IB Biology · Theme C: Interaction and interdependence · HL only
C2.1 Chemical signalling
A one-page summary of C2.1 Chemical signalling, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How do cells distinguish between the many different signals that they receive?
What interactions occur inside animal cells in response to chemical signals?
What C2.1 covers
Signalling chemicals and receptors
- C2.1.1A protein shaped to fit one signal
- C2.1.2When enough bacteria are listening
- C2.1.3Four categories, one job
- C2.1.4Three very different kinds of hormone
- C2.1.4One job, many different molecules
- C2.1.5Reaching one cell, or the whole body
Receptor types and signal transduction
- C2.1.6Two places a receptor can sit
- C2.1.7One binding event, a chain of responses
- C2.1.8A receptor that is also a channel
- C2.1.9The switch behind an entire receptor family
Epinephrine and insulin: two transmembrane pathways
- C2.1.10From one hormone to a flood of glucose
- C2.1.10One hormone, two names
- C2.1.11A receptor that phosphorylates itself
Steroid hormones and feedback regulation
- C2.1.12A hormone that becomes a transcription factor
- C2.1.13One hormone, one specified target
- C2.1.13A second named target, a different tissue
- C2.1.14Two ways a pathway can regulate itself
C2.1 Chemical signalling: summary
Receptors & ligands
- Receptor: protein with a binding site complementary to a specific ligand; binding causes a conformational change.
- Quorum sensing: Vibrio fischeri bioluminescence switches on once autoinducer crosses a density threshold.
Categories & chemistry
- Four functional categories: hormone, neurotransmitter, cytokine, calcium ion (Ca²⁺) — differ in range and role.
- Hormones: amine, protein, steroid. Neurotransmitters: amino acid, peptide, amine, nitric oxide (NO).
Distance & receptor location
- Hormones travel via blood to distant cells; neurotransmitters diffuse across a synaptic gap.
- Transmembrane receptors bind hydrophilic ligands; intracellular receptors bind hydrophobic ligands that cross the membrane.
Transduction & two receptor types
- Reception → transduction → response. Acetylcholine receptor: ligand-gated ion channel, changes membrane potential directly.
- GPCR: activates a G protein (GDP→GTP exchange) to relay the signal onward.
Epinephrine & insulin
- Epinephrine: GPCR → G protein → adenylyl cyclase → cAMP second messenger → glycogen breakdown, amplified.
- Insulin: tyrosine kinase receptor → phosphorylation → glucose transporter vesicles move to the membrane.
Steroids & feedback
- Intracellular receptors (oestradiol → hypothalamus/GnRH; progesterone → endometrium) bind DNA to promote transcription.
- Negative feedback stabilises (insulin/glucose); positive feedback reinforces (quorum sensing).
Key terms
- Ligand
- The IB term for any signalling chemical that binds specifically to a receptor.
- Signal transduction
- The sequence of responses set off inside a cell once a ligand has bound its receptor.
- Second messenger
- A small intracellular molecule, such as cAMP, that relays a signal onward once a receptor has been activated.
- GPCR
- A G protein-coupled receptor: a transmembrane receptor that activates an associated G protein once bound by its ligand.
- Quorum sensing
- Bacterial signalling in which gene expression changes once population density crosses a threshold.
- Tyrosine kinase receptor
- A transmembrane receptor, such as the insulin receptor, that phosphorylates tyrosine residues once activated.
- Intracellular receptor
- A receptor in the cytoplasm or nucleus, binding a ligand small and hydrophobic enough to cross the membrane.
- Positive feedback
- Regulation in which a pathway's output reinforces the original stimulus.
- Negative feedback
- Regulation in which a pathway's output reduces or reverses the original stimulus.
Sample exam questions
Three of the 42 multiple-choice questions for C2.1. Try each one before opening the answer.
Question 1. A receptor protein is best described as:
- A DNA sequence that responds to hormones
- A protein with a binding site complementary to a specific signalling chemical
- Any molecule secreted by a cell into the bloodstream
- An enzyme that always breaks down its signalling chemical after binding
Show the answer
Answer: B. A receptor is a protein whose binding site shape and chemistry is complementary to a specific ligand (or small family of related ligands). Binding causes a conformational change in the receptor, which is what starts the cellular response.
Question 2. Which physical event occurs in a receptor protein immediately after its ligand binds, and starts the cellular response?
- A conformational (shape) change in the receptor
- The receptor is broken down by the ligand
- The ligand is phosphorylated by the receptor
- The receptor moves to a different organelle
Show the answer
Answer: A. Ligand binding causes a conformational change in the receptor — this shape change is the physical event that alters the receptor's activity and starts the sequence of responses that follows (signal transduction).
Question 3. The IB syllabus term for a signalling chemical that binds specifically to a receptor is:
- Substrate
- Cofactor
- Enzyme
- Ligand
Show the answer
Answer: D. "Ligand" is the required term for any signalling chemical that binds a receptor, whether it is a hormone, neurotransmitter, cytokine or other signalling molecule.
Linking questions
Questions that connect C2.1 to other parts of the course, the kind that come up in Paper 2.
- Many hormones are proteins (B1.2). Using insulin as an example, explain how the primary and tertiary structure of a protein hormone enables specific binding to its receptor. (see B1.2)
- Signalling pathways often involve phosphorylation of target proteins. Explain how ATP from respiration (C1.2) provides the phosphate groups used in receptor tyrosine kinase signalling. (see C1.2)
- Neurotransmitters such as acetylcholine act as ligands in the nervous system (C2.2). Compare the speed and mechanism of neurotransmitter signalling with that of endocrine hormone signalling. (see C2.2)
- Gene expression (D2.2) is a common endpoint of signal transduction. Explain how a steroid hormone-receptor complex can directly activate transcription of specific genes. (see D2.2)
Practise C2.1
Study notes, every question and full markschemes for C2.1 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.