains are solid at room temperature, while many oils rich in unsaturated chains are liquid. This is a useful tendency, not the definition of saturation: chain length and the mixture of molecules also affect physical behaviour. Triglycerides are largely non-polarHaving no separated charges, so the molecule does not attract water and does not dissolve well in it. and do not dissolve readily in water.
Follow the process
Glycerol has three –OH groups and each fatty acid has one –COOH group → a condensation reaction joins one –OH to one –COOH → an ester bond forms and one water molecule leaves → the same reaction at the other two –OH groups gives a triglyceride with three ester bonds and three water molecules released.
Saturation is decided afterwards, by inspecting each hydrocarbon chain for C=C bonds. Zero C=C bonds means saturated; one means monounsaturated; more than one means polyunsaturated. The biological assembly order is not the only valid written order: you may define the fatty acids first if the question asks about melting point.
NOT: the bond is ester, not glycosidic. C=O does not make a chain unsaturated.
- 1 glycerol + 3 fatty acids → 1 triglyceride + 3 H₂O
- Saturated: no C=C in the hydrocarbon chain
- One C=C: monounsaturated. More than one: polyunsaturated
- Cis C=C bends the chain and can lower the melting point
Two traps this diagram blocks
- Do not call the bonds glycosidic or peptide bonds.
- An oil is not the definition of unsaturated. Count the C=C bonds.
From the diagram alone
Question
Describe how a triglyceride is formed. [3]
Show a model answer
One glycerol reacts with three fatty acids. Each hydroxyl group of glycerol forms an ester bond with the carboxyl group of a fatty acid by condensation. Three water molecules are released.
| Fatty acid type | C=C bonds per chain | Hydrogen saturation |
|---|---|---|
| Saturated | None | Maximum for that carbon skeleton |
| Monounsaturated | One | Fewer hydrogen atoms |
| Polyunsaturated | Two or more | Fewer hydrogen atoms |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: a triglycerideA lipid made of one glycerol and three fatty acids joined by ester bonds. It is an energy store. forms. Y: three condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. reactions each make an ester bondThe bond formed by condensation between glycerol and a fatty acid. between glycerol and a fatty acid. Z: three water molecules are released and the lipid is largely non-polarHaving no separated charges, so the molecule does not attract water and does not dissolve well in it.. Incomplete: “It is unsaturated because it is an oil.” Improved: “The chains contain C=C bonds, so the fatty acids are unsaturated; that can make packing less close and the melting point lower.” The improvement uses the bonds, not the appearance, as the definition.
Use the correct numbers
One glycerol + three fatty acids → one triglyceride + three water molecules. There are three ester bonds. Do not call them glycosidic bonds.
Key explanations
- One C=C bond makes it monounsaturated. More than one makes it polyunsaturated.
- The C=O in the carboxyl group is not counted when classifying saturation.
Avoid this misconception
The claim: A saturated fatty acidA fatty acid with no C=C double bonds in its hydrocarbon chain, so the chain is straight. has no double bonds anywhere.
The correction: It has no carbon–carbon double bonds in its hydrocarbon chain. Its carboxyl group still contains a carbon–oxygen double bond.
Order how a triglyceride forms
A triglyceride is built from one glycerol and three fatty acids by condensation reactions. Order the steps, from the separate molecules to the finished lipid.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Put the steps in order, from glycerol and fatty acids to one triglyceride.
0 of 5 in the right place
The three fatty acids need not be the same. Phospholipids form the same way but with one fatty acid replaced by a phosphate group.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Describe the formation of a triglyceride, naming its bond type and the small molecule released.
Show suggested answer
One glycerol reacts with three fatty acids in condensation reactions. Three ester bonds form and three water molecules are released.
Practice question
Retrieval and explanation · suggested answer
A fatty acid has two C=C bonds and one C=O bond. Classify it and explain your answer.
Show suggested answer
It is polyunsaturated because its chain contains more than one carbon–carbon double bond. The C=O bond is not counted when classifying saturation.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A student argues that an oil with more C=C bonds must have a higher melting point because double bonds are strong. Explain the flaw.
Show a hint
Which interactions are overcome when a substance melts?
Explore the reasoning
Melting mainly overcomes attractions between molecules; it does not break their covalent C=C bonds. Cis double bonds can prevent close packing, reducing intermolecular attractions and lowering melting point relative to comparable saturated chains.
Recall this lesson
Lesson recall questions
Question 1 of 5: Name the bond formed between glycerol and a fatty acid in a triglyceride.
Triglycerides and saturation
Practice question
Retrieval and explanation · suggested answer
Name the bond formed between glycerol and a fatty acid in a triglyceride.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
What does an oil label really tell you?
1A · LipidsFood chemists can compare unsaturation through reactions at carbon–carbon double bonds. This connects a label such as “unsaturated fat” to a measurable molecular feature, rather than simply the…Specification 1.5(ii)Read & use the scienceClose article
Why this matters
Food chemists can compare unsaturation through reactions at carbon–carbon double bonds. This connects a label such as “unsaturated fat” to a measurable molecular feature, rather than simply the appearance of the food. A food label compresses molecular information into categories such as saturated and unsaturated. A double bond changes a fatty-acid chain, but the nutritional implications depend on the mixture, the amount consumed and what food replaces what. Chemists can describe the bonding precisely while researchers still investigate health outcomes. This distinction helps when an advert turns one structural feature into a promise of health: identifying a molecule is not the same experiment as measuring the effects of eating it.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.5(ii) know the differences between saturated and unsaturated lipids
Use the science
A social-media post calls every liquid oil “healthy” because it is unsaturated. What can the molecular classification establish, and what can it not establish?
Explore a possible response
It can describe the presence of C=C bonds. On its own it cannot establish the overall health effect of a food or diet; amount consumed, other ingredients and evidence about outcomes also matter.
Explore the source
- Royal Society of Chemistry: measuring unsaturationRoyal Society of Chemistry
From used cooking oil to fuel
1A · LipidsUsed cooking oil can become a feedstock for biodiesel, but pouring it straight into an engine is not the same process. Triglycerides are chemically converted into smaller fatty-acid…Specification 1.5(i)Read & use the scienceClose article
Why this matters
Used cooking oil can become a feedstock for biodiesel, but pouring it straight into an engine is not the same process. Triglycerides are chemically converted into smaller fatty-acid esters and glycerol in a reaction called transesterification. You already know that triglycerides contain ester bonds linking fatty acids to glycerol; the industrial reaction rearranges those connections using an alcohol. It changes properties such as viscosity, which matters when a fuel must flow and spray reliably. Calling a fuel biological also does not establish its full environmental footprint: the feedstock, processing energy and competing land uses still matter.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.5(i) know how a triglyceride is synthesised by the formation of ester bonds during condensation reactions between glycerol and three fatty acids
Use the science
Why is producing biodiesel a chemical change rather than simply melting a fat? Refer to triglyceride structure.
Explore a possible response
Melting changes physical state without changing covalent bonding. Transesterification changes ester connections and produces different molecules, including glycerol and fatty-acid esters.
Explore the source
- Biodiesel Production and DistributionUS Department of Energy, Alternative Fuels Data Center