How does the structure of triglycerides and phospholipids relate to their functions?
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 forms an ester bond. The R group of a fatty acid may be saturated or unsaturated. In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group. The different structures of triglycerides and phospholipids relate to their different roles in living organisms. The emulsion test for lipids.
A focused answer to the AQA A-Level Biology 3.1 specification points on lipids. Covers triglyceride and phospholipid structure, ester bonds and condensation, saturated versus unsaturated fatty acids, the structure-function relationship, and the emulsion test.
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What this dot point is asking
AQA wants you to describe how triglycerides and phospholipids are built by condensation reactions that form ester bonds, distinguish saturated from unsaturated fatty acids, relate each molecule's structure to its function (energy storage versus membrane formation), and carry out the emulsion test for lipids.
Triglycerides
The reverse reaction, hydrolysis, adds three water molecules to break the three ester bonds.
Saturated vs unsaturated fatty acids
The R group (hydrocarbon chain) of a fatty acid can be:
- Saturated - no carbon-carbon double bonds; the chain holds the maximum hydrogen. Straight chains pack closely, so saturated fats are usually solid at room temperature (animal fats).
- Unsaturated - one or more C=C double bonds, which put kinks in the chain so molecules cannot pack as tightly. These are usually liquid (oils) at room temperature. One double bond is monounsaturated; more than one is polyunsaturated.
Why triglycerides make good energy stores
- High C - H bond ratio → release more energy per gram than carbohydrate when oxidised.
- Insoluble in water → no effect on water potential, no osmotic problems.
- High H:O ratio → yield metabolic water when respired (important for desert animals).
Phospholipids
A phospholipid is a modified triglyceride: one of the three fatty acids is replaced by a phosphate-containing group. This produces a molecule with two distinct ends:
- Hydrophilic ("water-loving") phosphate head - polar, attracted to water.
- Hydrophobic ("water-hating") fatty acid tails - two non-polar tails, repelled by water.
Because the molecule has both, it is amphipathic. In water, phospholipids spontaneously form a bilayer: heads point outwards towards the watery cytoplasm and extracellular fluid, tails point inwards away from water. This bilayer is the basis of every cell-surface membrane, forming a partially permeable barrier.
The emulsion test for lipids
To test a sample for the presence of a lipid:
- Add ethanol to the sample and shake to dissolve any lipid.
- Add the mixture to an equal volume of water in a test tube.
- A cloudy white emulsion forms if a lipid is present (the lipid comes out of solution as tiny droplets that scatter light). A clear solution means no lipid.
Try this
Q1. Name the reaction that joins glycerol to a fatty acid and the bond it forms. [2 marks]
- Cue. Condensation reaction; ester bond (one water released per bond).
Q2. Compare the structure of a triglyceride and a phospholipid. [2 marks]
- Cue. Both have glycerol; triglyceride has 3 fatty acids, phospholipid has 2 fatty acids and a phosphate group (so it is amphipathic).
Q3. A student tests a sample using the emulsion test and observes a cloudy white layer. What conclusion can they draw, and why does the cloudiness appear? [3 marks]
- Cue. A lipid is present. Lipid dissolves in ethanol; when added to water it comes out of solution as tiny droplets that scatter light, giving a white emulsion.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA Paper 1 (style)4 marksDescribe the structure of a phospholipid and explain how this structure allows it to form a bilayer in cell membranes.Show worked answer →
A 4-mark answer must cover the head, the tails, and how each behaves in water.
Structure.
- A phospholipid is formed from glycerol, two fatty acids and a phosphate-containing group (the phosphate replaces one fatty acid of a triglyceride).
- The phosphate head is hydrophilic (attracts water / is polar).
- The two fatty acid tails are hydrophobic (repel water).
Function link.
- In water, phospholipids arrange so the hydrophilic heads face outwards towards the watery solutions inside and outside the cell, while the hydrophobic tails face inwards away from water, forming a bilayer. This creates a partially permeable barrier that controls what enters and leaves the cell.
Markers reward identifying the hydrophilic head and hydrophobic tails and explaining the inward/outward orientation in the bilayer.
AQA Paper 1 (style)3 marksTriglycerides are good energy stores. Use their structure to explain why.Show worked answer →
A 3-mark answer links structural features to the storage role.
- Triglycerides have a high ratio of carbon-hydrogen bonds to carbon atoms, so they release a large amount of energy per gram when oxidised (more than carbohydrates).
- They are insoluble in water, so they do not affect the water potential of the cell and cause no osmotic water uptake.
- They have a high ratio of hydrogen to oxygen atoms, so they act as a source of metabolic water when respired.
Markers reward any of these structure-to-function points; two or three distinct correct points earn full marks.
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