Phospholipids vs Triglycerides: Emulsion Test
How the hydrophilic head and hydrophobic tails of a phospholipid make it form layers on water while triglycerides form droplets, plus the emulsion test for lipids. Includes the 5-mark exam question, a worked model answer and the examiner trap on the phosphate head.
Chapters (8)
Exam question [5 marks]
(a) Explain why phospholipids form a layer on the surface of water, but triglycerides form droplets.
(b) Describe how you would test a food sample for lipids.
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 | Phospholipid has a hydrophilic (phosphate-containing) head and hydrophobic fatty acid tails | "water-loving head, water-hating tails", "head attracted to water, tails repelled" | "head is hydrophobic"; "tails are hydrophilic"; "phospholipids dissolve" |
| 2 | Heads face the water and tails face away from the water (so a layer forms at the surface) | "tails stick out of the water / into the air", "heads in the water" | "heads and tails face the same way"; no reference to water |
| 3 | Triglyceride has no hydrophilic part / is entirely hydrophobic (insoluble), so molecules clump into a droplet | "no polar part", "non-polar / hydrophobic, so clump together", "insoluble in water, so clump together" | "triglycerides are too big"; "triglycerides are hydrophilic" |
| 4 | Shake / mix the sample with ethanol, then add (pour into) water | "dissolve in ethanol, then add water" | "add water first"; "add Benedict's / biuret" |
| 5 | A (milky) white emulsion shows lipid is present | "white cloudy emulsion", "milky white" | "cloudy" alone; "turns white" with no mention of emulsion/milky; "precipitate" |
Marks 1 to 3 are structure, then behaviour, then consequence; a statement of what happens with no structure scores nothing. In part (b) the order (ethanol, then water) is credited; the colour change alone is not enough without the word emulsion or milky. The role of triglycerides as an energy store and respiratory substrate is taught in the video but not credited here.
Model answer (5/5)
(a) A phospholipid has a hydrophilic phosphate-containing head and two hydrophobic fatty acid tails. On water, the heads face the water and the tails point away from it, so the molecules form a layer on the surface. A triglyceride has no hydrophilic part, so it is insoluble and entirely hydrophobic, and the molecules clump together into a droplet. (b) Shake the food sample with ethanol so any lipid dissolves, then add water. If a milky white emulsion forms, lipid is present.
Examiner trap
- Saying phospholipid heads are 'hydrophobic'
- Doing the emulsion test and forgetting to shake with ethanol then add water
- Writing 'cloudy' without 'white emulsion'
Key terms
- phospholipid
- hydrophilic head
- hydrophobic tail
- bilayer
- emulsion test
- ethanol
- milky white emulsion
- respiratory substrate
- insoluble
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:38Recap: the triglyceride
Last time, we built a triglyceride: one glycerol, and three fatty acids, joined by ester bonds. Each fatty acid is a hydrocarbon tail. So the whole molecule has no hydrophilic part. It is hydrophobic, and does not mix with water. It is insoluble, and it makes a very good energy store, which cells can use as a respiratory substrate.
Remember, a triglyceride is not a polymer, because it is not a chain of repeating monomers.
1:08The phospholipid
Now the phospholipid. It is built on the same glycerol backbone. But one of the three fatty acids is swapped out. In its place sits a phosphate group. So a phospholipid has two fatty acid tails, and one head that contains phosphate.
Same family, same backbone, same ester bonds. Just one swap, and a completely different behaviour.
1:32Hydrophilic head, hydrophobic tails
That head is charged, so it is attracted to water. We call it hydrophilic. The two fatty acid tails are hydrocarbons, so they are repelled by water. They are hydrophobic. The head is the water loving end, and the tails are the water hating end. One molecule, two behaviours.
1:52Layers and droplets in water
So what happens when you put phospholipids on water? The heads point into the water, and the tails point away from it, out into the air. That is a layer on the surface. Fully surrounded by water, they form a double layer, a bilayer. The heads face the water on both sides, and the tails are tucked inside, shielded from it. This is the basis of cell membranes, which we meet properly later.
But for this question, the key idea is simple. Heads toward water, tails away from water.
A triglyceride has no hydrophilic part to face the water. So the molecules just clump together into a droplet.
2:33The emulsion test practical
Now the practical, the emulsion test. Step one. Mix the sample with ethanol, and shake it, so any lipid dissolves in the ethanol. Step two. Pour that solution into water.
If there is a lipid, it is not soluble in water, so it comes out of the ethanol as tiny droplets. The mixture turns into a milky white emulsion. If there is no lipid, the liquid stays clear. So the order matters. Ethanol first, to dissolve it, then water.
3:48Examiner trap and recap
The trap. The phosphate head is hydrophilic, never hydrophobic. And in the test, write a white emulsion. Not just cloudy.
Recap. Phospholipids have a water loving head and water hating tails, triglycerides only have tails.
Next, we move on to proteins, starting with amino acids and the peptide bond.
Watch first
This lesson builds on:
- Triglycerides: Saturated vs Unsaturated Fats · AQA 7402 · 3.1.3
Keep going
More from 3.1 Biological molecules
- Triglycerides: Saturated vs Unsaturated Fats · 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-006.