How the cell membrane is built, and every way a substance can cross it — from simple diffusion to active transport.
Theme B · Form & functionLevel 2 · CellsB2.1
This is the Standard Level lesson, covering the core statements B2.1.1–B2.1.10. Taking Biology at Higher Level? The HL version adds membrane fluidity, the sodium–potassium pump, cotransport and cell adhesion.
Core content — SL & HL
👆 The diagrams are interactive — click parts and switch modes
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
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
Going to HL? The HL lesson adds why the membrane stays fluid (fatty-acid tails & cholesterol), the sodium–potassium pump (3 Na⁺ out : 2 K⁺ in), sodium–glucose cotransport, and cell adhesion molecules. → Open the HL version
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.
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.
Can large or charged molecules cross the bilayer directly?✗ Yes — anything small enough can diffuse straight through.✓ No. The hydrophobic core blocks ions and large/charged hydrophilic molecules; they need channel or pump proteins. Only small non-polar molecules diffuse directly.
Where are the carbohydrate chains of glycoproteins found?✗ On the inside (cytoplasmic) surface.✓ On the extracellular (outer) surface, where they act in cell recognition and as receptors.
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