This is the page where everything so far becomes one picture. It is also a skills topic, which is exam-speak for: you will be asked to draw it. Get the diagram right and you have banked marks that many students throw away.
📚 What you need to know
The fluid mosaic model was proposed in 1972 by Singer and Nicolson. It explains how biological molecules are arranged to form a membrane.
Fluid: the phospholipids and proteins can move about within their own layer.
Mosaic: the scattered pattern of proteins in the bilayer looks like a mosaic when viewed from above.
The four main components are phospholipids, cholesterol, glycoproteins and glycolipids, and integral and peripheral proteins.
Membranes form partially permeable barriers around the cell, around organelles and within organelles.
Substances cross by diffusion, facilitated diffusion, osmosis and active transport.
Membranes are the interface for cell signalling and cell-to-cell communication.
You must be able to draw and label a two-dimensional diagram of the model, including cholesterol positioned correctly.
Fluid, and mosaic
The name is a two-word summary of the whole model, so take the two words apart.
Fluid means the membrane is not a fixed structure. The phospholipids are not bonded to each other — they are simply held side by side by their attraction to water — so they can drift sideways past one another within their own layer. Proteins drift too, like boats on a pond. A membrane is more like a very thin layer of oil than a sheet of plastic.
Mosaic describes what you would see looking down on the membrane from above: proteins of many different shapes and sizes scattered irregularly through the phospholipids, like tiles set into a floor.
A common exam trap is asking what “fluid” refers to. It is not that liquid flows through the membrane. It means the molecules of the membrane itself can move within their layer. Say “the phospholipids and proteins can move laterally” and the mark is yours.
The four components
Component
Where it sits
What it is for
Phospholipids
Two layers, heads out and tails in
The basic structure and the permeability barrier
Cholesterol
Wedged between phospholipids, OH group by the heads
Controls fluidity and reduces permeability to water-soluble substances
Glycoproteins and glycolipids
Carbohydrate chains on the outer surface
Cell recognition, signalling and adhesion
Integral and peripheral proteins
Embedded in, or attached to, the bilayer
Transport, reception, catalysis and anchoring
The diagram you have to be able to draw
Your own version does not need to be beautiful. It needs the right components in the right places, each with a clear label line.
The examiner’s checklist
Drawing questions are marked against a list. Here is that list, in the order it is usually written.
Cholesterol is the one students forget, and its position is the detail examiners look for hardest.
🧩 Drawing the model under exam pressure
Draw two rows of circles first, one row along the top and one along the bottom, with a gap between them. These are the phosphate heads.
Add two wavy lines under each top circle and above each bottom circle, meeting in the middle. These are the fatty acid tails. Two per head, never one.
Leave a gap in both rows and draw one protein straight through it, from top to bottom. Label it “integral (channel) protein”.
Put a small blob on top of the heads at one side, touching but not entering the tails. Label it “peripheral protein”.
Add a branched chain of small circles rising from one protein into the space above. Label it “glycoprotein”.
Squeeze a thin cholesterol molecule between two phospholipids, with its OH group up at the head end. Label the OH.
Label the two sides: outside the cell above, cytoplasm below. Free marks, frequently missed.
What the model actually explains
A model earns its place by explaining observations. This one explains three big ones:
Passive and active movement between the cell and its surroundings. The bilayer sets what can cross unaided; the proteins provide routes for everything else.
Cell-to-cell interactions. Adhesion molecules and the carbohydrate coat let cells stick to and recognise each other, which is what makes tissues possible.
Cell signalling. Receptor proteins let a cell detect a hormone in the blood without that hormone ever entering the cell.
Membranes are not only found at the cell surface. They form the boundary of every organelle, and inside some organelles too — the inner mitochondrial membrane and the thylakoid membranes of a chloroplast are both compartments within compartments. This compartmentalisation lets a cell run several incompatible chemical processes at the same time in the same place.
A model, not a photograph. Singer and Nicolson proposed this in 1972 to fit the evidence available then, replacing an older model in which proteins were thought to coat the outside of the bilayer like a sandwich. Evidence from freeze-fracture microscopy, which split membranes open and revealed proteins buried inside, is what settled it.
Worked examples
WE 1
Explaining the name
Explain why the model of membrane structure is described as “fluid mosaic”. (2 marks)
Point 1: fluid
The phospholipids and proteins are not fixed in place and can move laterally within their own layer.
Point 2: mosaic
The proteins are scattered irregularly through the bilayer, which looks like a mosaic when the membrane is viewed from above.
fluid = the molecules move; mosaic = the proteins are scatteredtwo marks means one clean sentence for each word — do not spend both on “fluid”
WE 2
Spotting errors in a drawing
A student draws the fluid mosaic model. Their peripheral protein reaches into the middle of the bilayer, and their cholesterol is drawn lying flat across the centre. Identify what is wrong with each. (2 marks)
Error 1: the peripheral protein
A peripheral protein is hydrophilic, so it cannot enter the hydrophobic core. It must sit on the surface only.
Error 2: the cholesterol
Cholesterol sits upright between the phospholipids, with its OH group next to the phosphate heads and the rest of the molecule among the tails.
peripheral proteins stay on the surface; cholesterol stands uprightboth errors are about a molecule being in a place its chemistry will not allow
WE 3
Applying the model to evidence
Two cells, one human and one from a mouse, are fused together. Their surface proteins are labelled with different coloured markers. After 40 minutes at 37°C, the two colours are found evenly mixed across the whole fused cell. Explain what this shows. (3 marks)
Point 1: what the result means
The proteins from each original cell have spread across the entire membrane, so they must have moved.
Point 2: link to the model
This supports the idea that the membrane is fluid — proteins can move laterally within the bilayer rather than being fixed.
Point 3: why fluidity is possible
Phospholipids are held together by attraction to water rather than by bonds to each other, so molecules can drift past one another.
evidence that membrane proteins move laterally: the membrane is fluidthis is a real experiment (Frye and Edidin, 1970) and it turns up often as unseen data
💡 Exam tips
Learn the name and the year: Singer and Nicolson, 1972. It occasionally appears as a one-mark recall question.
When drawing, give every phospholipid two tails and draw a clear gap between the two layers of heads.
Label everything. An unlabelled diagram scores almost nothing, however accurate.
Draw cholesterol upright with the OH end by the heads. This exact detail is in the syllabus.
Show a peripheral protein not entering the hydrophobic region — that is what distinguishes it.
“Fluid” refers to the movement of the membrane molecules, not of substances through the membrane.
⚠ Common mistakes
Thinking fluid means liquid passes through. It means the components move sideways within their layer.
Leaving cholesterol out entirely. It is one of the four named components.
Drawing proteins sitting on top of the bilayer only. At least one must be shown embedded.
Putting carbohydrate chains on the inside. They belong on the extracellular surface.
Drawing phospholipids with one tail. Always two.
Forgetting that organelles have membranes too. The model applies to every membrane in the cell.
Up next: Membrane Fluidity — we have said the membrane is fluid. Now we find out what controls how fluid, and why organisms living in the cold have to do something about it.
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