IB DP Biology: Chemical signalling: ligands, receptors and signal transduction (HL) (DP Biology) – Revision Notes

These notes condense topic C2.1, Chemical signalling, statements C2.1.1 to C2.1.14, from the IB Diploma Programme Biology guide, first assessment 2025. All of C2.1 is HL only; the guide has no SL content here. For full explanations and worked examples use the study guide, then try the practice questions. Course pages: IB DP Biology hub, printable checklist and free 10-minute diagnostics.

Key definitions

Term	Meaning
Ligand	The signalling chemical that binds to a receptor
Receptor	Protein with a binding site specific to a ligand
Target cell	A cell with the receptor for a particular ligand
Signal transduction	Sequence of responses inside the cell after binding
Second messenger	Small molecule made inside the cell that spreads the signal (cAMP)
Kinase	Enzyme that adds a phosphate group to a protein
Autoinducer	Ligand released by bacteria in quorum sensing

Receptors in more detail (C2.1.1 and C2.1.7)

• A binding site is complementary to its ligand in shape and charge. Binding is reversible, so the response ends when the ligand is removed.
• A receptor changes shape when the ligand binds. This is the first step of signal transduction.
• A pathway usually has relay molecules, amplification (one enzyme makes many products) and an off switch.
• The same ligand can have different effects in different cells because the pathway behind the receptor differs.

Quorum sensing (C2.1.2)

1. Each bacterium releases a little autoinducer.
2. Low density: ligand diffuses away, concentration stays low.
3. High density: concentration passes a threshold and binds receptors.
4. Genes switch on across the population together.

Example: Vibrio fischeri makes light (bioluminescence) at high density, for example in a host light organ, and none in open seawater.

Four categories of signal (C2.1.3 to C2.1.5)

	Hormone	Neurotransmitter	Cytokine	Calcium ions
Source	Endocrine cells	Neurons	Immune and other cells	Stores and channels
Route	Blood, distant	Synaptic gap, about 20 nm	Short distance between cells	Inside the cell
Speed	Slow, lasting	Fast, brief	Intermediate	Very fast
Role	Coordinate body	Pass impulse	Immune regulation	Trigger exocytosis, contraction

Chemical groups to recall:

• Hormones: amines (epinephrine), proteins (insulin), steroids (oestradiol, progesterone, testosterone).
• Neurotransmitters: amino acids, peptides, amines, and nitric oxide, a gas. The guide text names the gas as nitrous oxide; the neurotransmitter is nitric oxide.
• Why so varied: different solubility, size, stability and speed suit different jobs, and many shapes give each ligand its own receptor.

Receptor location (C2.1.6)

	Transmembrane	Intracellular
Ligand	Hydrophilic or large, stays outside	Hydrophobic, enters the cell
Hydrophobic amino acids	In the membrane-spanning region	Not needed at the surface
Hydrophilic amino acids	Outside and inside parts	Surface, in cytoplasm or nucleoplasm
Examples	Acetylcholine, epinephrine, insulin	Oestradiol, progesterone, testosterone

Pathways at a glance

Receptor type	Steps	Time scale
Ligand-gated ion channel (acetylcholine, C2.1.8)	Ligand binds, channel opens, positive ions (mainly sodium) diffuse in, depolarisation, may reach threshold	Milliseconds
G protein-coupled (epinephrine, C2.1.9 and C2.1.10)	Binds, G protein swaps GDP for GTP, enzyme makes cAMP, kinase cascade	Seconds
Tyrosine kinase (insulin, C2.1.11)	Binds, tyrosine phosphorylation inside, relay reactions, vesicles with glucose transporters fuse with membrane	Minutes
Intracellular (steroids, C2.1.12)	Ligand crosses membrane, binds receptor, receptor binds DNA sequences, transcription	Hours

Acetylcholine receptor, step by step (C2.1.8)

1. Acetylcholine is released from the presynaptic neuron and diffuses across the synaptic gap.
2. It binds to the receptor on the postsynaptic membrane.
3. The receptor is also a channel, so it opens.
4. Positive ions, mainly sodium, diffuse into the cell down the gradient.
5. The inside becomes less negative: depolarisation.
6. If the threshold is reached, voltage-gated channels open and an action potential can follow.
7. The ligand is removed (for example by an enzyme that breaks it down), the channel closes and the potential returns to rest.

Method boxes

Describing epinephrine action in a liver cell

1. Epinephrine (first messenger) binds a receptor in the plasma membrane.
2. The receptor activates a G protein (GTP replaces GDP).
3. The G protein activates adenylyl cyclase, which converts ATP to cAMP.
4. cAMP (second messenger) activates protein kinase A and a cascade.
5. Glycogen is broken down, glucose is released.
6. Phosphodiesterase breaks down cAMP, and the signal ends.

Describing insulin action

1. Insulin binds the receptor; tyrosine kinase activity is switched on.
2. Tyrosine inside the cell is phosphorylated.
3. Relay reactions follow.
4. Vesicles with glucose transporters move to the plasma membrane and fuse.
5. More glucose enters by facilitated diffusion.

Describing a steroid hormone

1. It diffuses through the phospholipid bilayer.
2. It binds the receptor, which is activated.
3. The receptor binds specific DNA sequences.
4. Transcription of target genes is promoted; new protein is made.

Oestradiol and progesterone (C2.1.13)

• Oestradiol: target cells are GnRH-secreting cells of the hypothalamus. Lower levels reduce GnRH secretion; high, sustained levels increase it.
• Progesterone: target cells are endometrium cells; it maintains and prepares the lining for implantation.

Feedback (C2.1.14)

	Negative feedback	Positive feedback
Effect	Reverses the change	Amplifies the change
Result	Stable around a set point	Rapid movement away from start
Example	Glucose rises, insulin released, glucose falls, insulin falls	Cervix stretches, oxytocin released, stronger contractions, more stretch

Small worked reminders

Fold change versus percentage change. A rise from 4.0 to 31.0 units is 27.0/4.0 × 100 = 675% (7.75 times). Always say which one you are giving and match the question.

Membrane potential. Change = final minus initial. Going from −70 mV to −54 mV is +16 mV, so the cell is depolarised but still below a threshold of −50 mV.

Reading a time course. Compare the time to the first change. A response in seconds points to a membrane pathway and a second messenger; a response after hours points to changed gene expression. Quote values from the graph when you make the comparison.

Linking ideas across the topic

• Insulin and the acetylcholine receptor are both transmembrane receptors, but one is an enzyme-linked receptor and the other is an ion channel.
• Epinephrine and insulin both use membrane receptors, yet epinephrine uses a G protein and cAMP while insulin uses tyrosine phosphorylation.
• Steroids and the nitric oxide gas both pass through membranes without a transmembrane receptor.
• Negative feedback links to homeostasis; the glucose and insulin loop uses an insulin receptor to start it.

Command words for this topic

• Outline a pathway: give the main steps in order, with the name of each molecule that acts.
• Explain why a receptor is specific: link the shape and charge of the binding site to the ligand.
• Compare hormones and neurotransmitters: give a similarity and a difference for each feature, such as route, distance and speed.
• Distinguish two receptor types: state a difference for the same feature in both.
• Suggest a role for a molecule in a data question: use the data as evidence and say what the data does not show.

Must-know distinctions

• Hormone (blood, distant, slow) versus neurotransmitter (gap, local, fast).
• First messenger (ligand) versus second messenger (cAMP).
• Transmembrane receptor (ligand stays outside) versus intracellular receptor (ligand enters).
• Kinase (adds phosphate) versus phosphodiesterase (breaks cAMP).
• Switching on in a G protein: GTP bound means active.

Quick self-test

1. Name the term for a signalling chemical that binds a receptor.
2. Why do only some cells respond to a hormone in the blood?
3. What is the role of an autoinducer in V. fischeri?
4. State two chemical groups of hormones with one example each.
5. Why can oestradiol use an intracellular receptor while insulin cannot?
6. Resting potential is −65 mV. Acetylcholine depolarises the cell by 20 mV. Give the new potential.
7. cAMP rises from 5 to 135 units after epinephrine. By what factor does it rise?
8. What does phosphorylation of tyrosine in the insulin receptor lead to?
9. Which ions mainly enter through the open acetylcholine receptor?
10. Give one example each of positive and negative feedback.

Answers

1. Ligand.
2. Only target cells have the specific receptor.
3. It accumulates with cell density and, above a threshold, switches on light-producing genes.
4. Amines (epinephrine) and steroids (oestradiol); proteins (insulin) also fit.
5. Oestradiol is lipid-soluble and crosses the membrane; insulin is a protein and cannot.
6. −65 + 20 = −45 mV.
7. 135/5 = 27 times.
8. A sequence of reactions that ends with vesicles bringing glucose transporters to the plasma membrane.
9. Sodium ions (positive ions diffusing in).
10. Negative: glucose and insulin. Positive: oxytocin in labour.

Where marks are usually lost

• Writing “receptor fits the hormone” without naming shape or the binding site.
• Saying hormones travel only to target organs rather than through the whole blood system.
• Naming the signalling gas wrongly; the neurotransmitter gas is nitric oxide.
• Missing that a G protein must swap GDP for GTP to activate.
• Confusing cAMP as the hormone with cAMP as the second messenger.
• Forgetting an off switch in a pathway.
• Describing steroid action without mentioning DNA sequences and transcription.
• Quoting an example of feedback without saying which change is reversed or amplified.
• Leaving out units or the working in percentage change and fold-change calculations.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Topic C2.1, Chemical signalling (additional higher level).