Fluid-Mosaic Model: Cell Membrane Structure
The fluid-mosaic model: phospholipid bilayer, intrinsic and extrinsic proteins, glycoproteins, glycolipids, and the role of cholesterol in restricting movement. Includes the 4-mark exam question, a worked model answer and the examiner trap on never saying the membrane is fixed.
Chapters (9)
Exam question [4 marks]
Describe the fluid-mosaic model of membrane structure and explain the role of cholesterol in a membrane.
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 | Phospholipids form a bilayer, with hydrophilic (phosphate) heads facing outwards (towards water) and hydrophobic (fatty acid) tails facing inwards | "two layers of phospholipids", "heads towards water, tails away from water" | "a layer of lipids" with no arrangement; heads and tails the wrong way round |
| 2 | Proteins are scattered / embedded in the bilayer (intrinsic span or are embedded, extrinsic on the surface), giving a mosaic | "proteins dotted throughout", "some go all the way through" | "the bilayer is made of proteins"; "proteins form a layer on each side" only |
| 3 | Phospholipids (and proteins) can move within the layer, so the membrane is fluid | "phospholipids move sideways", "not fixed", "flexible" | "the membrane is fixed / rigid"; "the membrane moves" with no reference to its molecules |
| 4 | Cholesterol restricts the movement of phospholipids / other molecules, so it makes the membrane more stable (less fluid) | "fits between the tails", "reduces fluidity", "holds molecules in place" | "cholesterol makes the membrane more fluid"; "cholesterol makes the membrane waterproof" with nothing further |
Marks 1 to 3 describe the model (arrangement, then the two words in its name); mark 4 is the cholesterol role. Both "mosaic" and "fluid" must be linked to what the molecules are doing. Glycoproteins and glycolipids (carbohydrate chains on the outer surface) are credit-worthy detail but do not add marks beyond 4.
Model answer (4/4)
The membrane is a bilayer of phospholipids. The hydrophilic phosphate heads face outwards towards the water, and the hydrophobic fatty acid tails face inwards. Proteins are scattered through the bilayer, some embedded in it and some on the surface, which forms a mosaic. The phospholipids can move within their layer, so the membrane is fluid. Cholesterol fits between the phospholipids and restricts their movement, which makes the membrane more stable.
Examiner trap
- Saying the membrane is 'fixed'
- Saying cholesterol makes the membrane more fluid at all temperatures (it restricts movement)
- Saying the bilayer is made of proteins
Key terms
- fluid-mosaic model
- phospholipid bilayer
- intrinsic protein
- extrinsic protein
- glycoprotein
- glycolipid
- cholesterol
- fluid
- mosaic
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:26The phospholipid
Start with one phospholipid. It has a head containing a phosphate group, and two fatty acid tails. The head is hydrophilic, which means it is attracted to water. The tails are hydrophobic, which means they are repelled by water.
0:42The bilayer
Cells have water inside and water outside. So the phospholipids line up in two layers, called a bilayer. The hydrophilic heads face outwards, towards the water on each side. The hydrophobic tails face inwards, hidden away in the middle.
That is because the hydrophobic tails are repelled by water, so hiding them in the middle is the stable arrangement. This basic structure is the same in every cell membrane, whether it is the cell-surface membrane, or the membrane around an organelle.
1:14Proteins: the mosaic
Next, the proteins. They are scattered through the bilayer, like pieces in a mosaic. That is the first word in the name. Intrinsic proteins are embedded in the bilayer. Many of them span the whole thing, from one side to the other. Extrinsic proteins sit on the surface, on one side only. Different proteins have different jobs, such as channels, carriers and enzymes. You will study those jobs in the next few lessons.
1:44Glycoproteins and glycolipids
Some proteins have short carbohydrate chains attached. Those are glycoproteins. Some phospholipids do too. Those are glycolipids. Both have their carbohydrate chains on the outer surface of the membrane. You will meet their jobs later, such as cell recognition. For now, know where they are.
2:04Fluid: phospholipids move
Now the second word, fluid. The phospholipids are not fixed. They can move sideways within their own layer. The proteins drift about too, though more slowly, because they are bigger. Think of a crowd of people shuffling about, rather than a brick wall.
So the membrane is not a rigid wall. It is a flexible, moving sea of molecules.
A fluid, with a mosaic of proteins in it.
2:31Cholesterol
Finally, cholesterol. It may be present in some membranes. Cholesterol molecules fit between the phospholipid tails. They restrict the movement of the phospholipids and the other molecules around them. Without it, the phospholipids would move about more freely. With it, they are held more steadily in place.
That makes the membrane less fluid, and so more stable.
3:32Examiner trap and recap
The trap. Never say the membrane is fixed, and never say cholesterol makes it more fluid. Cholesterol restricts movement.
Recap. A bilayer of phospholipids, with scattered proteins, that can move, and cholesterol that holds them steady.
Next time, we see how substances actually cross this membrane, starting with diffusion.
Watch first
This lesson builds on:
- Phospholipids vs Triglycerides: Emulsion Test · AQA 7402 · 3.1.3
Keep going
More from 3.2 Cells
- Simple vs Facilitated Diffusion · AQA 7402 · 3.2.3
- Osmosis and Water Potential Explained · AQA 7402 · 3.2.3
- Active Transport and Co-transport (Ileum) · AQA 7402 · 3.2.3
- Why Cells Transport Faster: Microvilli · AQA 7402 · 3.2.3
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-034.