How the cell membrane is built, how it stays fluid, and every way a substance can cross it — from simple diffusion to the sodium–potassium pump.
Theme B · Form & functionLevel 2 · CellsB2.1Includes AHL content
This is the Higher Level lesson. Everything here is examinable at HL. The core statements B2.1.1–B2.1.10 are shared with SL; the extra HL-only statements B2.1.11–B2.1.17 are marked with a purple HL badge so you can see exactly what SL students leave out.
Core content — SL & HL
HL Additional Higher Level only
👆 Diagrams are interactive — click and drag the controls
1. What the membrane is made of SL
The membrane is a phospholipid bilayer studded with proteins — the fluid mosaic model (Singer & Nicolson, 1972). Each phospholipid is amphipathic: a hydrophilic phosphate head that faces the water, and two hydrophobic fatty-acid tails that hide from it. That single fact explains the whole structure — tails point inward, heads point outward, and the bilayer forms spontaneously in water.
Interactive diagram · click any part
Tap a labelled part of the membraneEach click explains what the structure is and the exact B2.1 statement it belongs to.
Parts: phospholipid · channel protein · pump (carrier) · peripheral protein · glycoprotein · glycolipid · cholesterol (HL)
Amphipathic phospholipidHydrophilic head (attracted to water) + two hydrophobic tails (repelled by water). B2.1.1
Bilayer as a barrierThe hydrophobic core stops ions and large/charged hydrophilic molecules crossing directly. B2.1.2 · B2.1.8
Integral vs peripheral proteinsIntegral proteins are embedded in / span the bilayer; peripheral proteins sit on one surface. B2.1.4
Glycoproteins & glycolipidsCarbohydrate chains on the outer surface — cell recognition and receptors. B2.1.9
2. Getting across the membrane SL
Choose a transport mechanism below. Watch what moves, which way it goes relative to the concentration gradient, and whether the cell has to spend ATP. This is the distinction examiners test most: passive transport (down the gradient, no ATP) versus active transport (against the gradient, ATP required).
Direction
Down the gradient
Energy (ATP)
Not needed
Uses a protein?
No — straight through
Simple diffusion: small non-polar molecules (O₂, CO₂) pass directly through the bilayer, down their concentration gradient. B2.1.3
Simple diffusionPassive; small non-polar molecules cross the bilayer directly. B2.1.3
Facilitated diffusionPassive; channel proteins let specific ions/hydrophilic molecules through, down the gradient. B2.1.6
OsmosisPassive movement of water from higher to lower water potential; aquaporins speed it up. B2.1.5
Active transportPump (carrier) proteins use ATP to move substances against the gradient. B2.1.7
3. What keeps the membrane fluid HL
A membrane has to stay fluid enough for proteins to move and for vesicles to form and fuse — but not so fluid that it falls apart. Two things tune this: the fatty-acid tails and, in animal cells, cholesterol. Drag the temperature slider and watch how packing changes.
❄ Low tempBody tempWarm 🔥
Body temperatureAt normal temperature saturated tails pack fairly tightly while unsaturated kinks keep gaps. Cholesterol sits between the tails, holding fluidity steady either way.
Saturated vs unsaturated tails B2.1.11Saturated tails are straight and pack tightly (less fluid). A cis C=C double bond kinks the tail, preventing tight packing (more fluid).
Cholesterol is a buffer B2.1.12At high temperature it restrains movement (reduces fluidity); at low temperature it stops tight packing (maintains fluidity). It is not simply a "fluidiser".
Vesicles form & fuse B2.1.13Because the membrane is fluid it can pinch off vesicles (endocytosis) and fuse them (exocytosis) — moving bulk material in and out.
Gated ion channels B2.1.14Voltage-gated and ligand-gated channels in neurons open/close to a stimulus — the basis of nerve impulses and synapses.
4. The sodium–potassium pump & cotransport HL
The sodium–potassium pump is the classic exchange transporter (antiporter): using one ATP it pumps 3 Na⁺ out and 2 K⁺ in, both against their gradients. Press play to step through the cycle. That Na⁺ gradient is then reused to drag glucose into the cell — indirect (secondary) active transport.
Na⁺ pumped out
0 / 3
K⁺ pumped in
0 / 2
ATP used
0
Ratio is fixed: 3 Na⁺ out : 2 K⁺ in per ATP hydrolysed. This builds the ion gradients that power nerves and cotransport. B2.1.15
Na⁺/K⁺ pump B2.1.15An exchange transporter (antiporter): 3 Na⁺ out and 2 K⁺ in per ATP, both against their gradients. Sets up the resting potential.
Na⁺–glucose cotransport B2.1.16A symporter (SGLT) uses the inward Na⁺ gradient to pull glucose in against its gradient — no ATP used directly here, so it is indirect active transport.
Where you meet this again: the Na⁺–glucose cotransporter is how the small intestine (C3.1) and kidney tubule (D3.3) absorb glucose, and the Na⁺/K⁺ pump underpins the resting potential in neural signalling (C2.2).
5. Cell adhesion molecules HL
Membranes don't just control what crosses them — they hold cells together. Cell adhesion molecules (CAMs) are membrane proteins (such as cadherins, integrins and selectins) that bind one cell to another and to the extracellular matrix. Coordinated adhesion is what turns a group of cells into an organised tissue. B2.1.17
CadherinsJoin like cells to like cells at junctions — calcium-dependent adhesion holding a tissue together.
IntegrinsAnchor cells to the extracellular matrix and relay signals across the membrane.
Common mistakes examiners see
The tempting wrong answer, and the mark-worthy right one:
Which way does water move in osmosis?✗ From low water concentration to high water concentration.✓ From higher water potential to lower water potential (i.e. from the more dilute to the more concentrated solution) across a partially permeable membrane.
Does facilitated diffusion use ATP?✗ Yes — it moves things through a protein, so it needs energy.✓ No. Facilitated diffusion is passive: it uses a channel/carrier but still moves substances down their gradient with no ATP. Only active transport uses ATP.
What is the stoichiometry of the sodium–potassium pump?✗ It swaps ions one-for-one, 1 Na⁺ out for 1 K⁺ in.✓ 3 Na⁺ out and 2 K⁺ in per ATP. The unequal exchange is why it is electrogenic.
Does cholesterol make the membrane more fluid?✗ Cholesterol is a fluidiser — more cholesterol, more fluid.✓ Cholesterol is a fluidity buffer: it reduces fluidity at high temperature and maintains it at low temperature. HL
Which part of a phospholipid is hydrophobic?✗ The phosphate head.✓ The two fatty-acid tails are hydrophobic; the phosphate head is hydrophilic. The molecule is amphipathic.
Does glucose cotransport use ATP directly?✗ Yes — glucose moves against its gradient, so ATP is hydrolysed at the cotransporter.✓ No ATP is used at the cotransporter. It runs on the Na⁺ gradient (which the Na⁺/K⁺ pump built using ATP) — hence indirect active transport. HL
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