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Educerie · IB Diploma · Biology
Theme B Form and function · B2.1 Membranes and membrane transport
What you must be able to do
| You must be able to | Level | What it looks like in the exam |
|---|---|---|
| Explain how phospholipids form bilayers and why a bilayer is a barrier | SL, HL | "Explain why ions cannot diffuse through the phospholipid bilayer" (2 marks) |
| Describe simple diffusion, using O₂ and CO₂ as examples | SL, HL | "Outline simple diffusion" (2 marks) |
| Distinguish integral and peripheral proteins | SL, HL | Multiple choice, or labelling on a membrane drawing |
| Explain osmosis and the role of aquaporins | SL, HL | Paper 1B: potato or tissue in solutions; "Explain the change in mass" (3–4 marks) |
| Explain facilitated diffusion through channel proteins | SL, HL | "Explain how channel proteins make membranes selectively permeable" (3 marks) |
| Explain active transport by pump proteins | SL, HL | "Distinguish between facilitated diffusion and active transport" (3 marks) |
| Explain why membrane permeability is selective | SL, HL | Data on uptake rates at different concentrations |
| Outline the structure and roles of glycoproteins and glycolipids | SL, HL | "Outline the role of glycoproteins in membranes" (2 marks) |
| Draw and label the fluid mosaic model | SL, HL | "Draw a labelled diagram of the fluid mosaic model" (4–5 marks) |
| Relate fatty acid composition to fluidity, with an example of adaptation | HL only | "Explain how membrane composition can be adapted to temperature" (3 marks) |
| Explain how cholesterol modulates fluidity | HL only | "Outline the role of cholesterol in membranes" (2–3 marks) |
| Explain endocytosis and exocytosis, with examples | HL only | "Explain how membrane fluidity allows exocytosis" (3 marks) |
| Describe neurotransmitter-gated and voltage-gated ion channels | HL only | Part of a Paper 2 extended response on neurons |
| Explain the sodium–potassium pump and its role in membrane potentials | HL only | "Explain how the sodium–potassium pump works" (4 marks) |
| Explain indirect active transport by sodium-dependent glucose cotransporters | HL only | "Explain how glucose is absorbed against its concentration gradient" (4 marks) |
| Outline how cell-adhesion molecules hold cells in tissues | HL only | One or two marks in a question on tissues |
Before you start
You need B1.1 section 12: a phospholipid is amphipathic, with a hydrophilic phosphate head and two hydrophobic tails, so in water phospholipids form a bilayer. You need the idea of diffusion from any earlier science course: particles move at random, and so spread from where they are more concentrated to where they are less. HL students also need B1.1 section 10 on saturated and unsaturated fatty acids, and B1.2 section 9 on where non-polar amino acids sit in a membrane protein.
1The idea in one paragraph
A cell membrane is a phospholipid bilayer with proteins set into it. The bilayer's hydrophobic core stops ions and large polar molecules, so the membrane separates two watery solutions and lets the cell keep its inside different from its outside. Small non-polar molecules such as oxygen slip straight through the lipids by simple diffusion. Water crosses by osmosis, much faster where aquaporins give it a channel. Everything else crosses only through proteins: channel proteins let particular ions diffuse through when they are open (facilitated diffusion), and pump proteins use energy from ATP to push particular particles against their concentration gradient (active transport). Because the proteins choose what passes, the membrane is selectively permeable. Carbohydrate chains on the outside let cells recognise and stick to each other, and the whole arrangement, fluid lipids with proteins scattered through them, is the fluid mosaic model.
2The lipid bilayer: the basis of the membrane, and a barrier
Phospholipids, and other amphipathic lipids, arrange themselves into continuous sheets two molecules thick whenever they are in water: heads out towards the water on both sides, tails in (B1.1). No energy or template is needed. That spontaneous lipid bilayer is the basis of every cell membrane and of the membranes around organelles. It is only about 7–10 nm thick.
The middle of the bilayer is a layer of hydrocarbon tails, and hydrocarbon is non-polar. So the core has low permeability to two kinds of particle.
- Hydrophilic particles: ions such as Na⁺, K⁺ and Cl⁻, and polar molecules such as glucose. They are attracted to water and surrounded by it; entering a non-polar layer would mean leaving that water behind, so they do not.
- Large molecules, of any kind, which cannot squeeze between the lipid tails.
That is what makes a membrane an effective barrier between two aqueous solutions, the cytoplasm on one side and the tissue fluid on the other. Figure 1 sorts the particles.
3Simple diffusion across membranes
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. Nobody pushes them. Particles move in every direction, but where there are more of them, more move away, so the net flow runs down the concentration gradient until the concentrations are equal.
Simple diffusion across a membrane is diffusion straight through the phospholipid bilayer, between the phospholipid molecules, with no protein involved. It works only for particles the core does not block: small and non-polar ones.
The guide's examples are the respiratory gases. Oxygen is used up inside a respiring cell, so its concentration inside stays lower than outside, and oxygen diffuses in continuously. Carbon dioxide is produced inside, so its concentration is higher inside, and it diffuses out. Both are small and non-polar, so both pass between the phospholipids with ease. The cell spends no energy on either movement; it only has to keep using oxygen and making carbon dioxide to keep the gradients in place.
4Membrane proteins: integral and peripheral
A membrane is roughly half protein by mass, and membrane proteins are very diverse in structure, position and function: channels, pumps, receptors, enzymes, anchors and recognition tags. The guide sorts them by where they sit.
- Integral proteins are embedded in the membrane, in one or both of the lipid layers. Many span the whole bilayer; these transmembrane proteins include every channel and pump. They are held in place because the part inside the bilayer is covered in hydrophobic amino acids (B1.2 section 9 at HL).
- Peripheral proteins are attached to one surface of the bilayer or the other, bound to the heads of phospholipids or to integral proteins. They do not enter the hydrophobic core, so they are more loosely held.
Both kinds appear in Figure 5, section 10.
5Osmosis, and the role of aquaporins
Osmosis is the net movement of water molecules across a partially permeable membrane from a region of lower solute concentration to a region of higher solute concentration.
It is diffusion of water, and the guide wants it explained with three ideas, all in Figure 2(a).
- Random movement. Water molecules move at random and cross the membrane in both directions all the time.
- The membrane is impermeable to the solute. The solute particles, sugar or ions, cannot cross, so they cannot even out their own concentration. Only the water can move.
- A difference in solute concentration. Solute particles attract water molecules around themselves, which leaves fewer water molecules free to move. Where there is more solute, fewer water molecules cross per second. So more water crosses from the side with less solute than returns from the side with more, and the net flow is towards the higher solute concentration.
Water is small but polar, so it crosses the bare bilayer only slowly. Many cells speed this up with aquaporins, integral channel proteins that form a pore just wide enough for water molecules to pass in single file, as in Figure 2(b). The pore is lined with hydrophilic groups, so water passes easily, and it is so narrow, and charged in the right places, that ions cannot follow. A membrane packed with aquaporins lets water cross many times faster than one without. Cells in the kidney that reabsorb water can change the number of aquaporins in their membranes and so control how much water they take back.
Reading osmosis data. The classic Paper 1B experiment puts pieces of plant tissue in a range of solute concentrations and measures their mass. Figure 3 plots an invented set of results for potato.
| Sucrose / mol dm⁻³ | 0.0 | 0.2 | 0.4 | 0.6 | 0.8 | 1.0 |
|---|---|---|---|---|---|---|
| Initial mass / g | 2.50 | 2.48 | 2.52 | 2.46 | 2.51 | 2.49 |
| Final mass / g | 2.95 | 2.69 | 2.49 | 2.20 | 2.06 | 1.95 |
| Change / % | +18.0 | +8.5 | −1.2 | −10.6 | −17.9 | −21.7 |
Pieces never start at exactly the same mass, so you compare percentage change, not change in grams. For the first and third samples:
Now read Figure 3. In pure water and 0.2 mol dm⁻³ sucrose the tissue gained mass: the solute concentration outside was lower than inside the cells, so water entered by osmosis. From 0.4 mol dm⁻³ upwards it lost mass: the outside was more concentrated, so water left. Where the line crosses zero, there was no net movement, so the solute concentration outside matched the tissue's own. Read it off the graph, or interpolate between the two readings either side of zero:
Say what limits the estimate: only one sample per concentration, and the crossing lies between two readings 0.2 mol dm⁻³ apart. Repeats and more concentrations between 0.2 and 0.4 would improve it.
6Channel proteins and facilitated diffusion
Ions and polar molecules cannot cross the bilayer, but many of them still need to get in or out. Facilitated diffusion is diffusion across a membrane through a protein. It is still diffusion: particles move down their concentration gradient, and the cell spends no energy on it.
A channel protein is an integral protein with a pore through its middle, shown in Figure 4(a). Two features of its structure make the membrane selectively permeable:
- The pore's diameter and the charges lining it fit one kind of ion. A potassium channel lets K⁺ through and not Na⁺, even though Na⁺ is smaller, because the pore is shaped around K⁺.
- The channel can be open or closed. Many channels have a gate that opens only in response to a signal. When the channel is open, its ion diffuses through, down its gradient; when it is closed, nothing passes.
So the cell controls which ions cross, and when, by which channels it builds and when it opens them.
7Pump proteins and active transport
Diffusion can only ever even out concentrations. A cell that needs to hold a concentration difference, more K⁺ inside than out, or to collect a mineral ion from a very dilute soil, needs to move particles the other way. Active transport is the movement of particles across a membrane against their concentration gradient, from lower to higher concentration, using energy from the cell.
It is done by pump proteins, shown in Figure 4(b). A pump binds its specific particle on one side, uses energy from ATP (adenosine triphosphate) to change its shape, and releases the particle on the other side. Because the energy comes from ATP, the pump can move particles against the gradient. Two consequences to use in data questions:
- Active transport stops if the cell cannot make ATP, for example when respiration is blocked by a poison or when oxygen runs out.
- Pumps are specific, like channels: each carries one kind of particle, or one set.
8Selectivity in membrane permeability
Put sections 3, 6 and 7 side by side.
| Simple diffusion | Facilitated diffusion | Active transport | |
|---|---|---|---|
| Route | between phospholipids | through a channel protein | through a pump protein |
| Direction | down the gradient | down the gradient | against the gradient |
| Energy from ATP | no | no | yes |
| Selective? | no | yes | yes |
| What decides it | size and polarity only | which channels exist, and whether they are open | which pumps exist |
Simple diffusion is not selective: any particle that is small enough and non-polar enough will cross, wanted or not, and the cell cannot stop it. Selectivity comes only from proteins. That is why the guide says facilitated diffusion and active transport are what make membranes selectively permeable, and why a cell's membrane proteins, not its lipids, decide what it takes in.
A useful data pattern: the rate of simple diffusion rises in proportion to the concentration difference, with no ceiling. The rate of facilitated diffusion or active transport rises and then levels off, because there is a fixed number of protein channels or pumps in the membrane, and once they are all working at full speed, more concentration cannot make them go faster.
9Glycoproteins and glycolipids
Some membrane proteins and lipids carry short, often branched, carbohydrate chains. A protein with a chain is a glycoprotein; a lipid with one is a glycolipid. In the plasma membrane the carbohydrate is always on the extracellular side, facing out.
Their roles, which the guide limits to two:
- Cell recognition. The chains differ between cell types and between individuals, so they act as identity markers. The immune system reads them to tell self from non-self, and the ABO blood group antigens from B1.1 are exactly such chains.
- Cell adhesion. Carbohydrate chains help cells attach to one another and to the material around them, so cells stay together in tissues.
10The fluid mosaic model
The fluid mosaic model is the accepted description of membrane structure, proposed in 1972. Figure 5 is the two-dimensional drawing you must be able to produce.
The name has two halves, and each describes something real.
- Fluid: the phospholipids are not fixed in place. They move sideways within their own layer all the time, and many proteins drift through the membrane like boats in a harbour. The membrane behaves like a thin liquid film, not a solid skin.
- Mosaic: seen from above, the proteins are scattered through the lipid like tiles in a mosaic, different kinds in different places.
What your drawing must show, and why each part is there:
- A phospholipid bilayer, heads outwards on both faces, tails meeting in the middle.
- Integral proteins, at least one spanning the whole bilayer; a channel protein makes a good second example.
- A peripheral protein, on one surface only.
- A glycoprotein, with its carbohydrate chain on the outside.
- Cholesterol between the phospholipids, in animal cell membranes.
- The hydrophilic regions (the two layers of heads) and the hydrophobic region (the tails) marked.
11HLFatty acids, cholesterol and membrane fluidity
SL students can skip to section 17.
A membrane must be fluid enough to be flexible, to let proteins move and to allow vesicles to form, but not so fluid that it leaks or falls apart. Two things set where it sits, and Figure 6 shows both.
Fatty acid composition. From B1.1: unsaturated fatty acids have kinks, pack loosely and have lower melting points; saturated fatty acids are straight, pack tightly and have higher melting points. In a membrane:
- phospholipids rich in unsaturated fatty acids keep the membrane fluid, and therefore flexible, at the temperatures the cell experiences, even cold ones;
- phospholipids rich in saturated fatty acids make the membrane stronger and more stable at higher temperatures, where an unsaturated membrane would become too fluid.
So organisms adjust the mix to their habitat. An example of adaptation: fish living in polar seas, at temperatures near 0 °C, have a higher proportion of unsaturated fatty acids in their membrane phospholipids than related fish from warm tropical waters, so their membranes stay fluid in the cold. Bacteria do the same within a lifetime: grown at a lower temperature, many species build membranes with a higher proportion of unsaturated fatty acids.
Cholesterol in animal cells. Cholesterol is a steroid (B1.1) and it sits between the phospholipids in each layer: its small –OH group lies among the phosphate heads, and its rings and short tail lie alongside the fatty acid tails. It acts as a modulator, an adjuster, of fluidity, and it works in opposite directions at the two extremes.
- At higher temperatures it restrains the movement of the phospholipid tails, stabilising the membrane so it does not become too fluid.
- At lower temperatures it gets in between the tails and stops them packing tightly together, preventing the membrane from stiffening.
The word the guide wants is modulator: cholesterol keeps fluidity within a working range, instead of simply raising or lowering it.
12HLFluidity, and making and fusing vesicles
A vesicle is a small sac of membrane with fluid inside. Because the bilayer is fluid, a patch of membrane can bend, pinch off as a vesicle, and later fuse with another membrane, and at each step the lipids flow to reseal it without a leak. Figure 7 shows the two directions.
Endocytosis takes material into the cell. The plasma membrane folds inwards around the material, the pocket deepens, and the membrane pinches off to form a vesicle inside the cell. Examples: a white blood cell engulfing a bacterium (phagocytosis, "cell eating"), and cells taking in droplets of tissue fluid (pinocytosis, "cell drinking").
Exocytosis releases material from the cell. A vesicle moves to the plasma membrane, the two membranes fuse, and the contents are released outside. Examples: pancreas cells secreting digestive enzymes; neurons releasing neurotransmitter into a synapse; and the secretion of hormones such as insulin.
Exocytosis also adds the vesicle's membrane to the plasma membrane, and endocytosis removes some, so the two together keep the cell surface in balance.
13HLGated ion channels in neurons
Neurons send signals by opening and closing ion channels, and the guide names two kinds of gate, both in Figure 8.
Neurotransmitter-gated channels open when a specific chemical binds. The example is the nicotinic acetylcholine receptor, found on muscle fibres and on some neurons where they receive signals at a synapse. When acetylcholine released by the neuron before the synapse binds to the receptor, the channel opens and positive ions, mainly Na⁺, flow in. The inside becomes less negative, which can start a new impulse. (It is called nicotinic because nicotine binds it too.)
Voltage-gated channels open when the voltage across the membrane changes. A resting neuron is more negative inside than outside. When part of the membrane becomes less negative (depolarised) past a threshold:
- voltage-gated sodium channels open, Na⁺ diffuses in, and the inside becomes positive;
- voltage-gated potassium channels open a fraction of a second later, K⁺ diffuses out, and the inside becomes negative again (repolarisation).
That sequence, repeated along the axon, is the nerve impulse (C2.2). Each channel is specific to its ion and opens only when its own signal arrives.
14HLThe sodium–potassium pump, an exchange transporter
The ion gradients that gated channels use have to be built first, and the sodium–potassium pump builds them. It is an exchange transporter: one protein moves two different ions in opposite directions, both against their gradients, using ATP. Figure 9 follows one cycle.
- The pump is open to the inside of the cell, and three Na⁺ ions from the cytoplasm bind to it.
- ATP is split and its phosphate group attaches to the pump, which changes shape.
- Now open to the outside, the pump releases the three Na⁺ outside and binds two K⁺ from outside.
- The phosphate is released, the pump returns to its original shape, and the two K⁺ are released inside.
Net, for each ATP: 3 Na⁺ out and 2 K⁺ in.
Why it matters for membrane potentials. The pump keeps Na⁺ concentrated outside and K⁺ concentrated inside. It also moves one more positive charge out than in on every cycle, which makes the inside slightly more negative directly. More important, the K⁺ it concentrates inside leaks back out through potassium channels, carrying positive charge out. Together these give the resting membrane potential of a neuron, about −70 mV inside compared with outside, and they keep the Na⁺ and K⁺ gradients ready for the next impulse. Neurons spend a large share of their ATP on these pumps.
15HLSodium-dependent glucose cotransporters: indirect active transport
Glucose sometimes has to be moved against its concentration gradient, and the cell does it without spending ATP on glucose directly. Indirect active transport uses the energy stored in one ion's gradient, made by a pump, to drive the movement of something else. Figure 10 shows it in the small intestine.
- On the side of the epithelium cell facing the blood, sodium–potassium pumps use ATP to pump Na⁺ out. This keeps the Na⁺ concentration low inside the cell.
- In the membrane facing the gut lumen, where Na⁺ is high, the sodium-dependent glucose cotransporter (SGLT) lets Na⁺ diffuse into the cell, down its gradient. It only works if it carries a glucose molecule in at the same time.
- So glucose is carried into the cell against its own concentration gradient, driven by the Na⁺ flowing down its gradient. The cotransporter uses no ATP itself; the ATP was spent by the pump. That is why it is called indirect active transport.
- Glucose then leaves the cell on the blood side by facilitated diffusion, and is carried away.
The guide names two places where it matters.
- Glucose absorption by epithelium cells of the small intestine, so that glucose from digested food is absorbed even when there is more of it in the cells than in the gut.
- Glucose reabsorption by cells of the proximal convoluted tubule of the nephron, so that the glucose filtered out of the blood in the kidney is taken back, and none is normally lost in urine.
16HLAdhesion of cells to form tissues
Cells in a tissue are held to each other by cell-adhesion molecules (CAMs), proteins in the plasma membrane that bind to CAMs on neighbouring cells or to the material between cells. There are different forms of CAM, and different forms are used for different types of cell–cell junction: some make tight seals that stop fluid leaking between cells, some make strong anchoring joins that resist stretching, as in skin and heart muscle. Many CAMs are glycoproteins. You do not need to name particular CAMs or junctions; you need the idea that adhesion between cells is made by specific membrane proteins, and that the type of CAM suits the type of join.
17Where marks are lost
"Water moves from high concentration to low concentration" in osmosis. Say solute: water moves from lower solute concentration to higher solute concentration. Leaving the word solute out makes the sentence ambiguous, and it scores nothing.
Saying osmosis happens through channel proteins "because water is polar". Water crosses the bilayer too, slowly. Aquaporins make it much faster; they are not the only route.
Saying facilitated diffusion needs energy. It is passive, like all diffusion. The protein provides a route, not energy.
"Active transport moves substances from high to low." Against the gradient: from lower to higher concentration. That is the whole point of it.
Calling simple diffusion selective. It depends only on size and polarity. Selectivity comes from proteins.
Comparing changes in mass in grams. Pieces start at different masses. Use percentage change.
Drawing the carbohydrate of a glycoprotein facing the cytoplasm. It is always on the outside of the plasma membrane.
(HL) "Cholesterol makes membranes less fluid." Only at higher temperatures. At lower temperatures it keeps them from stiffening. The word is modulates.
(HL) Writing the pump's ratio the wrong way round. Three sodium out, two potassium in, per ATP.
18Draw it right
The fluid mosaic model is the drawing the guide asks for by name. A top-mark drawing:
- Two rows of phospholipids, heads outward, tails meeting in the middle, drawn with a ruler-straight edge.
- At least one integral protein spanning the bilayer, drawn crossing both layers.
- One peripheral protein, on a surface, not in the core.
- A glycoprotein with a branched carbohydrate chain on the outside face; label "outside" or "extracellular".
- Cholesterol between phospholipids in one layer, small, among the tails.
- Hydrophilic and hydrophobic regions labelled, with a bracket or arrow to the right parts.
- Every label on a straight leader line that touches the structure it names.
- For transport drawings: show the concentration on each side (dots, or "high" and "low"), and the arrow's direction relative to that gradient. For a pump, write ATP.
19Try it
Marks in brackets. Answers and marker's notes are at the end.
Q1. Which substance crosses a phospholipid bilayer by simple diffusion? 1 mark
A. Glucose
B. Sodium ions
C. Oxygen
D. Haemoglobin
Q2. Cells were placed in solutions of two substances, A and B, at different concentrations, and the rate at which each was taken up was measured. The experiment for B was repeated with a poison that stops respiration. The data are invented. 7 marks
| External concentration / mmol dm⁻³ | 0 | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|---|
| Uptake of A / µmol g⁻¹ min⁻¹ | 0 | 1.1 | 2.2 | 3.2 | 4.3 | 5.4 |
| Uptake of B / µmol g⁻¹ min⁻¹ | 0 | 3.0 | 4.6 | 5.3 | 5.6 | 5.7 |
| Uptake of B with the poison / µmol g⁻¹ min⁻¹ | 0 | 3.0 | 4.5 | 5.3 | 5.6 | 5.6 |
(a) Calculate the percentage increase in the uptake of B when the external concentration rises from 2 to 10 mmol dm⁻³. 1 mark
(b) Identify, with a reason, which substance is taken up by simple diffusion. 2 marks
(c) Explain why the uptake of B levels off at higher concentrations. 2 marks
(d) Deduce, with a reason from the data, whether B is taken up by active transport. 2 marks
Q3. Draw a labelled diagram of the fluid mosaic model of a plasma membrane. 5 marks
Q4. Explain how water moves into a cell placed in pure water. 3 marks
Q5 (HL). Explain how glucose is absorbed into epithelium cells of the small intestine against its concentration gradient. 4 marks
Q6 (HL). Outline how cholesterol affects the fluidity of animal cell membranes. 3 marks
20In one breath
Phospholipids form bilayers in water on their own, and the hydrophobic core blocks ions, polar molecules and large molecules, so a membrane is a barrier between two watery solutions. Small non-polar molecules like O₂ and CO₂ cross between the phospholipids by simple diffusion, down their gradients, with no energy and no selectivity. Integral proteins sit in the bilayer, often across it; peripheral proteins sit on a surface. Osmosis is the net movement of water from lower to higher solute concentration across a membrane the solute cannot cross; aquaporins speed it up. Channel proteins let one kind of ion diffuse through when open (facilitated diffusion); pumps use ATP to move particles against their gradient (active transport); those proteins are what make membranes selective. Glycoproteins and glycolipids carry carbohydrate on the outside for recognition and adhesion. The fluid mosaic model: fluid lipids, proteins scattered like tiles. HL: unsaturated fatty acids keep membranes fluid in the cold, saturated ones strengthen them in the heat, and organisms adapt the mix to their habitat; cholesterol sits among the phospholipids and modulates fluidity both ways; fluidity lets membranes form vesicles and fuse, as in endocytosis and exocytosis. Acetylcholine opens the nicotinic receptor, voltage opens Na⁺ and K⁺ channels. The sodium–potassium pump moves 3 Na⁺ out and 2 K⁺ in per ATP and sets up the resting potential. SGLT lets Na⁺ flow in down the gradient the pump made and carries glucose in against its own, in the gut and the proximal convoluted tubule. CAMs of different forms hold cells together in different kinds of junction.
Answers
Q1. C. Oxygen is small and non-polar, so it passes between the phospholipids. Glucose is polar, sodium ions are charged, and haemoglobin is a very large protein. C only.
Q2. (a) (5.7 − 3.0) ÷ 3.0 × 100 = 90%. (b) A, because its rate of uptake rises in direct proportion to the external concentration (it roughly doubles when the concentration doubles) and does not level off, which is what happens when there is no protein involved. (c) B is taken up through membrane proteins, and there is a limited number of them. At higher concentrations all the proteins are in use at their maximum rate, so a further rise in concentration cannot raise the rate of uptake. (d) B is not taken up by active transport: stopping respiration, which stops ATP production, made almost no difference to its uptake (for example 5.6 against 5.7 at 10 mmol dm⁻³). It is taken up by facilitated diffusion. (a) A1 for 90%. (b) 1 for A, 1 for the reason about proportional increase with no plateau. (c) 1 for a limited number of channel or carrier proteins, 1 for all of them being occupied or saturated at high concentration. (d) 1 for "not active transport", 1 for the reason using the poison data, ideally with a quoted pair of values. A conclusion with no reference to the data scores 1.
Q3. Drawing as Figure 5. 1 each for any five of: phospholipid bilayer with heads outwards and tails inwards; integral protein spanning the bilayer; peripheral protein on one surface; glycoprotein with carbohydrate on the outer surface; cholesterol between the phospholipids; hydrophilic and hydrophobic regions correctly labelled. A single layer of phospholipids scores 0 for the first point.
Q4. Pure water has a lower solute concentration than the cytoplasm, which contains dissolved solutes. Water molecules move at random across the membrane in both directions, but the membrane is not permeable to the solutes inside the cell, so they stay in. More water molecules move into the cell than out, because inside, the solutes restrict the movement of water molecules, so there is a net movement of water into the cell by osmosis, partly through aquaporins. 1 for lower solute concentration outside or higher inside, 1 for the membrane being impermeable to the solutes or partially permeable, 1 for net movement of water in by osmosis. "Water moves from high to low concentration" with no mention of solute scores 0 for the first point.
Q5 (HL). Sodium–potassium pumps in the membrane facing the blood use ATP to pump Na⁺ out of the cell, keeping the Na⁺ concentration inside low. In the membrane facing the lumen, sodium-dependent glucose cotransporters let Na⁺ diffuse into the cell down its concentration gradient. Each carries glucose into the cell at the same time. So glucose moves in against its concentration gradient using the energy of the Na⁺ gradient; this is indirect active transport, because the cotransporter itself does not use ATP. 1 for Na⁺–K⁺ pumps using ATP to keep Na⁺ low inside, 1 for Na⁺ diffusing in through the cotransporter, 1 for glucose carried with it against its gradient, 1 for naming indirect active transport or explaining where the energy comes from.
Q6 (HL). Cholesterol molecules sit between the phospholipids in each layer of the membrane. At higher temperatures they restrict the movement of phospholipids, stabilising the membrane and stopping it becoming too fluid. At lower temperatures they prevent the fatty acid tails packing closely together, so the membrane does not become too rigid. Cholesterol therefore modulates fluidity, keeping it within a working range. 1 for position between phospholipids, 1 for the high-temperature effect, 1 for the low-temperature effect. "Cholesterol reduces fluidity" alone scores 1 at most.
Educerie · written from the published IB Diploma Programme Biology guide, first assessment 2025, section B2.1 Membranes and membrane transport. Original text, examples and questions. Diagrams drawn by Educerie. Last reviewed 25 September 2026.
Check your understanding
The main ideas of this note. Tick each one you could do now, in an exam, without looking back up. Anything you cannot tick yet is the part to read again.