This is the page where everything from this topic gets assembled into one diagram — and it is a diagram you may be asked to draw and label. The good news is that there are only four components. The marks are for putting each one in the right place and saying why it is there.
📘 What you need to know
Membranes form partially permeable barriers: around the cell, around organelles, and within organelles.
Substances cross them by diffusion, facilitated diffusion, osmosis and active transport.
Membranes are also an interface for cell signalling and communication between cells.
The fluid mosaic model was proposed in 1972 by Singer and Nicolson.
Fluid because phospholipids and proteins can move around within their own layers.
Mosaic because the scattered pattern of proteins in the bilayer looks like a mosaic from above.
Four components: phospholipids, cholesterol, glycoproteins and glycolipids, and integral and peripheral proteins.
Membranes are everywhere, not just around the cell
It is easy to think of “the membrane” as the outside of the cell. In fact the same structure is used over and over inside it, and that is what allows a cell to be organised at all.
Between the cell and its environment — the cell surface membrane.
Between the cytoplasm and an organelle — the nuclear envelope, a lysosome membrane, a vesicle.
Within an organelle — a mitochondrion has an inner and an outer membrane, with an intermembrane space between them.
Why compartments matter. Reactions that would interfere with each other can run at the same time in the same cell, because a membrane keeps them apart. Lysosome enzymes would digest the cell if they were loose in the cytoplasm; a membrane is the only thing keeping them where they belong.
The nuclear envelope is two bilayers, not one, and it is punctured by pores. A mitochondrion is two as well, which is why it has an intermembrane space.
Where the name comes from
Singer and Nicolson put the model forward in 1972, and the two words in the name each carry a specific meaning. Learn them separately, because questions ask about them separately.
WHY “FLUID”
The phospholipids and the proteins are not fixed in place. They move around within their own layer, drifting sideways past each other.
The membrane behaves more like a liquid film than a solid wall — which is how it flows back together when damaged.
WHY “MOSAIC”
Seen from above, the proteins are scattered through the bilayer in an irregular pattern.
That patchwork of different proteins set into a background of phospholipids looks like a mosaic made of tiles.
A neat check on whether you have understood “fluid”: proteins move within their layer, sideways. They do not flip from the outer layer to the inner one, because that would drag their hydrophilic ends through the hydrophobic core.
The diagram you need to be able to draw
Every component in this diagram has appeared earlier in the topic. What is being tested here is whether you can put them together in the right positions.
🧩 Drawing the fluid mosaic model: the checklist
Draw the bilayer first. Two rows of circles for the phosphate heads, facing outwards, with two tails hanging from each and pointing inwards. Make it obvious which part is head and which is tail.
Add an integral protein, e.g. a channel or carrier, drawn embedded in the bilayer.
Add a peripheral protein and make sure it does not extend into the hydrophobic region — that is the mark.
Add a glycoprotein with a carbohydrate chain, on the outside face only.
Add cholesterol, with its OH group next to the phosphate heads and the rest of it sitting among the tails.
Label everything. An unlabelled correct drawing scores very little.
🤔 What cholesterol is actually doing there
Cholesterol is amphipathic too, in its own way: a small hydrophilic OH group at one end and a large hydrophobic body. That is why it slots in with the OH beside the phosphate heads and the rest tucked among the tails. Sitting there, it packs the tails together and stops the membrane from becoming too fluid when warm, while also stopping them packing so tightly that the membrane becomes rigid when cold. It is a stabiliser rather than a structural necessity.
Worked examples
WORKED EXAMPLE
Explain why the membrane model is described as “fluid mosaic”. [3]
Point 1: fluid
The phospholipids and proteins are free to move around within their own layer.
Point 2: mosaic
The proteins are scattered irregularly through the bilayer.
Point 3
Viewed from above, that pattern of proteins set into the phospholipids resembles a mosaic.
Fluid = the parts move; mosaic = the proteins are scatteredAnswer the two words separately. Students often explain one and forget the other.
WORKED EXAMPLE
A student draws the fluid mosaic model with a peripheral protein extending halfway into the hydrophobic core. Explain why this is incorrect. [2]
Point 1
Peripheral proteins are hydrophilic across their whole surface.
Point 2
They cannot interact with the hydrophobic fatty acid tails, so they stay attached at the surface only.
Only amphipathic (integral) proteins can enter the coreThis is the most commonly lost mark on the drawing question.
WORKED EXAMPLE
State where cholesterol is positioned in a membrane and explain why it sits in that position. [3]
Point 1: position
Its hydroxyl (OH) group sits next to the phosphate heads, with the rest of the molecule among the fatty acid tails.
Point 2: why
The OH group is polar and hydrophilic, so it is stable near the heads and the water.
Point 3
The rest of the molecule is a non-polar hydrocarbon region, so it is stable among the hydrophobic tails.
Polar end up with the heads, non-polar body down with the tailsSame amphipathic logic as the phospholipids themselves. Notice the pattern.
WORKED EXAMPLE
Outline two roles of membranes inside a cell, other than forming the cell surface membrane. [2]
Role 1
They form barriers around organelles, separating their contents from the cytoplasm — for example the nuclear envelope or a lysosome membrane.
Role 2
They form compartments within organelles, such as the inner and outer membranes of a mitochondrion.
Compartmentalisation, inside and around organellesA named example each time turns a vague answer into a scoring one.
💡 Exam tip
Know the four components by heart: phospholipids, cholesterol, glycoproteins and glycolipids, integral and peripheral proteins.
When drawing, make the head and tail parts of the phospholipid clearly different. Vague blobs lose marks.
Peripheral proteins must not be drawn entering the hydrophobic region.
Put carbohydrate chains on the outside face only.
Cholesterol: OH next to the heads, body among the tails.
Credit the model properly if asked: Singer and Nicolson, 1972.
⚠ Common mix-up
Saying “fluid” means the membrane is liquid. It means the components move within their own layer.
Saying “mosaic” refers to the phospholipids. It refers to the scattered proteins.
Drawing proteins flipping between the two layers. They move sideways, not across.
Putting cholesterol in the middle of the core, lying flat. It stands upright with its OH at the head end.
Forgetting that organelles have membranes too. A question about “membranes” is not only about the cell surface.
Drawing one membrane around the nucleus. The nuclear envelope is two, with pores through it.
That completes Cell Membranes & Transport. Up next: Nucleic Acids — nucleotides, DNA and RNA, and how the instructions for every protein in this topic are stored.
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