Triglycerides — AQA A-level Biology

SPEC 3.1.3 - BIOLOGICAL MOLECULES

Lipids

Triglycerides, phospholipids, saturated and unsaturated fatty acids, and the emulsion test

What Are Lipids?

AQA SPEC (3.1.3)

Triglycerides and phospholipids are two groups of lipid.

Lipids are a diverse group of biological molecules. Unlike carbohydrates and proteins, lipids are not polymers — they are not built from repeating monomers joined by condensation into long chains.

PropertyDetail

ElementsCarbon, Hydrogen, Oxygen (like carbohydrates, but H:O ratio is much higher — far more C–H bonds)

SolubilityInsoluble in water; soluble in organic solvents (ethanol, acetone)

Polymer?No — lipids are NOT polymers; fatty acids are NOT monomers in the biological polymer sense

Energy valueMore energy per gram than carbohydrates (~39 kJ g⁻¹ vs ~17 kJ g⁻¹) due to higher proportion of C–H bonds

Common Error

Never call lipids "polymers of fatty acids." Triglycerides are not polymers — glycerol is not a monomer and the three fatty acids are not repeating identical subunits. This was covered in Lesson 1.01 and will be tested.

Fatty Acids — Structure

A fatty acid has the general formula RCOOH, where:

CARBOXYL GROUP

–COOH (the carboxylic acid group) — this is the reactive end that forms the ester bond with glycerol during condensation.

R-GROUP (HYDROCARBON TAIL)

R = a hydrocarbon chain of varying length (typically 14–20 carbons). This is the non-polar, hydrophobic tail. The R-group may be saturated or unsaturated.

AQA Requires You To Recognise

AQA asks students to recognise from diagrams whether a fatty acid is saturated or unsaturated. Look for C=C double bonds in the hydrocarbon R-group. If present → unsaturated. If all single bonds → saturated.

Saturated vs Unsaturated Fatty Acids

Saturated Fatty Acid

No C=C double bonds in the R-group

All carbon atoms bonded to maximum number of hydrogen atoms

Straight, linear chain

Chains pack closely together → stronger intermolecular forces → higher melting point

Solid at room temperature (e.g. butter, lard)

Unsaturated Fatty Acid

One or more C=C double bonds in the R-group

Monounsaturated = one C=C; polyunsaturated = two or more C=C

C=C double bonds create a kink (bend) in the chain

Kinks prevent close packing → weaker intermolecular forces → lower melting point

Liquid at room temperature (e.g. plant oils)

Examiner Alert — 2024 ER

When explaining melting point differences, refer specifically to C=C double bonds causing kinks. Do NOT say "more hydrogen atoms" — refer to carbon-carbon double bonds, not hydrogen content. AQA 2024 examiners penalised answers that described hydrogen molecules rather than specifying C–H bonds or C=C double bonds.

Triglyceride Formation

AQA SPEC (3.1.3)

Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid. A condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond.

The Ester Bond — Critical Detail for Exams

The ester bond forms between the –OH group of glycerol and the –COOH (carboxyl) group of the fatty acid. One water molecule is eliminated per bond formed.

3 condensation reactions → 3 ester bonds formed → 3 water molecules eliminated.

Examiner Alert — 2024 ER

In the ester bond formation, the –H comes from the –OH of glycerol and the –OH comes from the –COOH of the fatty acid. Together they eliminate as H₂O. A common error is to say "–H from the fatty acid" — this is incorrect. The hydrogen comes from glycerol's hydroxyl group; the hydroxyl comes from the fatty acid's carboxyl group.

Triglycerides — Structure & Function

Structure Summary

One glycerol molecule (3 –OH groups) + three fatty acid molecules

Joined by three ester bonds via three condensation reactions

Non-polar molecule overall → insoluble in water → does not affect water potential

The three fatty acid R-groups may be the same or different, and saturated or unsaturated

Functions of Triglycerides

Energy storage — yield more ATP per gram (~39 kJ g⁻¹) than carbohydrates or proteins due to the high proportion of C–H bonds (more hydrogen per carbon than carbohydrates)

Thermal insulation — subcutaneous fat layer reduces heat loss

Protection — fat pads around organs (e.g. kidneys) absorb mechanical shocks

Metabolic water — oxidation of fat during respiration releases more water per gram than carbohydrates (important in desert animals)

Buoyancy — fat is less dense than water; important in marine mammals

Why more energy than carbohydrates?

Lipids have a higher ratio of hydrogen to oxygen than carbohydrates. More C–H bonds means more electrons available for oxidative phosphorylation, yielding more ATP per gram. This is why fat stores much more energy per unit mass than glycogen.

Phospholipids — Structure

AQA SPEC (3.1.3)

In phospholipids, one of the fatty acids of a triglyceride is replaced by a phosphate-containing group.

Amphipathic — The Key Property

Phospholipids are amphipathic: they have both a hydrophilic (water-attracting) phosphate head and hydrophobic (water-repelling) fatty acid tails. This dual nature drives the spontaneous formation of the lipid bilayer in cell membranes.

Triglyceride vs Phospholipid

FeatureTriglyceridePhospholipid

ComponentsGlycerol + 3 fatty acidsGlycerol + 2 fatty acids + 1 phosphate group

Bonds3 ester bonds2 ester bonds (fatty acids) + 1 bond to phosphate

PolarityNon-polar throughout → entirely hydrophobicAmphipathic — hydrophilic head; hydrophobic tails

Solubility in waterInsolubleForms bilayer (not truly soluble — self-assembles)

FunctionEnergy storage; insulation; protectionCell membrane bilayer; transport vesicles

Where foundAdipose tissue; seeds; liverAll cell membranes (phospholipid bilayer)

Why does the phospholipid bilayer form spontaneously?

When placed in water, phospholipids self-assemble into a bilayer. The hydrophilic heads face outward (toward the aqueous environment) and the hydrophobic tails face inward (away from water). This arrangement minimises the contact between the hydrophobic tails and water — it is thermodynamically favourable.

Properties Linked to Structure

AQA requires you to explain the different properties of triglycerides and phospholipids related to their different structures. This is a common extended response question.

Triglyceride — Why suited to energy storage?

Insoluble → does not affect water potential of cells; does not diffuse out

High C:O ratio → more C–H bonds per molecule → more energy per gram (~39 kJ g⁻¹) than carbohydrates (~17 kJ g⁻¹)

Compact → stores large amounts of energy in small volume

Non-polar → chemically stable; not reactive with cellular machinery

Phospholipid — Why suited to membrane structure?

Amphipathic structure → hydrophilic head interacts with aqueous cytoplasm and extracellular fluid; hydrophobic tails face inward

Spontaneously forms a bilayer that acts as a selectively permeable barrier

Hydrophobic interior → prevents free movement of ions and polar molecules across the membrane

Fluid nature (especially with unsaturated fatty acid tails) → allows membrane to flex and proteins to move laterally

Emulsion Test for Lipids

Emulsion Test (Required Practical — AT f)

1

Place the sample in a dry, clean test tube. Add approximately 2 cm³ of absolute ethanol and shake vigorously to dissolve any lipid present

2

Pour the ethanol extract into a test tube containing approximately 2 cm³ of distilled water

3

Observe: a cloudy white emulsion forms if lipid is present. The solution remains clear if no lipid is present

Why does the emulsion form?

Ethanol dissolves the lipid (ethanol is a non-polar organic solvent). When poured into water (which is polar), the lipid cannot remain dissolved — it comes out of solution as tiny droplets dispersed throughout the water, creating a cloudy white emulsion.

Examiner Alert — Most Common Error

Students frequently describe adding the sample directly to water. The correct procedure is: dissolve in ethanol first, THEN pour into water. The ethanol step is essential and must be stated. Also: the test tube and sample must be dry — water in the tube before adding ethanol will give a false positive (cloudy) result.

Synoptic Links

Link to Lesson 1.01 — Condensation & Hydrolysis

Triglyceride formation = 3 condensation reactions → 3 ester bonds + 3 H₂O eliminated. Digestion of triglycerides by lipase = 3 hydrolysis reactions → 3 ester bonds broken → fatty acids + glycerol released. Water is used in each hydrolysis step.

Link to Cell Membranes (Spec 3.2.3)

The phospholipid bilayer is the structural foundation of all cell membranes. The hydrophobic interior prevents free diffusion of ions and polar molecules, creating a selectively permeable barrier. Unsaturated fatty acids in phospholipid tails increase membrane fluidity.

Link to Digestion (Spec 3.4)

Lipase hydrolyses ester bonds in triglycerides → fatty acids + glycerol. Bile salts emulsify fat droplets → increase surface area for lipase. Products packaged into micelles → absorbed by diffusion across ileum epithelium → reassembled into triglycerides → packaged into chylomicrons → enter lymph.

Key Examiner Point

Lipids are not polymers and fatty acids are not monomers. But ester bonds are formed by condensation and broken by hydrolysis — the same reaction principles from 3.1.1 apply here.

Common Exam Errors

Alert 1 — "Lipids are polymers"

Triglycerides are NOT polymers. Only 4 molecules join in formation (1 glycerol + 3 fatty acids). The fatty acids are not repeating identical monomers. Never use the term "polymer" for triglycerides.

Alert 2 — Emulsion test procedure

The correct sequence is: ethanol first (to dissolve lipid) → then pour into water (cloudy emulsion forms). Students who describe adding the sample to water directly, or forget the ethanol step, lose all marks for this question.

Alert 3 — Ester bond formation groups

The ester bond forms between the –OH of glycerol and the –COOH of the fatty acid. Water is eliminated: the –H comes from glycerol's –OH group, and the –OH comes from the fatty acid's –COOH group. A common error is writing "–H from the fatty acid" — this is incorrect and loses marks. The AQA 2024 ER specifically flagged students who failed to correctly identify which group contributes which atom.

Alert 4 — Saturated/unsaturated melting point explanation

Always say: unsaturated fatty acids have C=C double bonds that cause kinks in the chain, preventing close packing, so intermolecular forces are weaker, giving a lower melting point. Do not simply say "more double bonds = lower melting point" without explaining the mechanism.

DEEP Framework: Extended Answer

Question: "Explain why triglycerides are well suited to their role as energy storage molecules." [4 marks]

D

Define: A triglyceride is a lipid formed from one glycerol molecule and three fatty acid molecules joined by ester bonds via condensation reactions. It is non-polar and insoluble in water.

E

Explain mechanism: Triglycerides are insoluble — they do not dissolve in the cytoplasm and therefore have no osmotic effect on cells (do not lower water potential). They are compact — the non-polar fatty acid tails pack together in adipose cells. They yield more energy per gram (~39 kJ g⁻¹) than carbohydrates (~17 kJ g⁻¹) because their high proportion of C–H bonds provides more hydrogen for oxidative phosphorylation.

E

Evidence/Example: Adipose tissue stores triglycerides in fat droplets. During fasting or prolonged exercise, lipase hydrolyses ester bonds (hydrolysis), releasing fatty acids and glycerol for β-oxidation and respiration. Desert animals (e.g. camels) also use triglyceride oxidation to produce metabolic water.

P

Practical/Quantitative: The emulsion test (ethanol + water) confirms lipid presence — a positive cloudy white emulsion. Quantitative comparison: 1g of fat stores approximately 2.3× more energy than 1g of glycogen, meaning fat is a more efficient energy store per unit mass.

Practice Question

(a) Describe how a triglyceride is formed from glycerol and three fatty acids. Include the name of the bond formed and the type of reaction involved. [3 marks]

(b) Explain why a phospholipid is amphipathic and state why this property is important for cell membrane structure. [3 marks]

(c) A student performs the emulsion test on two samples: Sample A (olive oil) and Sample B (water). Describe the expected result for each sample and explain why the result differs. [3 marks]

D:

(a) Three condensation reactions occur between the –OH groups of glycerol and the –COOH groups of the three fatty acids (1). Three ester bonds are formed (1). Three water molecules are eliminated (1). [3]

E:

(b) A phospholipid has a charged, polar phosphate head (hydrophilic — attracted to water) (1) and two non-polar fatty acid tails (hydrophobic — repelled by water) (1). This amphipathic structure causes phospholipids to form a bilayer in aqueous environments, with tails facing inward and heads facing outward, creating a selectively permeable membrane barrier (1). [3]

E:

(c) Sample A: a cloudy white emulsion forms — olive oil dissolves in ethanol, and when poured into water the insoluble lipid disperses as tiny droplets causing cloudiness (1). Sample B: solution remains clear — water contains no lipid so no emulsion forms when mixed with water (1). The difference occurs because olive oil is non-polar and insoluble in water, whereas the sample B contains no lipid to form an emulsion (1). [3]

Biochemical Tests — All Four

SubstanceReagent / ProcedurePositive ResultNegative Result

Reducing sugarsBenedict's + heat (~80°C)Brick-red precipitateRemains blue

Non-reducing sugarsHCl + heat → NaHCO₃ neutralise → Benedict's + heatStage 1 blue; Stage 2 brick-redRemains blue after Stage 2

StarchIodine/potassium iodideBlue-blackYellow-brown

LipidsDissolve in ethanol → pour into waterCloudy white emulsionClear solution

Note — Biuret Test (Proteins, Lesson 1.04)

The biuret test for proteins (NaOH + CuSO₄ → purple/lilac colour) is covered in Lesson 1.04. The four tests above are the ones tested in this lesson.

Summary

Key Points

TopicKey Fact

Lipids — generalNot polymers; insoluble in water; C, H, O only; more energy per gram than carbohydrates

Fatty acid structureRCOOH; R = hydrocarbon tail; saturated = no C=C; unsaturated = one or more C=C (causes kink)

Melting pointsSaturated → higher (close packing); unsaturated → lower (kinks prevent close packing)

Triglyceride formationGlycerol + 3 fatty acids → 3 condensation reactions → 3 ester bonds + 3 H₂O eliminated

Ester bond groups–OH from glycerol + –COOH from fatty acid → ester bond + H₂O eliminated

PhospholipidTriglyceride with one fatty acid replaced by phosphate group; amphipathic; forms bilayer

Triglyceride functionEnergy storage (~39 kJ g⁻¹); insulation; protection; metabolic water; buoyancy

Emulsion testDissolve in ethanol → pour into water → cloudy white emulsion = positive

Exam Formula for Lipid Questions

For any "explain why lipids are suited to their function" question: (1) state the structural feature, (2) explain what that structural feature means chemically (e.g. non-polar, C–H bonds, amphipathic), (3) link to the functional consequence (insoluble, high energy yield, bilayer formation). Every mark = one structural–functional link.