IB DP Biology: Membranes and membrane transport (DP Biology)

This study guide covers section B2.1 (Membranes and membrane transport) of the IB Biology guide, first assessment 2025, for DP Biology. Statements B2.1.1 to B2.1.10 are for both SL and HL. Statements B2.1.11 to B2.1.17 are additional higher level, so they are HL only and are marked that way below.

Use it with the revision notes and the practice set. See also the IB DP Biology hub and the printable checklist. Not sure what to revise first? Try a free 10-minute diagnostic. For the wider theme, see Theme B: Form and Function.

What this unit covers

Statement	What you must be able to do	Level
B2.1.1	Explain why amphipathic lipids form bilayers in water	SL and HL
B2.1.2	Explain why a bilayer is a barrier to ions and polar molecules	SL and HL
B2.1.3	Describe simple diffusion, using oxygen and carbon dioxide	SL and HL
B2.1.4	Distinguish integral from peripheral proteins	SL and HL
B2.1.5	Explain osmosis and the role of aquaporins	SL and HL
B2.1.6	Explain facilitated diffusion through channel proteins	SL and HL
B2.1.7	Explain active transport by pumps	SL and HL
B2.1.8	Explain selective permeability	SL and HL
B2.1.9	Describe glycoproteins and glycolipids	SL and HL
B2.1.10	Draw and label the fluid mosaic model	SL and HL
B2.1.11	Link fatty acid composition to fluidity	HL only
B2.1.12	Explain how cholesterol modulates fluidity	HL only
B2.1.13	Link fluidity to vesicle formation and fusion	HL only
B2.1.14	Describe gated ion channels in neurons	HL only
B2.1.15	Explain sodium-potassium pumps	HL only
B2.1.16	Explain sodium-dependent glucose cotransporters	HL only
B2.1.17	Describe cell-adhesion molecules	HL only

B2.1.1 and B2.1.2: Bilayers and why they are barriers

A phospholipid is amphipathic. Its head is hydrophilic (it is attracted to water) and its two fatty acid tails are hydrophobic. Put phospholipids in water and they assemble on their own into a continuous sheet two molecules thick. The heads face the water on both sides and the tails point inwards, away from water. It forms without any energy input. Other amphipathic lipids behave in the same way.

The tails make a hydrocarbon core that has no charge. Bulky molecules and any hydrophilic particle, such as an ion or a polar molecule, cannot enter that core easily, so the bilayer separates the aqueous solution inside a cell from the aqueous solution outside it.

B2.1.3: Simple diffusion

Diffusion is the net movement of particles down a concentration gradient, high to low. In simple diffusion the particles pass directly between the phospholipids, with no protein involved and no ATP used. Oxygen and carbon dioxide are small and non-polar, so they dissolve in the hydrocarbon core and cross freely. In a respiring cell, oxygen is used up so it keeps diffusing in, and carbon dioxide is made so it diffuses out.

B2.1.4 and B2.1.9: Membrane proteins and carbohydrates

Membrane proteins have many different structures, positions and jobs.

• Integral proteins sit within the lipid layers, in one layer or across both. Those that span the whole bilayer have hydrophobic amino acids in the part inside the core and hydrophilic regions at the surfaces. Channels, pumps and many receptors are integral.
• Peripheral proteins are attached to one surface of the bilayer and do not enter the core. Some are enzymes; some anchor the cytoskeleton.

Carbohydrate chains attached to membrane proteins make glycoproteins; chains attached to lipids make glycolipids. The carbohydrate lies on the extracellular side. These molecules work in cell recognition (a cell’s carbohydrate pattern identifies it to other cells) and cell adhesion (they help cells stick to each other).

B2.1.5: Osmosis and aquaporins

Osmosis is the net movement of water across a membrane that is impermeable to the solute, from the side with lower solute concentration to the side with higher solute concentration. Water molecules move randomly in both directions. Where solute is more concentrated, there are fewer free water molecules per unit volume, so fewer water molecules cross from that side than arrive from the dilute side. The net flow is therefore towards the more concentrated solution, and it stops when the two concentrations are equal or the cell cannot expand further. Because the membrane is impermeable to the solute, the solute stays put and water does the moving.

Water is polar, so it crosses a bare bilayer slowly. Aquaporins are channel proteins with a pore that lets water molecules through quickly, in single file. They raise the rate of osmosis; they do not change its direction.

Worked example: finding the solute concentration that gives no net change

Potato cylinders were weighed, left in sucrose solutions, blotted and reweighed.

Sucrose (mol dm⁻³)	0.0	0.1	0.2	0.3	0.4	0.5
Initial mass (g)	2.10	2.05	2.08	2.12	2.06	2.10
Final mass (g)	2.46	2.19	2.10	1.98	1.82	1.77

Step 1. Percentage change = (final − initial) ÷ initial × 100. For 0.0: 0.36 ÷ 2.10 × 100 = +17.1%. Doing the same for the rest gives +6.8%, +1.0%, −6.6%, −11.7% and −15.7%.

Step 2. Percentage change is used because the starting masses differ.

Step 3. The sign changes between 0.2 and 0.3. Assuming a straight line between those points, the concentration at zero change is 0.2 + 0.1 × 0.96 ÷ (0.96 + 6.6) = 0.213 mol dm⁻³ (about 0.21).

Step 4. Interpretation: at about 0.21 mol dm⁻³ the sucrose concentration outside matches the effective solute concentration in the potato cells, so there is no net water movement. Below it, water enters the cells and mass rises. Above it, water leaves and mass falls.

B2.1.6 to B2.1.8: Channels, pumps and selectivity

Channel proteins form a water-filled pore through the bilayer. Each type has a pore whose size, shape and charged lining fit one kind of ion, so only that ion can go through, and only while the channel is open. This is facilitated diffusion: movement down a concentration gradient through a protein, with no ATP.

Pump proteins are different. A pump uses energy from ATP to move specific particles across the membrane, so it can move them against their concentration gradient. This is active transport.

Selectivity is therefore a matter of which route a particle takes. Simple diffusion is not selective; it depends only on a particle’s size and on whether it is hydrophobic or hydrophilic. Facilitated diffusion and active transport are selective, because each protein binds or admits only particular particles. A cell controls what crosses by changing which channels and pumps it has and whether channels are open.

B2.1.10: The fluid mosaic model

In this model the membrane is a fluid bilayer of phospholipids in which proteins are scattered like tiles in a mosaic. Phospholipids and many proteins can move sideways within their own layer. Be ready to sketch a flat cross-section that shows:

• two rows of phospholipids, each drawn as a round head and two tails, with the tails facing each other;
• cholesterol between the phospholipids;
• an integral protein spanning the bilayer and another embedded in one layer;
• a peripheral protein on a surface;
• a glycoprotein with a branched carbohydrate chain on the outer surface;
• labels showing the hydrophilic heads and the hydrophobic core.

Worked example: scaling an electron micrograph

A bilayer measures 17.5 mm across on a micrograph. Its real width is 7 nm. Magnification is image size divided by actual size, so both must be in one unit first: 17.5 mm = 17.5 × 10⁶ nm. Then 17.5 × 10⁶ ÷ 7 = 2.5 × 10⁶, so the magnification is ×2 500 000.

B2.1.11 and B2.1.12: Fluidity and cholesterol (HL only)

Fatty acid tails that are unsaturated have double bonds that put kinks in the chain. Kinked tails pack less tightly, so unsaturated fatty acids have lower melting points and the membrane stays fluid and flexible at the temperatures the cell experiences. Saturated tails are straight, pack closely and have higher melting points, giving stronger membranes at higher temperatures. Organisms can adjust their membrane composition to match their habitat. For example, cold-water fish tend to have a higher proportion of unsaturated fatty acids in their membranes than warm-water fish, which stops the membrane stiffening in the cold.

Cholesterol in animal cell membranes lies between phospholipids, with its small hydrophilic region near the heads and the rest in the tail region. It is a modulator of fluidity. At higher temperatures it restricts the movement of phospholipids and stabilises the membrane. At lower temperatures it separates the tails so they cannot pack together, which prevents the membrane stiffening.

Worked example: interpreting fluidity data

Cells of a fish were kept at 5 °C or 25 °C and the percentage of unsaturated fatty acid chains in the membrane phospholipids was found. At 5 °C it was 58% and at 25 °C it was 41%. The difference is 58 − 41 = 17 percentage points. As a relative change from the 25 °C value: 17 ÷ 41 × 100 = 41.5% more unsaturated chains in the cold. Explanation: more kinked tails at 5 °C keep the bilayer fluid when low temperature would otherwise pack the tails together.

B2.1.13: Fluidity and vesicles (HL only)

A fluid membrane can bend, pinch off and merge with another membrane without leaking. In endocytosis a region of plasma membrane folds inwards and pinches off to make a vesicle that carries material into the cell; phagocytosis of a bacterium by a white blood cell is an example. In exocytosis a vesicle fuses with the plasma membrane and releases its contents outside; release of neurotransmitter at a synapse is an example.

B2.1.14 and B2.1.15: Neurons (HL only)

Gated ion channels open or close in response to a signal. Nicotinic acetylcholine receptors are neurotransmitter-gated: when acetylcholine binds, the channel opens and cations flow through. Sodium and potassium channels in an axon are voltage-gated: they open or close as the membrane potential changes.

The sodium-potassium pump is an exchange transporter. Each cycle uses one ATP to move three sodium ions out and two potassium ions in. This keeps sodium concentration low inside and potassium high, and because three positive charges leave for two that enter, it helps to make the inside of the neuron negative. That charge difference is the membrane potential.

B2.1.16: Cotransport of sodium and glucose (HL only)

In indirect active transport, the energy comes from a gradient set up by a pump, not from ATP directly. A sodium-dependent glucose cotransporter lets sodium ions move into the cell down their gradient and uses that movement to bring glucose in with them, even when glucose is already more concentrated inside. This absorbs glucose in the small intestine and reabsorbs it from the filtrate in the nephron. Sodium-potassium pumps keep the sodium gradient going.

B2.1.17: Cell adhesion (HL only)

Cell-adhesion molecules (CAMs) are membrane proteins that bind cells to each other. Each kind of cell–cell junction relies on its own form of CAM. This is how cells hold together as tissues. You do not need individual CAM names.

Common errors

• Saying water moves “towards water”. Say it moves from lower to higher solute concentration.
• Writing that osmosis needs a protein. It does not; aquaporins speed it up.
• Saying facilitated diffusion uses ATP. It does not; only pumps use ATP directly.
• Drawing the carbohydrate chains of glycoproteins on the cytoplasmic side.
• Saying cholesterol only makes membranes rigid. It prevents stiffening in the cold as well.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization, section B2.1.