The big idea: Every cell is wrapped in a cell membrane (plasma membrane) — a thin, flexible barrier that controls what enters and leaves the cell.
Its basic structure is a phospholipid bilayer: two rows of phospholipid molecules.
Dotted all through this bilayer are proteins, glycoproteins and cholesterol. Because these parts can drift around and are scattered like tiles, the membrane is described as a fluid mosaic.
The fluid mosaic model: a phospholipid bilayer (hydrophilic heads facing the water on both sides, hydrophobic tails tucked into the core) with proteins, glycoproteins and cholesterol embedded like tiles in a mosaic.
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- Cell (plasma) membrane
- The thin barrier surrounding every cell that controls which substances cross into and out of it.
- Phospholipid
- The molecule that makes up most of the membrane: a hydrophilic phosphate head joined to two hydrophobic fatty-acid tails.
- Phospholipid bilayer
- Two rows of phospholipids — heads facing the water on both surfaces, tails facing inward — that form the basic membrane sheet.
- Amphipathic
- Having both a hydrophilic (water-loving) part and a hydrophobic (water-hating) part in the same molecule. Phospholipids are amphipathic.
- Fluid mosaic model
- The accepted model of the membrane: a fluid phospholipid bilayer with proteins and other molecules scattered through it like tiles in a mosaic.
Why 'fluid' and why 'mosaic': Fluid — the phospholipids and many proteins are not fixed; they can drift sideways within the bilayer, so the membrane behaves a bit like a liquid.
Mosaic — many different kinds of molecule (proteins, glycoproteins, cholesterol) are dotted through the bilayer, like the different tiles in a mosaic picture.
To understand why a membrane is a bilayer, you have to look at a single phospholipid.
Each phospholipid has two ends with opposite feelings about water — and that single fact is enough to make the whole bilayer form on its own.
One phospholipid is amphipathic: a hydrophilic (water-loving) phosphate head joined to two hydrophobic (water-hating) fatty-acid tails.
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- Hydrophilic head
- The phosphate end of a phospholipid. 'Hydrophilic' means water-loving — it is attracted to water.
- Hydrophobic tails
- The two fatty-acid ends of a phospholipid. 'Hydrophobic' means water-hating — they are repelled by water.
- Hydrophilic
- Water-loving — a part that is attracted to and mixes with water.
- Hydrophobic
- Water-hating — a part that is repelled by water and will not mix with it.
The cause → effect chain: There is water on both sides of a membrane — outside the cell and inside it (the cytoplasm).
The hydrophilic heads are attracted to that water, so they turn to face outward on both surfaces.
The hydrophobic tails are repelled by water, so they are pushed inward, away from it, where they meet in the middle.
The result is two rows of phospholipids — a bilayer — forming spontaneously, with a water-hating core sandwiched between two water-loving surfaces.
In water, phospholipids line up into a bilayer on their own: heads point outward to the water on both surfaces, tails point inward away from it.
Interactive diagram
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Hydrophilic heads
- Are attracted to water
- Face outward, toward the water on both surfaces
- Sit on the outer and inner edges of the membrane
- Make the membrane surfaces compatible with watery surroundings
Hydrophobic tails
- Are repelled by water
- Point inward, away from the water
- Meet in the middle to form a non-polar core
- This core blocks most polar and charged molecules from crossing
Why this core matters: Because the centre of the bilayer is hydrophobic, it acts as a barrier:
small non-polar molecules (such as oxygen and carbon dioxide) slip straight through, but large or charged/polar molecules (such as glucose and ions) cannot cross the tails freely.
That is why the membrane is selectively permeable — a direct consequence of the amphipathic phospholipids.
The mosaic of proteins: The bilayer is not bare. Scattered through it are different proteins:
Integral proteins are embedded right through the bilayer (these include the channel and carrier proteins that move substances across).
Peripheral proteins rest on one surface.
Glycoproteins carry a carbohydrate chain on the outer surface and act as cell-recognition tags. Cholesterol sits between the phospholipids and steadies the membrane's fluidity. All these different 'tiles' make it a mosaic.
| Membrane component | Where it sits | What it does |
|---|---|---|
| Phospholipid bilayer | Two rows of phospholipids forming the whole sheet | The basic structure of the membrane; controls what can cross |
| Hydrophilic heads | Facing the water on both surfaces (outside and inside) | Attracted to water, so they sit on the watery surfaces |
| Hydrophobic tails | Tucked into the middle, away from water | Repelled by water, forming a non-polar core that blocks polar molecules |
| Integral proteins (channel / carrier) | Embedded right through the bilayer | Transport molecules across; channels form pores, carriers change shape |
| Peripheral proteins | Resting on one surface of the membrane | Support, signalling and attachment roles on the membrane surface |
| Glycoproteins | Protein with a carbohydrate chain on the outer surface | Cell recognition and cell-to-cell signalling (cell 'identity tags') |
| Cholesterol | Wedged between the phospholipids | Stabilises fluidity and reduces leakiness to small molecules |
A memory hook: Heads love water, tails hate it. Heads point out to the water; tails hide in the middle.
Philic = friend of water (hydrophilic); phobic = afraid of water (hydrophobic).
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How this is tested: On Paper 2 a 3–4 mark Explain or Outline question is a regular: explain how the amphipathic properties of phospholipids allow a bilayer to form (heads to the water, tails away from it, bilayer forms spontaneously).
On Paper 1 a multiple-choice question shows a labelled fluid-mosaic diagram and asks you to identify which regions are hydrophilic, or which component does cell recognition (the glycoprotein), or to name labelled components X and Y.
You should also be ready to compare the older Davson–Danielli model with the fluid mosaic model, and to explain why the membrane is called a mosaic.
IB-style question — explain how amphipathic phospholipids form a bilayer
Explain how the amphipathic nature of phospholipids allows them to form the bilayer of a cell membrane. [4]
How to score all four marks
- Define amphipathic. Each phospholipid is amphipathic — it has a hydrophilic (water-loving) head and hydrophobic (water-hating) tails.
- Place the heads. There is water on both sides of the membrane, so the hydrophilic heads face outward toward the water on each surface.
- Place the tails. The hydrophobic tails are repelled by water, so they are pushed inward, away from the water, meeting in the middle.
- State the outcome. This produces two rows of phospholipids — a bilayer — which forms spontaneously in water, with a hydrophobic core between two hydrophilic surfaces. (Award 1 mark each for: amphipathic / heads + tails; heads to the water; tails away from water; bilayer forms.)
Final answer
Phospholipids are amphipathic; the hydrophilic heads face the water on both surfaces and the hydrophobic tails point inward away from water, so two rows line up spontaneously into a bilayer with a hydrophobic core.
✓ Why this scores full marks: Each step is a separate scoring point and the answer is a clear cause → effect chain: amphipathic structure causes the heads-out, tails-in arrangement, which causes the bilayer.
A common slip is to describe the bilayer without explaining that water on both sides is what pulls the heads out and pushes the tails in.
| Feature | Davson–Danielli model (older) | Fluid mosaic model (current) |
|---|---|---|
| Protein position | Two continuous protein layers coating both surfaces of the bilayer | Proteins scattered through and across the bilayer like tiles in a mosaic |
| Is the membrane fluid? | Treated as a fairly rigid 'sandwich' | Phospholipids and many proteins drift sideways — the membrane is fluid |
| Fits the evidence? | Did not match electron-microscope images of proteins inside the bilayer | Matches the images — proteins are seen embedded within the bilayer, not just coating it |