Amphipathic phospholipids form a bilayer on their own, and its oily core keeps most things out. Proteins in the bilayer decide what crosses, how fast, and whether energy is spent. Fluidity, cholesterol and fatty acid mix let the membrane bend, bud and fuse.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank ·
Specialist review in progress
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Assessed in Paper 1A (multiple choice), Paper 1B (data-based) and Paper 2 (short and extended response). IB Biology guide (first assessment 2025, updated May 2026 for 2028).
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B2.1.1 Phospholipids form a bilayer by themselves
A phospholipid is glycerol, two fatty acid tails and a phosphate head.
It is amphipathic: the head is hydrophilic, the tails hydrophobic.
In water, heads face the water and tails hide inside, forming a continuous bilayer.
Weak hydrophobic interactions hold it together; there are no covalent bonds between lipids.
Students often think enzymes must build the bilayer using ATP. In fact it assembles itself; the arrangement is simply the most stable one in water.
Students often think a phospholipid is wholly hydrophobic, like oil. In fact the phosphate head attracts water; only the tails avoid it.
B2.1.2 The bilayer is a barrier
The hydrophobic core is made of hydrocarbon tails: no water, no charges.
It has low permeability to ions, polar molecules and large molecules.
Small non-polar molecules cross readily; water only slightly; ions almost not at all.
So the membrane separates the aqueous solutions on its two sides.
Students often think only size matters, so ions and glucose pass easily. In fact ions are charged and hydrophilic; the core excludes them however small.
Students often think everything must cross through a protein. In fact oxygen and carbon dioxide slip straight between the phospholipids.
B2.1.3 Simple diffusion: no protein, no energy
Simple diffusion is net movement from higher to lower concentration by random motion.
No transport protein and no ATP are involved.
Oxygen and carbon dioxide cross this way, passing between the phospholipids.
Students often think diffusion stops when concentrations are equal. In fact molecules keep crossing both ways; only the net movement is zero.
Students often think a cell pulls in oxygen because it needs it. In fact respiration keeps the inside concentration low, and that gradient drives diffusion.
B2.1.4 Integral and peripheral proteins
An integral protein is embedded in the bilayer, in one layer or spanning both.
Its embedded surface carries hydrophobic R-groups that sit among the tails.
A peripheral protein sits on one surface, attached to heads or to an integral protein.
Membrane proteins vary widely: channels, pumps, receptors, enzymes and more.
Students often think every integral protein spans the whole membrane. In fact some sit in one layer only; only those spanning both are transmembrane.
Students often think all membrane proteins move substances. In fact many are enzymes, receptors or adhesion molecules.
B2.1.5 Osmosis and aquaporins
Osmosis is net water movement across a membrane permeable to water but not solute.
Water moves from lower to higher solute concentration.
Water moves randomly both ways; the low-solute side has more free water, so more cross.
Aquaporins are channel proteins with a narrow pore for water only; they greatly speed osmosis.
Students often think water moves from high solute to low solute. In fact it moves the other way, towards the higher solute concentration.
Students often think the solute moves and water follows. In fact the membrane blocks the solute; that is why water moves instead.
B2.1.6 Channel proteins: facilitated diffusion
Facilitated diffusion is passive movement down a gradient through a channel or carrier protein.
No ATP is used, but each protein admits only a specific particle.
A channel protein has a pore; its width and lining charges pick which ion passes.
Many channels are gated: ions pass only when the channel is open.
Students often think any protein-assisted transport needs ATP. In fact ions move by their own random motion; the channel only provides a route.
Students often think a channel can push ions against a gradient when needed. In fact gating controls whether ions move, never the direction.
B2.1.7 Pump proteins: active transport
Active transport moves specific particles against their concentration gradient.
Energy comes from ATP hydrolysis, which changes the pump's shape.
A pump protein is integral; it can do what no form of diffusion can.
It stops when the cell runs out of ATP.
Students often think active transport is just faster diffusion. In fact it moves particles up a gradient, which diffusion never does.
Students often think pumps keep working if respiration stops. In fact they need a continuous ATP supply and soon halt.
B2.1.8 Which transport is selective
Selective permeability means some substances can cross the membrane while others cannot.
Selectivity comes from proteins: each channel, carrier or pump handles specific particles.
Simple diffusion is not selective; it depends only on size and polarity.
Students often think the membrane chooses what diffuses according to need. In fact simple diffusion is blind; only protein-mediated transport is selective.
B2.1.9 Glycoproteins and glycolipids
A glycoprotein is a protein, a glycolipid a lipid, each with a carbohydrate chain attached.
The carbohydrate is always on the extracellular side.
Roles: cell recognition (the sugar pattern identifies the cell) and cell adhesion (cells bind together).
Students often think carbohydrate chains face both ways. In fact they project only from the outer surface, so the two faces differ.
Students often think surface carbohydrate is an energy store. In fact it is for recognition and adhesion, not respiration.
B2.1.10 The fluid mosaic model
Fluid: phospholipids and proteins move sideways within the plane of the bilayer.
Mosaic: integral and peripheral proteins are scattered irregularly through and on it.
A drawing shows heads out, tails in, both protein types, glycoproteins outside, cholesterol between lipids.
Both surfaces are hydrophilic; only the core of tails is hydrophobic.
Students often think a fluid membrane has gaps that leak. In fact movement is sideways and the sheet stays continuous.
Students often think the cytoplasm-facing tails point into the cell. In fact tails point into the core; both surfaces are hydrophilic heads.
B2.1.11 Fatty acids and fluidity (HL) HL
Fluidity is how freely lipids and proteins move; it depends on temperature and composition.
Unsaturated tails have kinks, pack loosely, melt lower: the membrane stays fluid when cold.
Saturated tails are straight, pack tightly, melt higher: the membrane stays strong when hot.
Cold-water fish have more unsaturated fatty acids in their membranes than warm-water relatives.
Students often think saturated tails are the kinked ones. In fact double bonds make the kinks, and those belong to unsaturated tails.
Students often think cold-habitat organisms need more saturated fat to stay firm. In fact they need more unsaturated tails so membranes do not stiffen.
B2.1.12 Cholesterol adjusts fluidity (HL) HL
Cholesterol sits between phospholipids: hydroxyl by the heads, rigid rings among the tails.
At higher temperatures it restricts phospholipid movement and stabilises the membrane.
At lower temperatures it stops tails packing closely and prevents stiffening.
So it is a modulator of fluidity, not simply a stiffener.
Students often think cholesterol only makes membranes stiffer. In fact it works both ways, depending on temperature.
Students often think cholesterol sits on the membrane surface. In fact it is embedded among the phospholipids.
B2.1.13 Vesicles form and fuse because membranes are fluid (HL) HL
A vesicle is a small membrane-bound sac budded off from a larger membrane.
Endocytosis: membrane surrounds outside material and pinches off inwards; a phagocyte engulfs a bacterium.
Exocytosis: a vesicle fuses with the plasma membrane and releases contents; pancreatic enzymes, neurotransmitter.
Both use ATP; both need a fluid bilayer that can bend, pinch off and reseal.
Students often think secreted protein leaves through a pore. In fact its vesicle fuses with the membrane and empties outside.
Students often think vesicle transport is passive because no pump is involved. In fact both processes use ATP to reshape the membrane.
B2.1.14 Gated ion channels in neurons (HL) HL
A neurotransmitter-gated channel opens when a specific chemical binds to it.
The nicotinic acetylcholine receptor opens when two acetylcholine molecules bind; sodium ions diffuse in.
A voltage-gated channel opens on a change in membrane potential, not chemical binding.
Voltage-gated sodium and potassium channels open on depolarisation; ions diffuse, no ATP used.
Students often think the receptor lets acetylcholine into the neuron. In fact acetylcholine binds outside; sodium ions are what pass through.
Students often think voltage-gated channels pump ions with ATP. In fact they are passive; ions diffuse down their gradients.
B2.1.15 The sodium–potassium pump (HL) HL
Per ATP hydrolysed, it moves three sodium ions out and two potassium ions in.
Both ions are moved against their concentration gradients, so ATP is needed.
It is an exchange transporter: one particle one way, another the other way.
Unequal charge movement and the gradients it maintains generate the membrane potential.
Sodium binding and phosphorylation change the pump's shape; potassium binding and phosphate loss restore it.
Students often think the pump swaps one sodium for one potassium. In fact it is three out, two in, so net charge leaves.
Students often think sodium goes in and potassium out. In fact it is the reverse: sodium out, potassium in.
B2.1.16 Glucose cotransport: indirect active transport (HL) HL
A sodium-dependent glucose cotransporter binds a sodium ion and a glucose outside, moving both in.
Sodium moves down its gradient; that energy carries glucose against its gradient.
ATP is used only indirectly, by the sodium–potassium pump that keeps the sodium gradient.
These cotransporters absorb glucose in the small intestine and reabsorb it in the nephron.
Students often think moving glucose uphill must hydrolyse ATP directly. In fact the sodium gradient supplies the energy; ATP was spent earlier by the pump.
Students often think glucose is absorbed by simple diffusion until concentrations equalise. In fact cotransport moves it against its gradient.
B2.1.17 Cells stick together with CAMs (HL) HL
A cell-adhesion molecule (CAM) is a membrane protein that binds a neighbouring cell's CAM.
Bound CAMs hold cells together so they can form a tissue.
Different forms of CAM build different types of cell–cell junction.
Students often think neighbouring membranes fuse where cells touch. In fact CAMs in each membrane bind to each other; the membranes stay separate.
Students often think one universal CAM builds every junction. In fact different CAMs are used for different junction types.
Diagnostic a bearings check, not a test
10 questions, one per part of the topic where we can. Answer them, then see which statements you own and which to read.
1 Why do phospholipids form a bilayer when they are mixed with water?
Answer and reasoning
Hydrophilic heads face the water and hydrophobic tails are shielded from it, so the sheet forms spontaneously. — Phospholipids are amphipathic. In water the most stable arrangement is a continuous sheet two molecules thick with heads towards the water on both sides and tails in the middle, so the bilayer forms naturally with no energy input.
Enzymes in the cell join the phospholipids together into a sheet, using energy released from ATP. — A student who thinks membranes must be built like other cell structures picks this. In fact no enzyme or ATP is needed: amphipathic lipids form bilayers spontaneously in water.
The whole molecule repels water, so the phospholipids gather away from it as a droplet of oil. — A student who treats a phospholipid as an ordinary fat picks this. In fact the phosphate head is hydrophilic, so the molecules form a sheet with heads in contact with water rather than an oil droplet.
Covalent bonds form between neighbouring phospholipids, linking them into one rigid sheet of molecules. — A student who expects a membrane to be a covalent polymer picks this. In fact the phospholipids stay separate molecules held together by weak interactions, which is why the bilayer is fluid.
2 Which of these particles crosses a pure phospholipid bilayer, with no proteins present, least readily?
Answer and reasoning
A carbon dioxide molecule, which is larger than a water molecule — A student who ranks particles by size alone picks this. In fact carbon dioxide is non-polar, so it passes readily between the hydrophobic tails despite its size.
An oxygen molecule, which needs a protein to cross the bilayer — A student who thinks nothing crosses the bilayer without a protein picks this. In fact oxygen is small and non-polar and crosses by simple diffusion between the phospholipids.
A sodium ion, which is small but charged and hydrophilic — The hydrophobic core has very low permeability to hydrophilic particles, including ions and polar molecules. A charged sodium ion cannot enter the hydrocarbon core, so it has the lowest permeability of the four.
A water molecule, which is very small and also polar — A student who thinks water cannot cross a bilayer at all without aquaporins picks this. In fact water is small enough to pass slowly between the phospholipids; a charged ion is far less permeable than water.
3 Which statement about membrane proteins is correct?
Answer and reasoning
Integral proteins are embedded in one or both lipid layers, while peripheral proteins are attached to a surface. — This is the distinction the guide makes: integral proteins have part of their structure within the bilayer, in one or both lipid layers; peripheral proteins are attached to one or other surface of the bilayer.
Integral proteins all span the whole bilayer, while peripheral proteins are embedded in only one layer. — A student who equates 'integral' with 'transmembrane' picks this. In fact integral proteins may be embedded in only one layer, and peripheral proteins are not embedded at all.
Integral and peripheral proteins are both transport proteins, differing only in the substance they carry. — A student who thinks all membrane proteins transport substances picks this. In fact membrane proteins have diverse functions, including enzymes, receptors and adhesion molecules.
Glycoproteins carry their carbohydrate on both faces of the membrane, so the two surfaces are identical. — A student who pictures a symmetrical membrane picks this. In fact the carbohydrate of glycoproteins is found only on the extracellular side, so the two faces differ.
4 A potassium channel in a cell membrane is open. Which statement describes the movement of ions through it?
Answer and reasoning
Potassium ions and any other ions of similar size diffuse through the open pore. — A student who sees a channel as a hole picks this. In fact the pore's diameter and the charges lining it allow only potassium ions through, even excluding the smaller sodium ion.
Potassium ions are moved through in whichever direction the cell requires. — A student who thinks 'facilitated' means the cell can choose the direction picks this. In fact diffusion through a channel is passive and net movement is only down the concentration gradient.
Potassium ions are carried through using energy from the hydrolysis of ATP. — A student who thinks protein-mediated transport always needs ATP picks this. In fact channels are used for facilitated diffusion, which is passive; only pumps use ATP.
Potassium ions diffuse through, down their gradient, with no ATP used. — A channel provides a hydrophilic route through the hydrophobic core for one specific ion. The ions move by facilitated diffusion, down their concentration gradient, and no energy from ATP is needed.
5 Root cells absorb nitrate ions from soil water in which the nitrate concentration is much lower than in the cell. When the roots are deprived of oxygen, nitrate uptake falls sharply. What explains these observations?
Answer and reasoning
Nitrate is taken up by pumps using ATP from aerobic respiration, so without oxygen the supply of ATP falls and active transport slows. — Uptake against a concentration gradient requires active transport, which uses energy from ATP. ATP is regenerated by cell respiration, so depriving the roots of oxygen reduces ATP supply and the pumps slow down.
Nitrate is taken up by active transport in the same direction as diffusion but faster, and the lack of oxygen slows the diffusion. — A student who thinks active transport is just faster diffusion picks this. In fact nitrate is moving from a lower to a higher concentration, which diffusion cannot do in any direction; only pumps using ATP can.
Nitrate is taken up by pumps using ATP already present in the cell, so removing oxygen must have affected the channel proteins instead. — A student who does not connect the ATP used by pumps to respiration picks this. In fact the cell's ATP is regenerated continuously by respiration and is used up within seconds if respiration stops, so removing oxygen from an aerobic cell soon halts active transport.
Nitrate is taken up by facilitated diffusion through channels, which use ATP to move ions across against the gradient. — A student who thinks channels use ATP picks this. In fact facilitated diffusion is passive and cannot move ions against a gradient; that is the role of pumps.
6 Which statement about glycoproteins in the plasma membrane is correct?
Answer and reasoning
Their carbohydrate is on the cytoplasmic side and is respired when the cell lacks glucose. — A student who treats every carbohydrate as an energy store picks this. In fact membrane carbohydrates face outwards and serve in recognition and adhesion; they are not respired.
Their carbohydrate projects from both faces of the membrane so that either face can be recognised. — A student who pictures a symmetrical membrane picks this. In fact the carbohydrate is only on the extracellular face; the cytoplasmic face carries no carbohydrate.
Their carbohydrate is on the extracellular side and is used in cell recognition and adhesion. — Glycoproteins are proteins with a carbohydrate chain attached. The carbohydrate is located on the extracellular side of the membrane, where its roles are cell recognition and cell adhesion.
They are transport proteins whose carbohydrate forms the channel through which sugars enter the cell. — A student who assumes every membrane protein is a transporter, and reads 'glyco' as meaning it carries sugar, picks this. In fact the carbohydrate of a glycoprotein is a short chain attached to the outer face of the protein for recognition and adhesion; it forms no channel, and sugars cross through separate transport proteins.
7 Why does a lipid bilayer that contains a high proportion of unsaturated fatty acids remain fluid at low temperatures? HL
Answer and reasoning
The double bonds hold neighbouring tails rigidly together, giving the bilayer a fixed structure. — A student who reasons that a double bond makes the tails stiffer and more rigid picks this. In fact the double bond kinks the tail so the tails pack less closely, which lowers the melting point.
The kinked tails cannot pack closely, so the lipids have a lower melting point. — Each double bond puts a kink in the hydrocarbon chain, so neighbouring phospholipids cannot pack tightly. Lipids with unsaturated fatty acids therefore have lower melting points, and the membrane stays fluid and flexible at temperatures that would solidify a saturated bilayer.
The bilayer's fluidity does not depend on temperature, so its fatty acids make no difference. — A student who thinks fluidity is fixed picks this. In fact a bilayer stiffens as temperature falls, and the fatty acid composition determines the temperature at which this happens.
The unsaturated tails are straighter than saturated tails, so they slide past one another easily. — A student who has the two structures the wrong way round picks this. In fact saturated tails are the straight ones and pack closely; the double bonds of unsaturated tails kink them, and it is the kinks, not straightness, that keep the tails apart.
8 Which statement describes cholesterol in an animal cell membrane? HL
Answer and reasoning
It lies within the bilayer between phospholipids and acts solely to reduce membrane fluidity. — A student who knows cholesterol reduces fluidity at high temperatures but not the rest picks this. In fact at low temperatures it prevents the tails from packing and so stops the membrane stiffening.
It is attached to the outer surface of the bilayer and makes the membrane stiffer and stronger. — A student who pictures cholesterol as an extra on the surface picks this. In fact it is a mostly hydrophobic lipid embedded among the fatty acid tails, and its effect on fluidity depends on temperature.
It is attached to the outer surface of the bilayer and carries carbohydrate used in cell recognition. — A student who places cholesterol outside the bilayer and confuses it with a glycolipid picks this. In fact cholesterol carries no carbohydrate and sits within the bilayer.
It lies in the bilayer between phospholipids and adjusts fluidity in both directions. — Cholesterol is positioned between the phospholipids, hydroxyl group towards the heads and rings among the tails. It is a modulator of fluidity: it stabilizes the membrane at higher temperatures and prevents stiffening at lower temperatures.
9 Which statement about voltage-gated sodium channels in the membrane of a neuron is correct? HL
Answer and reasoning
They open in response to a change in membrane potential and allow sodium ions to diffuse down their gradient. — A voltage-gated channel responds to depolarization of the membrane, not to a chemical signal. When open it allows facilitated diffusion of sodium ions down their concentration gradient, using no ATP.
They open when acetylcholine binds to them and allow sodium ions to diffuse down their gradient. — A student who has merged the two types of gated channel picks this. In fact acetylcholine opens the nicotinic receptor; voltage-gated channels respond only to the membrane potential.
They open in response to a change in membrane potential and pump sodium ions in using ATP. — A student who thinks these channels need energy picks this. In fact they are channels for passive diffusion; ATP is used by the sodium–potassium pump, not by the channel.
They open whenever any small ion is present and allow it to pass through into the neuron. — A student who sees channels as unselective holes picks this. In fact the channel is specific for sodium ions and opens only in response to a change in membrane potential.
10 How do cells lining the nephron reabsorb glucose from the filtrate when the glucose concentration inside the cells is already higher than in the filtrate? HL
Answer and reasoning
Glucose diffuses in through the lipid bilayer down its own concentration gradient — A student who thinks glucose crosses membranes by simple diffusion picks this. In fact glucose is polar, and here it is moving from a lower to a higher concentration, which diffusion cannot do.
A cotransporter hydrolyses ATP directly to move glucose in against its gradient — A student who applies 'against the gradient means ATP' to the cotransporter itself picks this. In fact the cotransporter uses the sodium gradient; ATP is used only by the sodium–potassium pump.
Sodium–potassium pumps carry glucose into the cell together with the potassium — A student who is unsure what the pump transports picks this. In fact the pump moves only sodium and potassium ions; glucose is carried by a separate cotransporter.
A cotransporter moves glucose in with sodium ions moving down their gradient — Sodium-dependent glucose cotransporters bind a sodium ion and a glucose molecule together. Sodium moving down its concentration gradient, maintained by sodium–potassium pumps, provides the energy to move glucose into the cell against its gradient. This is indirect active transport.
Read the ones marked not yet in Learn, then Verify.
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16 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 A lipid is found to form a continuous sheet-like bilayer when shaken with water. Which property must the lipid have?
Answer and reasoning
It must be entirely hydrophobic, so that all of its molecules cluster together away from water. — A student who thinks lipids are simply water-repelling picks this. In fact an entirely hydrophobic lipid would separate as a droplet; a sheet needs a hydrophilic part that stays in contact with water.
It must be amphipathic, with a hydrophilic region and a hydrophobic region in each molecule. — Only amphipathic lipids form bilayers: the hydrophilic regions face the water on each side and the hydrophobic regions form the core. Phospholipids are the main example in cell membranes.
It must be able to form covalent bonds with neighbouring lipid molecules in the sheet. — A student who expects the sheet to be a covalent polymer picks this. In fact a bilayer is held together only by hydrophobic interactions and other weak forces.
It must be acted on by an enzyme that assembles the molecules into the bilayer. — A student who thinks bilayers must be built picks this. In fact the bilayer forms spontaneously; the test in the stem involves only lipid and water, with no enzyme.
2 A cell is respiring rapidly, so its internal oxygen concentration is low. How do oxygen molecules enter the cell?
Answer and reasoning
They diffuse through a channel protein that is specific for oxygen and opens when needed — A student who thinks every substance needs a protein picks this. In fact oxygen is small and non-polar and needs no channel; it crosses between the phospholipids.
They are moved in by a pump protein using ATP, because the cell needs oxygen urgently — A student who links proteins and urgency with energy use picks this. In fact oxygen enters by simple diffusion down its concentration gradient, with no protein and no ATP.
They move in and out randomly until the concentrations are equal, then stop moving — A student who thinks diffusion ends when concentrations equalise picks this. In fact molecules keep crossing in both directions; only the net movement becomes zero.
They pass between the phospholipids by simple diffusion down their gradient — Oxygen and carbon dioxide are the guide's examples of simple diffusion across membranes: the small non-polar molecules pass directly between the phospholipids, with net movement from higher to lower concentration.
3 An animal cell with a cytoplasmic solute concentration of 0.30 mol dm⁻³ is placed in a solution with a solute concentration of 0.10 mol dm⁻³. The membrane is impermeable to the solute. What happens?
Answer and reasoning
Water leaves the cell by osmosis, because water moves from the higher to the lower solute concentration. — A student who applies 'high to low' to the solute picks this. In fact net water movement is from the lower solute concentration (0.10) to the higher (0.30), so water enters the cell.
Net water movement is into the cell, because more free water molecules hit the membrane from the outside. — The solution outside has the lower solute concentration and so a higher proportion of free water molecules. Water molecules move randomly in both directions, but more cross inwards than outwards, so the net movement is into the cell.
Solute diffuses out of the cell until both sides reach 0.20 mol dm⁻³, and water then follows the solute out. — A student who treats osmosis as solute diffusion picks this. In fact the stem says the membrane is impermeable to the solute, so the solute cannot move; it is water that crosses.
No water crosses the membrane, because the cell membrane has no aquaporins to let water pass. — A student who thinks aquaporins are the only route for water picks this. In fact water can cross the bilayer slowly without them, so osmosis still occurs; aquaporins only increase its rate.
4 Some cells lining the kidney tubules have large numbers of aquaporins in their plasma membranes. What is the effect of these aquaporins?
Answer and reasoning
They pump water across the membrane against its gradient, using ATP energy. — A student who assumes any protein-mediated transport uses ATP picks this. In fact aquaporins are channels: water diffuses through them passively.
They allow water to cross the membrane, which is otherwise impossible for the bilayer. — A student who thinks water cannot cross a bilayer at all picks this. In fact water crosses slowly between phospholipids; aquaporins increase the rate rather than making it possible.
They greatly increase the rate at which water can cross the membrane by osmosis. — Aquaporins are channel proteins with a pore that water molecules, but not solutes, can pass through. They make the membrane much more permeable to water, so osmosis is rapid in cells that have many of them.
They let water move in the direction the cell needs, whatever the solute concentrations. — A student who thinks a channel can set the direction of movement picks this. In fact water still moves by osmosis, from lower to higher solute concentration; the aquaporin cannot reverse this.
5 Artificial vesicles were made from a pure phospholipid bilayer. Oxygen crossed the bilayer rapidly, glucose very slowly and potassium ions hardly at all. When a potassium channel protein was inserted into the bilayer, potassium crossed rapidly but permeability to sodium ions and to glucose did not change. Which conclusion is supported?
Answer and reasoning
The pure bilayer was selecting substances that a cell would need, letting oxygen in and keeping out potassium. — A student who thinks the membrane makes choices picks this. In fact the bilayer has no way to know what a cell needs; oxygen crossed because it is small and non-polar, potassium did not because it is an ion.
The selectivity of the membrane came from the channel protein, while the bilayer discriminated only by size and polarity. — Permeability by simple diffusion depends only on the size and hydrophilic or hydrophobic properties of particles. Adding a protein raised permeability to one specific ion and nothing else, which is selective permeability by facilitated diffusion.
The channel raised permeability to potassium because it is simply a small hole, so sodium ions should have crossed as well. — A student who sees the channel as an unselective hole picks this, but the data show sodium permeability unchanged. In fact the channel's structure admits only potassium ions.
The bilayer ranked substances by size, which is why the small potassium ion crossed faster than the larger glucose. — A student who thinks size is the only factor picks this, but the data show potassium crossing more slowly than glucose, not faster. In fact the charge on the ion makes it the least permeable of the three.
6 A student draws a two-dimensional representation of the fluid mosaic model. Which feature of the drawing is correct?
Answer and reasoning
The tails of the inner layer point into the cytoplasm, as the cytoplasm is the inside of the cell. — A student who confuses the interior of the bilayer with the interior of the cell picks this. In fact the cytoplasm is aqueous, so the inner layer's heads face it and both sets of tails meet in the middle.
The proteins are drawn at regular, equal spacings, like tiles set in a fixed pattern across the sheet. — A student who takes 'mosaic' to mean a fixed tiled pattern picks this. In fact the proteins are scattered irregularly and can move laterally.
Glycoproteins are drawn with their carbohydrate chains on the cytoplasmic side of the membrane. — A student who has not learnt that the two faces differ picks this. In fact the carbohydrate of a glycoprotein is drawn on the extracellular side only.
Cholesterol is drawn among the phospholipids within the bilayer, not on its surface. — Cholesterol is one of the components the drawing must include, and it is positioned within the bilayer among the phospholipids. The hydrophilic heads form both surfaces and the hydrophobic tails form the core.
7 Membrane proteins from a mouse cell and from a human cell were labelled with different coloured markers and the two cells were fused. After about 40 minutes the two colours were fully intermixed over the surface of the fused cell. What does this show?
Answer and reasoning
Proteins can move laterally through the bilayer, so the membrane is fluid and proteins are not held in a fixed pattern. — If the proteins had been fixed in place, each colour would have stayed on its own half. Their mixing shows that proteins move sideways through the fluid phospholipid bilayer, which is what 'fluid' means in the fluid mosaic model.
The membrane developed gaps as it became more fluid, allowing the labelled proteins to leak through them. — A student who thinks 'fluid' means leaky picks this. In fact the bilayer stays continuous; proteins mixed by moving sideways within the plane of the membrane, not through gaps in it.
The proteins from the two cells were covalently bonded into a single new continuous sheet during fusion. — A student who expects membrane components to be covalently linked picks this. In fact no bonds form between the proteins; they mix because the bilayer is fluid and they can diffuse through it.
The proteins remained in fixed positions but the mosaic pattern was rearranged by the fusion event. — A student who thinks 'mosaic' means a fixed arrangement picks this. In fact the proteins moved: fusion joins the membranes but does not move a protein from one half to the other.
8 In an investigation, fish of one species were kept at 5 °C or at 25 °C for several weeks. The membrane phospholipids of the fish kept at 5 °C contained a higher proportion of unsaturated fatty acids than those of the fish kept at 25 °C. What is the best explanation? HL
Answer and reasoning
At 5 °C unsaturated fatty acids are needed to make the membranes more rigid, so that they hold their shape firmly in the cold. — A student who has swapped the effects of the two types of fatty acid picks this. In fact unsaturated fatty acids lower the melting point and make membranes more fluid, not more rigid.
At 5 °C the extra unsaturated fatty acids insulate the cells against heat loss, as fat does in polar animals. — A student who thinks in terms of insulating fat picks this. In fact a membrane two molecules thick provides no insulation; the change in composition is about keeping the bilayer fluid.
At 5 °C membranes rich in saturated fatty acids would stiffen, so the fish increase unsaturated ones to stay fluid. — Unsaturated fatty acids have lower melting points, so a higher proportion keeps the membrane fluid and flexible at the low temperature. Organisms living in cold habitats show the same pattern as an adaptation: cold-water fish have a higher proportion of unsaturated fatty acids in their membranes than related warm-water fish.
At 5 °C the fish's membranes are already more fluid because of the cold, so unsaturated fatty acids are added to make them stronger. — A student who reverses the effect of temperature on fluidity picks this. In fact cold makes a bilayer less fluid, not more, and it is saturated fatty acids that add strength.
9 Two artificial membranes are made from the same phospholipids, one with cholesterol added and one without. Both are cooled to a temperature at which the membrane without cholesterol becomes stiff. What is expected for the membrane with cholesterol? HL
Answer and reasoning
It remains more fluid than the membrane without cholesterol, because cholesterol prevents the tails from packing closely. — At lower temperatures cholesterol, sitting between the phospholipids, keeps the fatty acid tails from packing together, so it prevents the membrane from stiffening. This is the low-temperature half of its role as a modulator of fluidity.
It becomes stiffer than the membrane without cholesterol, because cholesterol reduces fluidity at any temperature. — A student who thinks cholesterol always reduces fluidity picks this. In fact its effect depends on temperature: it prevents stiffening in the cold and reduces fluidity only at higher temperatures.
It is unchanged, because cholesterol on the membrane surface cannot affect the packing of the tails inside. — A student who places cholesterol on the surface picks this. In fact cholesterol lies among the tails, which is exactly why it can change how closely they pack.
It is unchanged, because a membrane's fluidity depends only on its fatty acids and not on temperature. — A student who thinks fluidity is fixed picks this, but the stem itself says cooling stiffened the cholesterol-free membrane. In fact temperature, fatty acids and cholesterol all affect fluidity.
10 A phagocytic white blood cell engulfs a bacterium, enclosing it in a vesicle inside the cell. Which statement about this process is correct? HL
Answer and reasoning
It is endocytosis, in which the membrane is deformed and pinched off without needing any ATP. — A student who thinks only pump-based transport is active picks this. In fact endocytosis requires ATP to reshape the membrane and move the vesicle.
It is endocytosis, which depends on the membrane being fluid enough to bend, pinch off and re-seal. — Engulfing a bacterium is an example of endocytosis. The phospholipid bilayer must be fluid so that it can surround the bacterium, pinch off as a vesicle and re-form a continuous plasma membrane; the process uses ATP.
It is endocytosis, in which the bacterium is pulled in through a large channel protein into a vesicle. — A student who fits all transport into the channel picture picks this. In fact nothing passes through the bilayer: the membrane wraps around the bacterium and forms the vesicle.
It is exocytosis, in which a section of rigid membrane breaks away cleanly around the bacterium. — A student who sees vesicle formation as a rigid piece snapping off picks this, and has also reversed the terms. In fact material entering the cell is endocytosis, and it needs a fluid, not a rigid, membrane.
11 At a synapse, neurotransmitter stored in vesicles is released into the synaptic cleft. How does the neurotransmitter leave the presynaptic neuron? HL
Answer and reasoning
The vesicle bursts inside the neuron and the neurotransmitter diffuses out through the lipid bilayer. — A student who pictures 'release' as bursting picks this. In fact the vesicle fuses with the plasma membrane and its contents are released outside without crossing a bilayer.
The vesicle docks at a channel protein through which the neurotransmitter is passed out of the cell. — A student who expects all export to go through a protein picks this. In fact exocytosis involves fusion of two fluid membranes, not passage through a channel.
The vesicle membrane fuses with the plasma membrane by exocytosis, releasing the contents outside. — Neurotransmitter release is an example of exocytosis: the vesicle moves to the plasma membrane, the two fluid bilayers merge and the neurotransmitter is released into the synaptic cleft. The process requires ATP.
The vesicle fuses with the plasma membrane, a passive process since no pump protein is involved. — A student who thinks 'no pump' means 'passive' picks this. In fact exocytosis requires ATP to move the vesicle and bring about fusion, so it is an active process.
12 Acetylcholine released at a synapse binds to nicotinic acetylcholine receptors on the postsynaptic membrane. What happens as a result? HL
Answer and reasoning
The receptor opens and the acetylcholine molecules pass through it into the postsynaptic neuron. — A student who thinks the neurotransmitter travels through the channel picks this. In fact acetylcholine stays bound on the outside; it is sodium ions that pass through the open pore.
The receptor uses ATP to pump sodium ions into the postsynaptic neuron, depolarizing it. — A student who thinks ion movement in neurons is powered by ATP picks this. In fact the receptor is a channel, not a pump: sodium ions diffuse in down their gradient.
The receptor stays closed until the membrane potential changes, then it opens. — A student who has merged the two kinds of gated channel picks this. In fact the nicotinic receptor is opened by acetylcholine binding; it is the voltage-gated channels that respond to the change in potential.
The receptor's channel opens and sodium ions diffuse into the neuron down their gradient. — The nicotinic acetylcholine receptor is a neurotransmitter-gated ion channel. Binding of acetylcholine changes its shape and opens a pore, and sodium ions diffuse into the cell down their concentration gradient, depolarizing the membrane.
13 Which sequence correctly describes one cycle of the sodium–potassium pump? HL
Answer and reasoning
Three potassium ions bind inside; ATP phosphorylates the pump; they are released outside; two sodium ions then bind and are released inside. — A student who has the directions reversed picks this. In fact it is sodium ions that bind inside and are released outside, and potassium ions that bind outside and are released inside.
Three sodium ions bind inside; ATP phosphorylates the pump; they are released outside; two potassium ions bind and are released inside. — Sodium ions bind on the cytoplasmic side, ATP phosphorylates the pump and changes its shape so the sodium is released outside; potassium ions then bind, the phosphate is lost and the pump returns to its original shape, releasing potassium inside. The exchange is 3 sodium out for 2 potassium in.
One sodium ion binds inside; ATP phosphorylates the pump; it is released outside; one potassium ion binds and is released inside. — A student who takes 'exchange' to mean a one-for-one swap picks this. In fact the ratio is three sodium ions out to two potassium ions in, which is why the pump moves net charge.
Three sodium ions bind inside; the pump opens when the membrane potential changes; they diffuse out; two potassium ions diffuse in. — A student who confuses the pump with a voltage-gated channel picks this. In fact the pump is driven by ATP hydrolysis, not by a change in potential, and it moves both ions against their gradients.
14 Epithelial cells from the small intestine were treated with a substance that inhibits sodium–potassium pumps. Uptake of glucose from the lumen by the cells fell sharply, although the sodium-dependent glucose cotransporters themselves were not affected by the substance. What explains this result? HL
Answer and reasoning
The inhibitor stopped the cotransporters hydrolysing ATP, which they need to move glucose against its gradient. — A student who thinks the cotransporter uses ATP directly picks this, but the stem says the cotransporters were unaffected. In fact they never hydrolyse ATP; they use the sodium gradient.
Glucose is normally absorbed by simple diffusion, and the inhibitor made the lipid bilayer impermeable to it. — A student who thinks glucose diffuses through the bilayer picks this. In fact glucose is polar and cannot cross the bilayer; it needs the cotransporter, and the bilayer was not changed.
Without the pumps, sodium accumulated in the cells, so there was no sodium gradient to drive glucose uptake. — The cotransporter moves glucose against its gradient only because sodium ions flow in down theirs. Sodium–potassium pumps maintain that gradient by using ATP to remove sodium from the cell, so inhibiting the pumps removes the gradient and glucose uptake fails. This is why the transport is called indirect active transport.
The pumps normally carry glucose into the cell, so inhibiting them removed the main route for glucose. — A student who has not learnt what the pump carries picks this. In fact the pump moves only sodium and potassium ions; glucose enters through the cotransporter, which depends on the pump indirectly.
15 How do adjacent cells adhere to one another to form a tissue? HL
Answer and reasoning
Cell-adhesion molecules in each cell's membrane bind to those on the neighbouring cell, with different CAMs used for different junctions. — Adhesion is due to CAMs, membrane proteins that bind to matching CAMs on the adjacent cell to form cell–cell junctions. Different forms of CAM are used for different types of junction, giving tissues junctions with different properties.
The fluid plasma membranes of the two cells fuse where they touch, so the two cells share one continuous membrane. — A student who extends membrane fusion from vesicles to whole cells picks this. In fact the membranes of adjacent cells remain separate and are linked by proteins.
A single universal cell-adhesion molecule links every cell to its neighbours in every type of tissue in the body. — A student who takes 'CAM' to be one molecule picks this. In fact there are different forms of CAM, and different forms are used for the different types of cell–cell junction.
The carbohydrate chains of glycoproteins on one cell are covalently bonded to the lipid bilayer of the adjacent cell. — A student who reaches for covalent bonds to explain any strong attachment picks this. In fact adhesion depends on non-covalent binding between CAM proteins on the two cells.
16 A neuron is treated with a drug that inhibits sodium–potassium pumps. Over the following hours its resting membrane potential declines steadily towards zero. Why does inhibiting the pump have this effect? HL
Answer and reasoning
The drug must also be blocking ion channels, because the pump exchanges one sodium ion for one potassium ion and so cannot affect the potential. — A student who takes 'exchange' to mean a one-for-one swap picks this. In fact the exchange is 3:2, so the pump moves net positive charge out of the cell, and the gradients it maintains are what the potential depends on.
The pump normally exports three sodium ions for every two potassium ions imported, so its net export of positive charge and the ion gradients it maintains are lost. — Sodium–potassium pumps are important in generating membrane potentials: each cycle moves three positive ions out and two in, and the sodium and potassium gradients this builds are what the resting potential depends on. Without the pump the gradients run down and the potential decays.
The pump normally moves sodium ions in and potassium ions out, so without it sodium ions no longer enter and the inside of the cell loses positive charge. — A student who has the directions reversed picks this. In fact the pump moves sodium out and potassium in; without it sodium leaks in and potassium leaks out, and the potential collapses.
The pump normally opens in response to a change in membrane potential, so without it the voltage-gated channels cannot restore the resting state after an impulse. — A student who confuses the pump with a voltage-gated channel picks this. In fact the pump is driven by ATP hydrolysis, not by a change in potential, and it maintains the gradients that the channels rely on.
That was your twenty minutes. Real practice on B2.1 is past-paper questions marked against the mark scheme.
What the exam asks of B2.1
Paper 1A asks you to name the transport method from a description, or to read a diagram of the fluid mosaic model. Paper 1B may give rates of uptake against concentration or with respiration blocked, and ask what the shape shows about active or passive transport. Paper 2 uses *draw* for the membrane model, *distinguish* for simple and facilitated diffusion or channels and pumps, and *explain* for osmosis, where the answer must mention random movement, solute impermeability and the solute difference. At HL, *explain* the sodium–potassium pump or glucose cotransport step by step, naming the gradient and where the ATP is spent.
Compiled from the IB Biology guide (first assessment 2025, updated May 2026 for 2028) and our question bank · Specialist review in progress. How these pages are made ·
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