IB Biology · Theme C Interaction and interdependence · Cells
C2.1 Chemical signalling HL only
Cells communicate with chemicals called ligands, which bind receptor proteins and change their shape. Hydrophilic ligands stay outside and signal through transmembrane receptors; hydrophobic ones enter and act on genes. Receptor activation starts a transduction pathway inside the cell, and feedback regulates the whole system.
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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C2.1.1 Receptors are proteins that bind one specific ligand HL
A receptor is a protein with a binding site complementary to one signalling chemical.
The signalling chemical is the ligand; hormones, neurotransmitters and cytokines are all ligands.
Binding is reversible and changes the receptor's shape; that change starts the response.
A cell responds only if it has a receptor for that ligand.
Students often think a hormone acts wherever blood carries it. In fact only cells with the matching receptor respond.
Students often think the binding site converts the ligand like an active site. In fact the ligand leaves unchanged; the receptor's shape change is the signal.
C2.1.2 Bacteria count themselves with a shared secreted signal HL
In quorum sensing, each bacterium secretes an autoinducer into its surroundings.
Its concentration rises with population density.
Past a threshold, it binds receptors and switches on gene expression across the population.
Vibrio fischeri glows only at high density, in the light organ of the Hawaiian bobtail squid.
There the autoinducer promotes transcription of the lux genes, which encode luciferase, the light-producing enzyme.
Students often think the bacteria respond to darkness or to the squid. In fact they respond to the concentration of their own secreted signal.
Students often think signalling needs a nervous or endocrine system. In fact a bacterial population coordinates gene expression through a shared chemical.
C2.1.3 Four functional categories of animal signalling chemicals HL
Hormones: secreted into blood by endocrine cells; act on distant targets over minutes to hours.
Neurotransmitters: released across a synaptic gap; local, fast and short-lived.
Cytokines: small proteins, mostly from immune cells; act on nearby cells.
Calcium ions: kept low in cytoplasm; channels open, Ca²⁺ floods in and triggers fast responses.
These are categories of function, not chemistry: noradrenaline is a neurotransmitter at synapses and a hormone in blood.
Students often think a substance must be either hormone or neurotransmitter. In fact the same chemical can be both, depending on route.
Students often think calcium ions are released by a gland into blood. In fact they enter the cytoplasm through channels and act within the cell.
C2.1.4 Signalling chemicals are chemically diverse, and that is useful HL
Amine hormones derive from one amino acid: epinephrine and noradrenaline from tyrosine.
Protein hormones are amino acid chains: insulin has 51 amino acids in two chains.
Steroid hormones are lipids from cholesterol: oestradiol, progesterone, testosterone.
Neurotransmitters include amino acids, peptides, amines and the gas nitric oxide (NO). The IB guide prints this as "nitrous oxide"; the signalling gas is nitric oxide.
Distinct chemicals with distinct receptors let many messages share one bloodstream without cross-talk.
Students often think all hormones are proteins. In fact amines, proteins and steroids are all hormone groups.
Students often think epinephrine is a steroid because the adrenal gland makes it. In fact it is an amine from tyrosine.
C2.1.5 Hormones act far away; neurotransmitters act next door HL
Hormones travel in blood to every tissue; several distant organs can respond at once.
The effect develops over seconds to hours and lasts until liver or kidneys remove the hormone.
A neurotransmitter diffuses across a synaptic gap about 20 nm wide in under a millisecond.
Its effect is confined to one postsynaptic cell and ended by breakdown or reuptake.
Students often think neurotransmitters travel in blood. In fact they diffuse across a tiny gap and never enter blood.
Students often think blood delivers a hormone to one target organ. In fact it reaches everywhere; only receptor-bearing cells respond.
C2.1.6 Surface receptors for ligands that stay out; internal receptors for ligands that get in HL
A transmembrane receptor spans the plasma membrane; its ligand stays outside.
Its membrane-spanning region is hydrophobic; the outer and inner parts are mainly hydrophilic.
An intracellular receptor sits in cytoplasm or nucleus, with a hydrophilic surface and hydrophobic pocket.
Hydrophobic ligands such as steroids cross the bilayer to reach it.
Students often think a transmembrane receptor is all hydrophobic. In fact only the membrane-spanning part is; the rest is hydrophilic.
Students often think insulin and epinephrine enter the cell. In fact they are hydrophilic and bind outside; only the signal goes in.
C2.1.7 Receptor activation starts a chain of events inside the cell HL
Signal transduction is the sequence of responses set off when a ligand binds.
The receptor's shape change activates proteins, makes second messengers and phosphorylates enzymes.
It ends in a response: an enzyme activated, a channel opened, a gene transcribed.
Each activated component acts on many of the next, so the signal is amplified.
Students often think the ligand does the work inside the cell. In fact the ligand usually stays outside; the pathway does the work.
Students often think one ligand molecule gives one unit of response. In fact amplification means a few molecules give a large response.
C2.1.8 Neurotransmitter receptors open channels and change the voltage HL
The acetylcholine receptor is a transmembrane receptor that is itself an ion channel.
Two acetylcholine molecules bind outside; the channel opens; positive ions (mainly Na⁺) diffuse in.
The inside becomes less negative: depolarisation of the membrane potential.
If threshold is reached, an action potential or muscle contraction may follow.
Students often think acetylcholine passes through the channel. In fact it stays outside; ions pass through.
Students often think opening channels makes the inside more negative. In fact positive ions enter and the inside becomes less negative.
C2.1.9 Some receptors pass the signal to a G protein HL
A G protein-coupled receptor has seven membrane-spanning helices.
Ligand binding outside changes its shape; its cytoplasmic part activates a G protein.
The G protein swaps GDP for GTP, switches on an effector such as adenylyl cyclase.
It hydrolyses GTP back to GDP soon after, so the signal is not left on.
Humans have many hundreds of GPCRs, responding to hormones, neurotransmitters, odours and light; many drugs target them.
Students often think the G protein is the receptor. In fact they are separate proteins; the ligand never reaches the G protein.
Students often think every transmembrane receptor works via a G protein. In fact ion channels and tyrosine kinase receptors work differently.
C2.1.10 Epinephrine acts through a G protein and cAMP HL
Epinephrine (adrenaline) is an amine hormone from the adrenal glands.
It binds a GPCR on cells such as liver cells and stays outside.
The activated G protein switches on adenylyl cyclase, which makes cAMP from ATP.
cAMP is the second messenger: it activates kinases that switch on glycogen breakdown.
Both names mean "by the kidney": Latin ad ren, Greek epi nephros. Both persist in different regions.
Students often think the second messenger is another hormone. In fact it is a small molecule made inside the cell.
Students often think the G protein makes cAMP. In fact adenylyl cyclase does, once the G protein switches it on.
C2.1.11 Insulin's receptor phosphorylates itself, and glucose transporters arrive HL
A tyrosine kinase receptor has a cytoplasmic domain that is a kinase.
Insulin binds outside; the receptor phosphorylates tyrosine residues on its own inner domain.
A sequence of reactions follows.
Vesicles holding glucose transporters move to and fuse with the plasma membrane.
More transporters mean faster glucose uptake from the blood.
Students often think insulin carries glucose into the cell. In fact insulin stays outside; transporters added to the membrane let glucose in.
Students often think new transporter genes are switched on. In fact pre-formed transporters in vesicles are moved to the membrane within minutes.
C2.1.12 Steroid hormones enter the cell and switch on genes HL
Oestradiol, progesterone and testosterone diffuse through the plasma membrane.
Each binds a site on an intracellular receptor in cytoplasm or nucleus, activating it.
The hormone-receptor complex binds specific DNA sequences and promotes transcription.
The response is new proteins, so it takes minutes to hours.
Students often think the steroid binds DNA itself. In fact the activated hormone-receptor complex does.
Students often think steroids act fast because they enter directly. In fact transcription and translation make them slower than epinephrine.
C2.1.13 What oestradiol does in the hypothalamus and progesterone in the endometrium HL
Oestradiol acts on hypothalamic cells that secrete GnRH.
At moderate levels it inhibits GnRH secretion.
At high sustained levels it stimulates GnRH, driving the LH surge.
Progesterone, after ovulation, maintains and thickens the endometrium, stimulating glands and blood supply.
When progesterone falls, the endometrium can no longer be maintained and is shed.
Students often think oestradiol only inhibits GnRH. In fact a high sustained level switches it to stimulation.
Students often think progesterone causes menstruation. In fact it maintains the endometrium; its fall causes shedding.
C2.1.14 Negative feedback opposes change; positive feedback amplifies it HL
Negative feedback: the response reduces the stimulus, returning the system towards a set level.
Example: oestradiol inhibits the GnRH-secreting cells, limiting its own production.
Positive feedback: the response increases the stimulus until an end point.
Students often think negative means harmful and positive means good. In fact the words describe direction of effect on the stimulus.
Students often think negative feedback only lowers things. In fact it opposes change either way, raising what has fallen too.
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 Which statement correctly describes the relationship between a ligand and its receptor? HL
Answer and reasoning
The ligand binds reversibly to a binding site on the receptor protein that is complementary to it, and the receptor changes shape. — A receptor is a protein with a binding site specific to its ligand. Binding is reversible and does not alter the ligand; the conformational change in the receptor is what initiates the cell's response.
The receptor's binding site converts the ligand into a product, in the way an enzyme's active site converts a substrate. — A student who transfers the enzyme model to receptors picks this. A receptor does not catalyse a change in its ligand; the ligand dissociates unchanged and it is the receptor that changes shape.
Any ligand reaching a cell binds to it, because every cell carries receptors for every signalling chemical in the body. — A student who thinks arrival of a chemical is what causes a response picks this. Cells carry receptors only for particular ligands, so a signalling chemical acts only on cells with a complementary binding site.
The ligand passes through a channel in the receptor into the cytoplasm, where it brings about the response directly. — A student who pictures the signalling chemical doing the job itself picks this. Binding to the receptor sets off a sequence of responses inside the cell; the ligand itself usually never enters.
Bacteria counting neighbouring cells through direct contact between the surface proteins of their cell walls. — A student who expects a head-count to need contact picks this. Density is sensed indirectly, through the concentration of a chemical that every cell secretes into the surroundings.
Bacteria of one species each responding on their own to the concentration of nutrients in their environment. — A student who thinks unicellular organisms do not signal to each other picks this. Quorum sensing is a response to a chemical made by the bacteria, coordinated across the population.
Bacteria secreting a chemical that itself gives out light once enough of it has built up around the cells. — A student who thinks the signalling chemical performs the response picks this. The autoinducer is a ligand: it binds a receptor that promotes transcription of the lux genes, and the enzyme luciferase makes the light.
A change in gene expression across a population when a secreted signal reaches a threshold concentration. — Each cell secretes an autoinducer; its concentration rises with population density, and above a threshold it binds receptors and alters gene expression in all the cells, as in bioluminescence in Vibrio fischeri.
3 Which statement about the functional categories of signalling chemicals in animals is correct? HL
Answer and reasoning
Cytokines are small proteins, secreted mainly by cells of the immune system, that act mostly on nearby cells. — Cytokines are produced by ordinary cells rather than endocrine glands and coordinate local responses, especially among immune cells; this local action distinguishes them from hormones.
Calcium ions are secreted by an endocrine gland into the blood and carried to distant target organs. — A student who pictures calcium as a substance carried in blood picks this. Calcium ions signal within cells: a rise in cytoplasmic Ca2+ when channels open triggers responses such as contraction.
Cytokines are secreted by endocrine glands into the blood, from which they act on distant organs. — A student who files every new chemical messenger as a hormone picks this. Cytokines come from cells such as immune cells, not glands, and act mainly locally.
Neurotransmitters are carried in the blood from a neuron to the target cells of distant organs. — A student who thinks all chemical messengers travel in blood picks this. A neurotransmitter diffuses across a synaptic gap of about 20 nm to the adjacent cell and acts locally.
4 Chemical X is released from one cell, diffuses about 20 nm and acts on a single adjacent cell within a millisecond before being broken down. Chemical Y is released into the blood and, over several minutes, acts on cells in the liver, heart and skeletal muscle. Which statement is correct? HL
Answer and reasoning
X is a neurotransmitter acting locally across a synaptic gap, and Y is a hormone acting at a distance after transport in the blood. — The 20 nm diffusion distance, millisecond timing and single target cell are the marks of a neurotransmitter; transport in the blood and effects on cells in several organs over minutes are the marks of a hormone.
Both are hormones, because a chemical released from a cell to act on another cell has to travel through the blood. — A student who thinks every chemical messenger uses the blood picks this. X crosses a synaptic gap by diffusion and never enters the blood.
Y is a neurotransmitter, because a hormone would be delivered by the blood to a single target organ rather than to several. — A student who thinks the blood routes a hormone to a single organ picks this. The blood distributes a hormone everywhere, so acting on several organs is typical of a hormone, not evidence against it.
X acts faster because it enters the target cell and carries out the response itself, whereas Y has to bind a receptor. — A student who pictures the signalling chemical doing the job itself picks this. X binds a receptor on the adjacent cell; its speed comes from the tiny distance it diffuses, not from entering the cell.
5 Insulin is a hydrophilic protein and oestradiol is a hydrophobic steroid. Where are their receptors, and do the hormones enter their target cells? HL
Answer and reasoning
Both stay outside and bind receptors on the cell surface, because all receptors are proteins embedded in the plasma membrane. — A student who has only seen surface receptors picks this. Steroid hormones penetrate the cell and bind intracellular receptors, which then act on DNA.
Insulin stays outside and binds a transmembrane receptor; oestradiol passes through the membrane and binds an intracellular receptor. — A hydrophilic protein cannot cross the phospholipid bilayer, so its receptor spans the plasma membrane and the ligand remains outside. A hydrophobic steroid dissolves through the bilayer to reach a receptor in the cytoplasm or nucleus.
Both enter the cell, because a signalling chemical has to reach the cytoplasm in order to bring about any response at all. — A student who believes hormones enter cells to act picks this. Insulin, being a hydrophilic protein, cannot cross the membrane; its signal is passed inside by the receptor.
Insulin stays outside and binds a transmembrane receptor; oestradiol is carried through the membrane by a transport protein to a receptor inside. — A student who thinks hydrophobic molecules are excluded by the membrane picks this. Steroids dissolve through the hydrophobic core of the bilayer by simple diffusion; no carrier is needed to reach the intracellular receptor.
6 Acetylcholine released at a neuromuscular junction binds to acetylcholine receptors in the membrane of a muscle fibre. What happens next? HL
Answer and reasoning
An ion channel in the receptor opens, positive ions diffuse into the fibre and the membrane potential becomes less negative. — The acetylcholine receptor is a ligand-gated ion channel. Binding opens the channel, Na+ diffuses in down its concentration gradient, and the resulting depolarization may trigger contraction.
Acetylcholine passes through the channel in the receptor, carrying its positive charge into the fibre with it. — A student who thinks the channel is for the ligand picks this. Acetylcholine stays bound on the outside; it is Na+ that diffuses through the opened channel.
An ion channel in the receptor opens, positively charged ions leave the fibre and the inside becomes more negative. — A student who has the direction of ion movement or the sign convention reversed picks this. Na+ is concentrated outside, so it enters, and inflow of positive charge depolarizes the membrane.
Acetylcholine enters the fibre and binds directly to the contractile proteins, causing them to shorten at once. — A student who thinks the signalling chemical performs the response itself picks this. Contraction follows a change in membrane potential set off by the receptor; acetylcholine never enters the fibre.
7 How does a G protein-coupled receptor convey a signal into a cell? HL
Answer and reasoning
The ligand passes through a channel in the receptor and binds directly to the G protein, which lies in the cytoplasm just beneath the membrane. — A student who thinks the ligand travels through the receptor to its target picks this. Nothing passes through a GPCR; the signal crosses as a change in protein shape.
The G protein is released from the membrane into the blood as a second messenger that carries the signal to neighbouring cells of the tissue. — A student who reads 'second messenger' as another circulating hormone picks this. The G protein stays on the inner face of the membrane, and second messengers such as cAMP act within the cell.
Binding of the ligand outside changes the receptor's shape, activating a G protein on the inner face, which then activates an effector. — The receptor spans the membrane; the conformational change caused by binding is transmitted to its cytoplasmic part, which activates the G protein. The activated G protein switches on an effector such as adenylyl cyclase. Humans have many hundreds of such receptors.
The activated G protein converts ATP into cAMP directly, and the cAMP then brings about the response inside the cell. — A student who collapses the pathway picks this. The G protein activates the enzyme adenylyl cyclase, and it is this enzyme that converts ATP into cAMP.
8 Which sequence correctly describes the events after epinephrine (adrenaline) binds to its receptor on a liver cell? HL
Answer and reasoning
A G protein is activated, adenylyl cyclase converts ATP to cAMP, and cAMP activates kinases that switch on glycogen breakdown. — The epinephrine receptor is a G protein-coupled receptor. The activated G protein switches on adenylyl cyclase, the second messenger cAMP activates protein kinases, and the phosphorylated enzymes break down glycogen to release glucose.
Epinephrine enters the liver cell and directly activates the enzymes that break down glycogen into glucose. — A student who thinks hormones enter their target cells picks this. Epinephrine is hydrophilic and stays outside; the message is carried inside by the G protein and cAMP.
cAMP is released from the liver cell into the blood as a second messenger that acts on other liver cells. — A student who reads 'second messenger' as a second hormone picks this. cAMP is made and acts within the cell; it is called a second messenger because it relays the hormone's message inside the cell.
The activated G protein converts ATP into cAMP, and the cAMP then breaks glycogen down directly into glucose. — A student who collapses the pathway picks this. Adenylyl cyclase makes cAMP, and cAMP acts by activating kinases; it does not hydrolyse glycogen itself.
9 How does a steroid hormone such as testosterone change the expression of genes in a target cell? HL
Answer and reasoning
It diffuses through the membrane into the nucleus and binds directly to the DNA at the start of the genes it switches on. — A student who leaves out the receptor picks this. The hormone has no DNA-binding ability; it is the activated receptor protein that recognises specific DNA sequences.
It binds a receptor in the plasma membrane, and this receptor then detaches and travels to the nucleus to bind the DNA. — A student who places every receptor on the cell surface picks this. Steroid receptors are intracellular, in the cytoplasm or nucleus, and the hormone enters the cell to reach them.
It is pumped across the membrane by a transport protein, since steroids cannot cross the bilayer, and then activates a receptor inside. — A student who thinks hydrophobic molecules are excluded by the membrane picks this. Steroids dissolve through the hydrophobic core of the bilayer by simple diffusion; no transport protein is needed.
It enters the cell and activates a receptor inside it, which binds specific DNA sequences to promote transcription. — Steroids are hydrophobic and penetrate the cell. Binding to a site on the receptor activates it; the hormone-receptor complex binds specific DNA sequences and promotes transcription of particular genes.
10 What is the effect of progesterone on the cells of the endometrium? HL
Answer and reasoning
It causes the endometrium to break down and be shed, which is why menstruation happens while progesterone is being secreted. — A student who attaches shedding to the hormone rather than to its withdrawal picks this. Progesterone maintains the endometrium; menstruation follows the fall in progesterone at the end of the cycle.
It binds directly to the DNA of the endometrial cells and switches on the genes that cause growth of the lining. — A student who omits the receptor picks this. Progesterone binds and activates an intracellular receptor, and it is the receptor that binds specific DNA sequences.
It maintains and thickens the endometrium, making its glands secrete and increasing its blood supply for implantation. — Progesterone acts on endometrial cells after ovulation to keep the lining thick, secretory and well supplied with blood so that an embryo can implant; a fall in progesterone leads to menstruation.
It binds to receptors on the surface of endometrial cells and raises cAMP inside them, which makes the lining grow much thicker. — A student who places every receptor on the cell surface picks this. Progesterone is a steroid: it enters the cell and acts through an intracellular receptor that affects gene expression.
Read the ones marked not yet in Learn, then Verify.
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15 more questions. Every wrong answer here is a real misconception, and you see why it is wrong straight away.
1 Epinephrine (adrenaline) released into the blood reaches every organ. Liver cells respond to it by breaking down glycogen, but some other cell types show no response at all. What best explains this difference? HL
Answer and reasoning
The blood carries epinephrine straight to the liver, so none reaches the other cell types. — A student who thinks the circulation routes a hormone to one target organ picks this. The stem states that the hormone reaches every organ; the difference lies in the cells, not in delivery.
Only the liver cells have receptor proteins whose binding sites are complementary to epinephrine. — All the cell types are exposed to the hormone, but a cell responds only if it possesses a receptor with a binding site specific to that ligand. Cells without the receptor are unaffected however much hormone reaches them.
Epinephrine can pass through the membrane of a liver cell but not through the membranes of the other cells. — A student who believes hormones enter cells to act picks this. Epinephrine is a hydrophilic amine that stays outside all cells and acts on a transmembrane receptor; membrane permeability is not the difference.
Only liver cell receptors convert epinephrine into the product that triggers glycogen breakdown. — A student who treats the receptor as an enzyme acting on its ligand picks this. The receptor binds epinephrine reversibly and unchanged; it is the conformational change in the receptor, not a product made from the hormone, that starts the response.
2 Vibrio fischeri produces light when packed at high density inside the light organ of a squid, but not when living at low density in seawater. Which explanation is correct? HL
Answer and reasoning
The squid releases a chemical into its light organ that switches on the light-producing genes of the bacteria living there. — A student who looks outside the bacteria for the signal picks this. The signalling chemical is made and secreted by the bacteria themselves; the host provides the dense population, not the signal.
Each bacterium switches on its light-producing genes independently when its supply of nutrients in the light organ is high enough. — A student who assumes unicellular organisms act cell by cell picks this. The switch is coordinated across the population by a shared secreted signal, which is what makes it cell signalling.
Autoinducer secreted by each cell only reaches the threshold that switches on the light-producing genes when cells are dense. — This is quorum sensing. Every cell secretes autoinducer; its concentration tracks population density, and only above the threshold does it bind receptors and promote transcription of the genes for luciferase.
The autoinducer accumulating in the light organ is itself the substance that gives out light when it reaches high concentration. — A student who thinks the signalling chemical performs the response itself picks this. The autoinducer is a ligand: it binds a receptor that promotes transcription of the lux genes, and the enzyme luciferase produces the light.
3 Noradrenaline is released by sympathetic neurons into synaptic gaps and is also released by the adrenal glands into the blood. What does this show about the categories hormone and neurotransmitter? HL
Answer and reasoning
Noradrenaline must be a hormone rather than a neurotransmitter, since a chemical belongs to only one category. — A student who treats the categories as chemical classes picks this. The categories are functional, and the stem shows the same substance being used in both ways.
They are functional categories, so one chemical can belong to either depending on how it is released and reaches its target. — A hormone is defined by transport in the blood to distant cells and a neurotransmitter by diffusion across a synaptic gap. The definitions are about delivery, not chemistry, so noradrenaline is a neurotransmitter at synapses and a hormone in the blood.
In both cases the noradrenaline is carried in the blood to the cells it acts on, so the two categories cannot be distinguished. — A student who thinks neurotransmitters travel in blood picks this. At a synapse noradrenaline diffuses across the synaptic gap and never enters the blood; the two routes are what separate the categories.
The blood carries adrenal noradrenaline only to the same cells that the sympathetic neurons already reach. — A student who thinks the blood delivers a hormone to a single target picks this. Noradrenaline in the blood reaches every organ and acts on all cells that carry its receptors.
4 To which chemical groups do the hormones epinephrine, insulin and testosterone belong? HL
Answer and reasoning
Epinephrine, insulin and testosterone are all proteins made of amino acids. — A student who generalises from insulin picks this. Only insulin is a protein; epinephrine is a small amine and testosterone is a lipid.
Epinephrine is a steroid, insulin is a protein and testosterone is a steroid as well. — A student who links epinephrine to the adrenal gland's steroids picks this. Epinephrine is an amine made from tyrosine, not a steroid from cholesterol.
Epinephrine is an amine, insulin is a protein and testosterone is a steroid. — These are the three chemical groups of hormones the guide requires: amines derived from an amino acid (epinephrine), proteins (insulin) and steroids derived from cholesterol (testosterone).
Epinephrine is an amine, insulin is a protein and testosterone is a small protein. — A student who thinks of steroids as protein-builders and hence as proteins picks this. Testosterone is a steroid, a lipid with a four-ring carbon skeleton.
5 Why is it advantageous for animals to use a wide range of chemically different substances as signalling chemicals rather than a single one? HL
Answer and reasoning
The chemical nature of a signal does not matter, because a receptor recognises the message rather than the shape of the molecule. — A student who separates the message from the molecule picks this. Specificity comes from the complementary shape and chemistry of ligand and binding site, so distinct messages need chemically distinct ligands.
Hormones must be large molecules and neurotransmitters small ones, so each category needs its own chemical class. — A student who treats the functional categories as chemical classes picks this. Both categories contain small amines and larger peptides; chemistry does not define the category.
Each receptor breaks its ligand down into a different product, so a different chemical is needed for every product required. — A student who treats receptors as enzymes acting on ligands picks this. Receptors do not convert their ligands; the response comes from a transduction pathway inside the cell.
Many messages can travel in the same fluid at once, each read only by cells with the matching receptor. — Receptor binding sites are specific, so chemically distinct ligands let many signals coexist in the blood and tissue fluid without cross-talk. Different chemistries also suit different speeds, durations and receptor locations.
6 The amino acid sequence of a newly discovered receptor shows seven stretches of about 20 mostly hydrophobic amino acids, separated by hydrophilic regions. Its ligand, a short peptide, binds to one of the hydrophilic regions. What can be concluded? HL
Answer and reasoning
It is an intracellular receptor, because a protein containing hydrophilic regions could not remain embedded within the lipid bilayer. — A student who expects a membrane protein to be hydrophobic throughout picks this. Transmembrane receptors are hydrophobic only where they cross the bilayer and hydrophilic where they project into watery surroundings.
It is a transmembrane receptor whose ligand enters the cell through the hydrophilic regions to act in the cytoplasm. — A student who thinks the signalling chemical must get in picks this. A ligand that binds a transmembrane receptor stays outside; the receptor's change of shape carries the signal across.
It is a transmembrane receptor: the hydrophobic stretches span the bilayer and the ligand binds a hydrophilic part outside. — Membrane-spanning regions are made of hydrophobic amino acids that sit among the phospholipid tails, while the parts exposed to water on each side are hydrophilic. A ligand binding to a hydrophilic region on the outer face remains outside the cell.
The seven hydrophobic stretches form a channel through which the ligand passes into the cell to activate a G protein on the inner face. — A student who thinks the ligand travels through a G protein-coupled receptor picks this. Seven membrane-spanning helices are typical of a GPCR, but nothing passes through it; a conformational change activates the G protein.
7 What is meant by the initiation of a signal transduction pathway by a receptor? HL
Answer and reasoning
The ligand enters the cytoplasm after binding and itself performs the response inside the cell. — A student who pictures the signalling chemical as the agent of the response picks this. The response is carried out by components of the pathway inside the cell; the ligand usually stays outside.
The receptor converts the ligand into a product, and the product produced is the cell's response. — A student who treats the receptor as an enzyme acting on its ligand picks this. The ligand is not changed; the receptor changes shape and the response arises from the cascade this triggers.
The receptor makes exactly one molecule of response product for each ligand molecule that binds to it. — A student who reasons one-to-one, as in a chemical equation, picks this. Transduction pathways amplify the signal, so one bound ligand can lead to a very large response.
Ligand binding changes the receptor's shape, setting off a sequence of responses in the cell. — The receptor initiates the pathway: the conformational change caused by binding is passed on through a series of intracellular events, such as activation of proteins and production of second messengers, that ends in the response.
8 A muscle fibre has a resting membrane potential of -70 mV. When acetylcholine is applied, the potential changes to -50 mV within 1 ms. If sodium ions are first removed from the fluid outside the fibre, acetylcholine still binds but the potential stays at -70 mV. What do these results show? HL
Answer and reasoning
The change to -50 mV is caused by positive ions leaving the fibre, a movement that needs sodium outside to draw them out. — A student who reverses the direction of ion movement picks this. Loss of positive ions would make the inside more negative, not less; the shift towards zero shows positive charge entering.
The change to -50 mV is caused by Na+ diffusing into the fibre through channels opened by acetylcholine binding. — The potential becomes less negative (depolarization) only when Na+ is available outside, so inflow of these positively charged ions through the opened receptor channels is what changes the voltage across the membrane.
Acetylcholine itself enters through the channel, but it can only do so if it is carried in by sodium ions. — A student who thinks the ligand passes through its receptor picks this. Acetylcholine binds and stays outside; the dependence on external Na+ shows it is Na+ that moves in.
Acetylcholine binding activates a G protein that makes cAMP, and the cAMP needs sodium to raise the potential. — A student who applies one mechanism to every receptor picks this. The acetylcholine receptor is itself an ion channel; no G protein or second messenger is involved in this millisecond response.
9 A hormone binds to a G protein-coupled receptor on cells, and an enzyme inside the cells becomes active. A toxin that locks the G protein in its inactive form is added. The hormone still binds to the receptor, but the enzyme is no longer activated. What does this show? HL
Answer and reasoning
The toxin has blocked the channel in the receptor through which the hormone normally enters the cell. — A student who thinks the ligand goes through the receptor picks this. The hormone never enters; it binds outside and the stem shows binding is unaffected.
The hormone has to reach the enzyme itself to activate it, and the toxin prevents the hormone from entering. — A student who pictures the hormone performing the response picks this. The hormone acts only on the receptor; the enzyme is activated through the intracellular pathway, which the toxin interrupts at the G protein.
The G protein converts the bound hormone into cAMP, so without it no second messenger can be formed. — A student who gives the G protein the enzyme's job picks this. cAMP is made from ATP by adenylyl cyclase, and the hormone is never converted into anything; the G protein's role is to activate the effector.
The G protein is needed to convey the signal from the occupied receptor to the enzyme inside. — Binding is unaffected, so the receptor is intact; the response fails only because the G protein cannot be activated. This isolates the G protein's role as the link between receptor and effector.
10 Binding of a few molecules of epinephrine to receptors on a liver cell leads to the release of many thousands of glucose molecules. Which feature of the signalling pathway explains this? HL
Answer and reasoning
The receptor converts each epinephrine molecule it binds into an active enzyme, and that enzyme releases the glucose. — A student who treats the receptor as an enzyme acting on its ligand picks this. Epinephrine is not converted into anything; the amplification comes from each component of the intracellular cascade acting on many molecules of the next.
Each activated G protein, adenylyl cyclase and kinase acts on many molecules of the next component, amplifying the signal. — One receptor can activate several G proteins, each adenylyl cyclase makes many cAMP molecules, and each activated kinase phosphorylates many enzyme molecules, so a small number of ligand molecules produces a very large response.
The pathway cannot amplify the signal, so the cell must in fact have bound many thousands of epinephrine molecules at the same time. — A student who reasons one ligand to one product picks this. Signal transduction cascades amplify at several steps, which is exactly why hormones are effective at very low concentrations.
cAMP leaves the cell and recruits neighbouring liver cells, which together release the large amount of glucose. — A student who thinks the second messenger is exported picks this. cAMP acts inside the cell that made it; the amplification happens within a single cell's cascade.
11 Which sequence correctly describes how insulin increases glucose uptake by a muscle cell? HL
Answer and reasoning
Insulin enters the cell bound to glucose and releases it in the cytoplasm; the empty insulin then returns to the blood to collect more glucose. — A student who imagines insulin as a carrier for glucose picks this. Insulin is a protein that cannot cross the membrane; glucose enters through transporter proteins that the insulin signal adds to the membrane.
Insulin binds the receptor, which phosphorylates free tyrosine in the cytoplasm; the phosphorylated tyrosine then binds glucose molecules and pulls them into the cell. — A student who reads 'phosphorylation of tyrosine' as the free amino acid picks this. The tyrosines phosphorylated are residues within the receptor and other proteins, and glucose enters through transporters, not bound to tyrosine.
Insulin binds the receptor outside; tyrosines on its cytoplasmic part are phosphorylated; a cascade moves vesicles with glucose transporters to the membrane. — The insulin receptor has tyrosine kinase activity: binding of the hormone outside causes phosphorylation of tyrosine inside the cell, and the resulting cascade ends with vesicles containing glucose transporters fusing with the plasma membrane, so more glucose enters.
Insulin binds the receptor; the signal activates transcription of the transporter genes, and the newly synthesized glucose transporters are then inserted into the membrane. — A student who assumes more protein in the membrane requires new gene expression picks this. The transporters already exist in vesicles; the response is their movement to the plasma membrane, which is why it is fast.
12 Cells respond to testosterone only after about 30 minutes, and the response is abolished by a drug that blocks transcription. The same cells respond to epinephrine within seconds, and this response is unaffected by the drug. Which conclusion is best supported? HL
Answer and reasoning
Testosterone acts via an intracellular receptor that promotes transcription, so its effect needs new mRNA; epinephrine acts on enzymes already present. — Dependence on transcription and a delay of minutes are the signature of a steroid hormone changing gene expression via an activated receptor; a response in seconds that survives transcription block shows epinephrine altering existing proteins via cAMP.
The drug prevents testosterone itself from binding to the DNA of the target genes, which is why the response to testosterone is lost. — A student who thinks the hormone binds DNA directly picks this. Testosterone binds its receptor, and the receptor binds DNA; the drug blocks transcription, not hormone binding.
Testosterone is slow because it must be carried across the membrane by a transport protein, and the drug blocks that carrier. — A student who believes hydrophobic molecules cannot cross the bilayer picks this. Steroids diffuse through the membrane freely; the delay reflects the time needed for transcription and translation.
Steroids act as quickly as other hormones because they enter the cell directly, so the 30-minute delay must be an artefact of the experiment. — A student who expects entering the cell to make a hormone faster picks this. A response that depends on making new mRNA and protein necessarily takes minutes to hours; the delay is real and diagnostic.
13 For most of the menstrual cycle, oestradiol reduces the secretion of GnRH by cells in the hypothalamus. Late in the follicular phase, when oestradiol has been at a high level for about two days, GnRH secretion increases sharply. What best explains these observations? HL
Answer and reasoning
Oestradiol can only inhibit GnRH secretion, so the sharp increase must have been caused by a different hormone. — A student who treats negative feedback as oestradiol's only possible action picks this. The same hormone stimulates GnRH secretion when its level is high and sustained.
Oestradiol's effect on GnRH-secreting cells switches from inhibition at moderate levels to stimulation at high sustained levels. — Oestradiol exerts negative feedback on the hypothalamic GnRH-secreting cells at low to moderate concentrations and positive feedback once it is high and sustained; the switch drives the surge that triggers ovulation.
The early reduction was a harmful side effect of oestradiol and the later increase is its beneficial action. — A student who reads negative feedback as 'harmful' picks this. Both effects are normal regulation: the inhibition keeps hormone levels stable and the stimulation drives ovulation.
Negative feedback lowers GnRH, so the sharp rise means the feedback loop has simply been switched off, with no new effect of oestradiol. — A student who equates negative feedback with 'lowering' and its absence with a rise picks this. Removing inhibition would allow a return to baseline; the sharp surge shows active stimulation, which is positive feedback.
14 What is the difference between negative feedback and positive feedback in the regulation of signalling pathways? HL
Answer and reasoning
Negative feedback produces harmful effects in the body, whereas positive feedback produces beneficial effects. — A student who carries the everyday value-judgement meaning of the words into biology picks this. The terms describe the direction of the effect on the stimulus, and both forms are normal regulation.
Negative feedback lowers the level of a variable, whereas positive feedback raises the level of a variable. — A student who maps negative onto 'decrease' picks this. Negative feedback opposes change in either direction, raising a variable that has fallen as readily as lowering one that has risen.
Negative feedback is used in healthy regulation, whereas positive feedback occurs only when regulation has failed. — A student who has met positive feedback only as a vicious circle picks this. Positive feedback is a normal mechanism for driving processes such as ovulation and birth to completion.
In negative feedback the response reduces the stimulus; in positive feedback the response increases it. — Negative feedback counteracts change and keeps a variable near a set level, as when oestradiol inhibits GnRH secretion; positive feedback amplifies change and drives a process to completion, as with oxytocin during birth.
15 During birth, stretching of the cervix stimulates secretion of oxytocin, which strengthens contractions of the uterus, which stretch the cervix further, until the baby is delivered. How should this regulation be described? HL
Answer and reasoning
Positive feedback, because each response increases the stimulus that caused it until an end point is reached. — The response (stronger contractions) amplifies the stimulus (stretching), so the loop drives the process to completion; delivery removes the stimulus and ends the loop.
Negative feedback, because the outcome of the process is beneficial to both the mother and the baby. — A student who judges feedback by whether the outcome is good picks this. The type of feedback depends on whether the response increases or reduces the stimulus, not on its benefit.
Negative feedback, because the whole process is switched off once the baby has been delivered. — A student who equates negative feedback with stopping picks this. The loop ends because delivery removes the stimulus; while it runs, each response increases the stimulus, which is positive feedback.
A failure of regulation, because positive feedback of this kind does not occur in any healthy process. — A student who knows positive feedback only as a runaway fault picks this. Birth is a normal process in which positive feedback is used deliberately to drive contractions to completion.
That was your twenty minutes. Real practice on C2.1 is past-paper questions marked against the mark scheme.
What the exam asks of C2.1
This is an HL-only topic. Paper 1A asks you to classify ligands and receptors, and to recognise the steps of a named pathway. Paper 1B may give data on receptor binding or on *Vibrio fischeri* light output against density and ask you to interpret it. Paper 2 uses *outline*, *explain* and *compare*: compare transmembrane with intracellular receptors, explain how epinephrine or insulin conveys a signal, distinguish negative from positive feedback with one example of each. Name the ligand, the receptor, where each is, and what changes inside the cell.
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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