Lipids share carbon, hydrogen, and oxygen with carbohydrates but are insoluble in water. Two classes sit at this code: triglycerides for long-term energy storage, phospholipids for membrane structure. Both are built from glycerol and fatty acids joined by ester bonds. Unlike carbohydrates and proteins, lipids are not polymers.
A triglyceride is one glycerol joined to three fatty acids by ester bonds.
Lipids are not polymers; they are discrete molecules with a fixed sub-component count. A triglyceride is built from one glycerol and three fatty acids, joined by ester bonds formed in condensation reactions. Both reactants and the count must be named for the reactants mark. Three water molecules are released in total, one per ester bond.
- One glycerol molecule lines up with three fatty acid molecules. Glycerol carries three hydroxyl (–OH) groups; each fatty acid carries a carboxyl (–COOH) group at one end.
- Three condensation reactions occur in sequence. Each forms an ester bond between a glycerol –OH and a fatty acid –COOH, releasing one water molecule per bond.
- The complete triglyceride holds one glycerol joined to three fatty acids by three ester bonds. Three water molecules have been released in total.
The bond in a triglyceride is an
ester bond.Glycosidic,peptide, andhydrogen bondare all explicit rejects. Glycosidic is the most common error.
The water molecule released in each condensation step is built from the H of glycerol's –OH group and the –OH of the fatty acid's –COOH group. On diagram-annotation questions, circling an H from a C–H bond on glycerol misidentifies the reactive atom; circling both full –OH groups misses the point that only one H and one –OH leave.
Three water molecules are released, not one. The count must be named.
Condensation reactionwithout the water count is a partial answer.
Saturated fatty acids are straight; unsaturated ones kink and lower the melting point.
AQA 3.1.3 tests only the saturated / unsaturated distinction. Saturated fatty acids contain no carbon-carbon double bonds and carry the maximum number of hydrogen atoms; the chain is straight. Unsaturated fatty acids contain one or more C=C double bonds, and each double bond produces a kink in the chain.
Saturated and unsaturated fatty acids compared.
| Type | Double bonds | Chain shape and packing | Melting point and state at room temperature |
|---|---|---|---|
| Saturated | None | Straight, packs tightly | Higher; solid (animal fats) |
| Unsaturated | One or more (C=C) | Kinked, packs loosely | Lower; liquid (plant oils, fish oils) |
Write
melting point, notboiling point. AQA explicitly rejects boiling point on fatty acid property questions.
Three directional forms all earn the mark. As C=C double bonds increase, melting point decreases. Or: as saturation increases, melting point increases. Or: as hydrogen atoms increase, melting point increases. Reversing direction within one statement breaks the mark.
Write
hydrogen atoms, nothydrogen molecules. H₂ is not the structural feature being described.
Triglycerides are dense, insoluble, non-osmotic, metabolic-water-yielding energy stores.
Triglycerides are the body's primary long-term energy reserves. Four structural properties account for the storage role, and each is independently credited on energy-store questions.
High energy yield per gram. The high C–H to C–O bond ratio in fatty acid chains means more energy released per gram than carbohydrate on oxidation. Low mass-to-energy ratio. The same energy reserve can be stored in less mass, which matters for animals carrying their reserves while moving. Insoluble and non-osmotic. Triglycerides do not dissolve in cytoplasm, so they have no osmotic effect and do not affect cell water potential. Metabolic water on oxidation. The hydrogen-rich tails yield water on complete oxidation, significant for organisms in arid environments such as the camel.
Insolubleandnon-osmoticare separate credit points. The credited phrasing for the osmotic property isdoes not affect cell water potential.
The two-step chain for triglyceride breakdown: lipase catalyses the hydrolysis of the ester bonds, releasing fatty acids and glycerol; the fatty acids are then respired to yield ATP. Hydrolysis releases energy is a zero-mark drop. The hydrolysis step releases the fatty acids; respiration is what releases the energy.
Phospholipids replace one fatty acid with a phosphate head, then self-assemble into the bilayer.
A phospholipid carries one glycerol (still present, frequently dropped from comparative descriptions), two fatty acid tails down from three, and one phosphate head in place of the third fatty acid. Same ester bonds, same condensation chemistry as a triglyceride. The result is amphipathic: hydrophilic head, hydrophobic tails.
Glycerolis in both triglycerides and phospholipids. Don't omit it from the phospholipid structure; it is a frequent comparative mark point that students drop.
The phosphate head carries a negative charge and is hydrophilic; it interacts with water. The two fatty acid tails are non-polar and hydrophobic; they cannot interact with water. The molecule combines both regions in one structure. Amphipathic, polar, and amphiphilic all describe this asymmetry.
In an aqueous environment, phospholipids spontaneously arrange so that the hydrophilic heads face the water on both surfaces and the hydrophobic tails point inward, shielded from water in the centre. No enzyme catalyses this; it is thermodynamic self-assembly. The bilayer is the structural foundation of every biological membrane. Glycolipids, where a carbohydrate chain attaches to the phosphate head, sit on the outer surface and act as cell-surface recognition molecules.
Membrane fluidity, the fluid mosaic model, and membrane proteins belong to 3.2.1, not 3.1.3. The 3.1.3 answer stops at the bilayer structure.
Pitfall — Compare and contrast means paired statements
On comparative questions about triglycerides and phospholipids, every mark point requires an explicit comparative term.
Words such as
both,whereas,but, orcompared toanchor each comparison. Two separate descriptions of each molecule do not earn comparison marks even if every individual fact is correct; examiners will not infer links between juxtaposed sentences. The 2018 P1 Q10.2 paired-statement failure was the largest discrimination sink on this topic.
The emulsion test uses ethanol and water to detect lipids.
The emulsion test exploits the solubility split: lipids dissolve in ethanol but not in water. Mixing the two produces a milky dispersion if lipid is present in the sample.
- Place the sample in a grease-free test tube. Any lipid on the glassware would produce a false positive.
- Add approximately 5 cm³ of ethanol. Shake thoroughly to dissolve any lipid present.
- Add approximately 5 cm³ of water. Shake gently.
- Observe the colour. A white or milky emulsion indicates lipid is present. Clear indicates no lipid.
- Run a grease-free control tube with water in place of the sample to confirm no false positive from the reagents.
The credited descriptors are
whiteormilky.Cloudyhas not been credited for some time and is an explicit reject.
Key terms
- fatty acids
- glycerol
- ester bond
- melting point
- condensation reaction
- hydrolysis
- saturated
- unsaturated
- double bond
- bilayer
- lipase
- saturation