IB DP Biology: Membranes and membrane transport (DP Biology) – Revision Notes

These revision notes condense section B2.1 (Membranes and membrane transport) of the IB Biology guide, first assessment 2025, for DP Biology. B2.1.1 to B2.1.10 are SL and HL. B2.1.11 to B2.1.17 are additional higher level, so they are HL only. For full explanations, start with the study guide, then test yourself with the practice set.

Related pages: the IB DP Biology hub, the printable checklist, a free 10-minute diagnostic and Theme B: Form and Function revision notes.

Key terms

Term	Meaning
Amphipathic	Having a hydrophilic part and a hydrophobic part
Phospholipid bilayer	Two layers of phospholipids, heads out, tails in
Integral protein	Embedded in one or both lipid layers
Peripheral protein	Attached to one surface only
Glycoprotein / glycolipid	Carbohydrate chain attached to a protein / lipid, on the outer surface
Simple diffusion	Net movement down a gradient directly between phospholipids
Facilitated diffusion	Net movement down a gradient through a channel protein
Active transport	Movement by a pump using ATP, which can be against a gradient
Osmosis	Net movement of water across a membrane that is impermeable to the solute, towards the higher solute concentration
Aquaporin	Channel protein that lets water through quickly

Structure (B2.1.1, B2.1.2, B2.1.10)

• Amphipathic lipids assemble into sheet-like bilayers in water: heads towards water, tails together.
• The hydrocarbon core is a barrier to large molecules and to hydrophilic particles such as ions and polar molecules.
• Fluid mosaic model: a fluid bilayer with proteins scattered through it; lipids and many proteins can move sideways.

Method for the drawing, in steps:

1. Draw two parallel rows of phospholipids, tails inwards.
2. Add cholesterol between phospholipids.
3. Add an integral protein across both layers and a second embedded in one layer.
4. Add a peripheral protein on a surface.
5. Add a glycoprotein with a branched carbohydrate chain on the outer face.
6. Label hydrophilic heads, hydrophobic core and each protein type.

Transport routes (B2.1.3, B2.1.6, B2.1.7, B2.1.8)

Feature	Simple diffusion	Facilitated diffusion	Active transport
Protein needed	No	Channel protein	Pump protein
ATP used	No	No	Yes
Direction	Down gradient	Down gradient	Can be against gradient
Selective	No (size, polarity only)	Yes	Yes
Example	Oxygen, carbon dioxide	Specific ions	Specific ions or molecules

A channel allows its specific ion through only when open. A cell regulates its permeability by altering which channels and pumps are present and whether channels are open.

Osmosis (B2.1.5)

• Water molecules move randomly in both directions.
• The membrane is impermeable to the solute, so only water can move.
• The more concentrated solution has fewer water molecules per unit volume, so net movement is towards it.
• Aquaporins increase the rate; they do not decide the direction.

Method for a percentage change in mass:

1. Subtract: final − initial.
2. Divide by the initial mass.
3. Multiply by 100 and keep the sign.
4. Plot against concentration; the concentration at which the line meets zero is where water movement balances.

Small reminder: a cylinder goes from 4.00 g to 4.28 g. Change = 0.28 ÷ 4.00 × 100 = +7.0%.

Magnification reminder

Magnification is image size ÷ actual size, with matching units. A membrane image of 28 mm for a real width of 8 nm: 28 mm = 28 × 10⁶ nm, and 28 × 10⁶ ÷ 8 = 3.5 × 10⁶, so ×3 500 000. To find a real size, rearrange: actual = image ÷ magnification.

Model sentences to learn

• Barrier: “The hydrophobic core of fatty acid tails repels ions and polar molecules, so the bilayer is a barrier between aqueous solutions.”
• Osmosis: “The membrane is impermeable to solute, so water alone moves; more water molecules arrive from the dilute side than leave the concentrated side, giving net movement towards the higher solute concentration.”
• Selectivity: “Simple diffusion depends only on size and polarity, whereas channels and pumps admit only particular particles.”
• HL only, fluidity: “Unsaturated tails are kinked, pack loosely and melt at lower temperatures, keeping the membrane fluid and flexible.”

Reading data questions

1. Say what is on each axis and the units before you describe a trend.
2. Cite figures and units from the data when you describe a change.
3. Use percentage change when starting values differ.
4. Link the pattern back to the mechanism (osmosis, packing of tails, pump activity).
5. State a conclusion that matches the data and no more.

Glycoproteins and glycolipids (B2.1.9)

Carbohydrate is always on the extracellular side. Roles: cell recognition and cell adhesion. Nothing more is needed.

HL only: fluidity and cholesterol (B2.1.11, B2.1.12)

Fatty acid tails	Packing	Melting point	Effect
Unsaturated (kinked)	Loose	Lower	Fluid, flexible at cell temperatures
Saturated (straight)	Tight	Higher	Stronger at higher temperatures

• Habitat link: organisms in colder habitats tend to have more unsaturated fatty acids in membranes.
• Cholesterol sits between phospholipids in animal cell membranes, with its small hydrophilic region by the heads.
• Cholesterol modulates fluidity: it stabilises the membrane when warm and stops stiffening when cold.

Worked reminder, HL only: a cell kept at 10 °C has 52% unsaturated chains in its membrane lipids and at 30 °C it has 40%. The drop is 12 percentage points, which is 12 ÷ 52 × 100 = 23.1% of the cold value. With fewer unsaturated chains at the warm temperature, the membrane stays strong rather than becoming too fluid.

HL only: vesicles (B2.1.13)

• Endocytosis: membrane folds in and pinches off a vesicle (example: phagocytosis by a white blood cell).
• Exocytosis: vesicle fuses with the plasma membrane and releases contents (example: neurotransmitter release).
• Both need a fluid, flexible bilayer.

HL only: neurons and cotransport (B2.1.14 to B2.1.17)

• Nicotinic acetylcholine receptor: a neurotransmitter-gated ion channel.
• Sodium and potassium channels in axons: voltage-gated.
• Sodium-potassium pump: each cycle uses 1 ATP, moves 3 Na⁺ out and 2 K⁺ in, and helps to generate the membrane potential.
• Sodium-dependent glucose cotransporter: sodium moves in down its gradient and brings glucose with it. This is indirect active transport. Used in the small intestine (absorption) and the nephron (reabsorption).
• Cell-adhesion molecules (CAMs): different forms for different cell–cell junctions; they bind cells into tissues.

Worked reminder: if 900 glucose molecules enter with 2 Na⁺ each, 1800 Na⁺ must later be pumped out. Each pump cycle removes 3 Na⁺, so 1800 ÷ 3 = 600 cycles, which is 600 ATP.

Examples to quote

• Simple diffusion: oxygen into a respiring cell, carbon dioxide out.
• Facilitated diffusion: a specific ion through its own open channel.
• Active transport: a pump moving a specific particle using ATP.
• Cell recognition: a carbohydrate pattern on the outer surface that other cells detect.
• HL only: insulin or neurotransmitter release by exocytosis, and LDL or bacteria uptake by endocytosis.

Must-know distinctions

• Integral and peripheral: depth in the bilayer.
• Facilitated diffusion and active transport: ATP and direction.
• Direct and indirect active transport: ATP at the pump, or a sodium gradient made by a pump.
• Endocytosis and exocytosis: into or out of the cell.

Quick self-test

1. Why do phospholipids form bilayers in water?
2. Name two substances that cross a membrane by simple diffusion.
3. Which type of membrane protein is attached to only one surface?
4. State the direction of net water movement in osmosis.
5. A cylinder falls from 3.50 g to 3.15 g. Calculate the percentage change.
6. State one function of an aquaporin.
7. Why can a pump move particles against a gradient?
8. On which side of the membrane are carbohydrate chains found?
9. HL only: Why do unsaturated tails keep a membrane fluid in the cold?
10. HL only: How many net positive charges leave a cell after 1000 sodium-potassium pump cycles?

Answers

1. Heads are hydrophilic and tails hydrophobic, so the tails cluster away from water and the heads face it.
2. Oxygen and carbon dioxide.
3. Peripheral protein.
4. From lower solute concentration to higher solute concentration.
5. (3.15 − 3.50) ÷ 3.50 × 100 = −10.0%.
6. It lets water molecules cross the membrane quickly.
7. It uses energy from ATP.
8. The extracellular side.
9. The kinks stop the tails packing closely, so the bilayer does not stiffen.
10. 3000 out and 2000 in leaves 1000 net positive charges outside.

Where marks are usually lost

• Writing “water moves to where there is more water” without comparing solute concentrations.
• Forgetting to say that the membrane is impermeable to the solute in osmosis answers.
• Omitting the sign on a percentage change.
• Saying that channels and pumps are the same thing.
• Not stating that glycoprotein carbohydrate is extracellular.
• Drawing a membrane with proteins only on the surface and none spanning the bilayer.
• Describing cholesterol as simply stiffening the membrane (HL only).
• Saying the cotransporter uses ATP directly (HL only).
• Giving the pump ratio the wrong way round (HL only).

Official syllabus

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