Membranes and membrane transport — IB Diploma Biology HL
IB Biology B2.1: lipid bilayers, simple diffusion, osmosis and aquaporins, channels and pumps, the fluid mosaic model and HL membrane transport.
Revision notes
6 short notes, in the order of the specification. Each one in short:
A lipid bilayer is a double layer of amphipathic phospholipids that forms naturally in water, with hydrophilic phosphate heads facing the water on both sides and hydrophobic hydrocarbon tails forming the core. The core has low permeability to ions, polar molecules and large molecules, but small non-polar molecules such as oxygen and carbon dioxide cross it by simple diffusion.
Osmosis is the net movement of water across a partially permeable membrane from a lower to a higher solute concentration; aquaporins are channels that greatly speed it up. Channel proteins let specific ions diffuse through when open, and pump proteins use ATP to move specific particles against a concentration gradient. Both make membranes selectively permeable.
The fluid mosaic model describes a membrane as a fluid phospholipid bilayer with a mosaic of proteins in it. Integral proteins are embedded in one or both lipid layers; peripheral proteins are attached to one surface. Glycoproteins and glycolipids carry carbohydrate chains on the extracellular side, where they work in cell adhesion and cell recognition.
Membrane fluidity depends on the fatty acids in the bilayer: unsaturated fatty acids have lower melting points and keep membranes fluid and flexible, while saturated fatty acids make membranes stronger at high temperatures. Cholesterol modulates fluidity in animal cells. Fluidity lets membranes form vesicles in endocytosis and fuse with vesicles in exocytosis.
Gated ion channels open only in response to a stimulus: nicotinic acetylcholine receptors open when acetylcholine binds, and sodium and potassium channels open when the membrane voltage changes. The sodium–potassium pump uses ATP to exchange three Na⁺ out for two K⁺ in, generating membrane potentials. Sodium-dependent glucose cotransporters use the resulting Na⁺ gradient to absorb glucose.
Cell-adhesion molecules (CAMs) are proteins in the plasma membrane that bind cells to each other so that they form tissues. Different forms of CAM are used for different types of cell–cell junction, for example junctions that seal cells together or junctions that anchor them firmly. CAMs on adjacent cells bind to each other, holding the tissue together.
Exam questions
8 exam-style questions (27 marks), each with its mark scheme.
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25 cards: flip them, mark what you knew, and practise the rest.
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The whole of cells (form and function) on one page, so you can see where this subtopic fits.
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Quick check questions
Which part of a phospholipid is hydrophobic?
Show answer
The two hydrocarbon (fatty acid) tails.
Name the channel protein that lets water cross membranes rapidly.
Show answer
Aquaporin.
What does a pump protein use to move particles against a concentration gradient?
Show answer
Energy from ATP.
On which side of the plasma membrane are the carbohydrate chains of glycoproteins?
Show answer
The extracellular side.
HL only How many Na⁺ and K⁺ ions does the sodium–potassium pump move in one cycle?
Show answer
Three Na⁺ out and two K⁺ in.
Frequently asked questions
Why can oxygen diffuse through the cell membrane but ions cannot?
Oxygen is small and non-polar, so it can pass between the phospholipids through the hydrophobic core of the membrane by simple diffusion. Ions are charged and hydrophilic, so they cannot easily enter the hydrocarbon core. They cross only through specific channel or pump proteins, which makes the membrane selectively permeable.
How does osmosis differ from diffusion?
Osmosis is a special case of diffusion: it is the net movement of water only, across a partially permeable membrane, from a lower to a higher solute concentration. Simple diffusion is the net movement of any particle from a higher to a lower concentration of that particle. Both are passive and caused by random movement of particles.
What is the difference between facilitated diffusion and active transport?
Facilitated diffusion is passive movement of specific particles down their concentration gradient through channel proteins, with no ATP used. Active transport uses energy from ATP and pump proteins to move specific particles against their concentration gradient. Both are selective, because each protein only lets certain particles across.
What is the fluid mosaic model?
The fluid mosaic model describes the membrane as a phospholipid bilayer in which the lipids and many proteins move sideways (fluid), with proteins scattered through it like tiles (mosaic). It includes integral and peripheral proteins, glycoproteins, glycolipids and cholesterol, with hydrophilic surfaces and a hydrophobic core.
How does cholesterol affect membrane fluidity?
HL only Cholesterol is a modulator of membrane fluidity in animal cells. At higher temperatures it restricts the movement of phospholipid tails, stabilising the membrane. At lower temperatures it stops the tails packing closely, so the membrane does not stiffen. Its hydroxyl group sits among the phosphate heads and its rings lie among the tails.
Written and checked against the IB Biology HL specification · Updated October 2026