IB Biology HL C2.1 Chemical signalling [HL only] Questions

Practise IB Biology HL C2.1 by comparing ligands, receptors, hormones, quorum sensing and intracellular signalling pathways.

40 questions in this syllabus scope, with question parts counted together.

Syllabus
First assessment 2025
Course
Biology HL
Level
HL

Exam analysis

HL · 2010–2025
Papers with this topic33 / 117in the analysed sample
Latest appearanceNovember 2025
Tagged question parts39
Paper types5

Most tested objectives

  1. C2.1.6 (HL)—Transmembrane vs. intracellular receptors13 papers
  2. C2.1.2 (HL)—Quorum sensing in bacteria6 papers
  3. C2.1.3 (HL)—Functional categories in animals4 papers
  4. C2.1.4 (HL)—Chemical diversity4 papers
  5. C2.1.1 (HL)—Receptors as proteins3 papers

Common question formats

  • Calculation
  • Comparison
  • Data analysis
  • Diagram interpretation
  • Process explanation
  • Structured response
  • Definition or recall

Recent exam appearances

November 2025Paper 2 HL · TZ3Q9(a)[ 4 ]
November 2025Paper 1A HL · TZ3Q17[ 1 ]
November 2025Paper 1A HL · TZ1Q17[ 1 ]

117 papers analysed · Tagging incomplete

Exam points

  • Identify receptor proteins as specific ligand-binding sites and explain signalling specificity.
  • Explain quorum sensing through autoinducer thresholds and coordinated bacterial behaviour.
  • Compare animal signalling categories and local or distant communication, using leptin.
  • Classify hormones and distinguish peptide versus steroid receptor routes.
  • Compare transmembrane and intracellular receptor pathways for hydrophilic and steroid ligands.
  • Trace signal transduction through second messengers, gene-regulatory proteins and phosphorylation cascades.
  • Trace epinephrine GPCR signalling through G protein, adenylyl cyclase and cyclic AMP.
  • Explain intracellular steroid or thyroxine-like receptor complexes regulating transcription.
  • Explain negative feedback as an opposite response returning a regulated variable toward prior levels.

Covered syllabus points

C2.1.1 (HL)—Receptors as proteinsC2.1.2 (HL)—Quorum sensing in bacteriaC2.1.3 (HL)—Functional categories in animalsC2.1.4 (HL)—Chemical diversityC2.1.5 (HL)—Localized vs. distant effectsC2.1.6 (HL)—Transmembrane vs. intracellular receptorsC2.1.7 (HL)—Signal transduction pathwaysC2.1.8 (HL)—Neurotransmitter receptors and membrane potentialC2.1.9 (HL)—G protein-coupled receptorsC2.1.10 (HL)—Epinephrine (adrenaline) mechanismC2.1.11 (HL)—Tyrosine kinase receptorsC2.1.12 (HL)—Intracellular receptorsC2.1.13 (HL)—Effects of oestradiol and progesteroneC2.1.14 (HL)—Regulation by feedback

Exam points

  • Identify receptor proteins as specific binding sites for signalling ligands.
  • Explain how complementary ligand-receptor shape and chemistry determine signalling specificity.
  • Relate ligand binding to receptor activation, signal transduction and responses in target cells.

Question 1

[Maximum number: 4]

The complementarity of shapes and chemical properties allows molecules to bind to each other.

Describe, with examples, the types of molecule within cell-to-cell signalling systems that are complementary to each other.

Exam points

  • Define quorum sensing as population-density-dependent bacterial communication and behaviour.
  • Explain how secreted signalling molecules or autoinducers accumulate to a threshold and activate coordinated responses.
  • Use Vibrio fischeri bioluminescence, biofilm formation and bacterial-density data as examples of quorum sensing.

Question 1

[Maximum number: 1]

Mutualistic bacterium Vibrio fischeri lives in the light organ of some squid species. As a result of quorum sensing, bioluminescence is produced, which camouflages squid from predators.

How does quorum sensing trigger bioluminescence?

A

At high population density, bacteria secrete ligands that bind to receptors in the squid's light organ.

B

Above a critical V. fischeri density, signalling molecules activate bacterial genes involved in bioluminescence.

C

A high concentration of bacterial signalling molecules causes the expression of bioluminescence genes in the squid DNA.

D

When at risk, the squid's light organ sends signals to stimulate production of bioluminescence in bacteria.

Exam points

  • Compare hormonal and nervous communication by signal type, route, speed, duration and target range.
  • Use leptin to explain how a hormone released by adipose tissue acts on hypothalamic target cells to reduce food intake.

Question 1

[Maximum number: 1]

Leptin is a hormone which initiates the process to reduce food intake. What is the location of the glands that produce leptin and where are its target cells found?

Location of glands

Location of target cells

pancreas

hypothalamus

pancreas

lining of stomach

adipose tissue

lining of stomach

adipose tissue

hypothalamus

Exam points

  • Name examples of peptide/protein, steroid and tyrosine-derived amine hormones.
  • Distinguish peptide and steroid hormones by polarity and solubility.

Question 1

[Maximum number: 2]

State one example of a steroid hormone and one example of a protein hormone.

Steroid hormone:

Protein hormone:

No public sample questions are available for this syllabus point yet.

Exam points

  • Explain that peptide hormones bind transmembrane receptors because they do not enter target cells.
  • Relate peptide-hormone receptor binding to second messengers, reaction cascades and altered enzyme activity.
  • Explain that steroid hormones cross the plasma membrane, bind intracellular receptors and regulate gene transcription.

Question 1

[Maximum number: 6]

Compare and contrast the mode of action of peptide hormones and steroid hormones.

Exam points

  • Trace epinephrine signalling from a membrane receptor through cAMP to an enzyme response.

Question 1

[Maximum number: 3]

The diagram shows an ECG trace with the QRS complex indicated.

Figure for Question 1 — IB Biology HL

Heart rate is affected by the hormone epinephrine. The action of epinephrine is mediated by a chemical called a second messenger. Explain the mechanism of action of a second messenger.

No public sample questions are available for this syllabus point yet.

No public sample questions are available for this syllabus point yet.

Exam points

  • Trace epinephrine from adrenergic GPCR binding through G-protein activation and cAMP to a cellular response.

Question 1

[Maximum number: 1]

The graph shows changes in blood plasma glucose concentration before exercise, during exercise and during recovery with and without epinephrine (adrenaline) infusion. Epinephrine infusion was started 30 minutes into the exercise routine.

Figure for Question 1 — IB Biology HL

[Source: Kreisman, S.H., Ah Mew, N., Arsenault, M., Marliss, E.B., et al., 2000. AJP Endocrinology and Metabolism 278(5): E949-57. https://doi.org/10.1152/ajpendo.2000.278.5.E949. Reference redacted. Source adapted.]

Which action of epinephrine would result in the observed changes to plasma glucose concentration?

A

Second messenger is deactivated.

B

A G protein is activated, and cyclic AMP (cAMP) is produced.

C

Second messenger stimulates the production of ATP.

D

Cyclic AMP (cAMP) deactivates the G protein.

Exam points

  • Identify steroid and thyroxine-like hormones as ligands for intracellular receptor proteins.
  • Explain how steroid hormones cross the plasma membrane and bind receptor proteins in the cytoplasm.
  • Relate the hormone-receptor complex to nuclear DNA binding, gene transcription and production of specific proteins.

Question 1

[Maximum number: 3]

The action of thyroxin is similar to steroid hormones. Describe the action of steroid hormones.

No public sample questions are available for this syllabus point yet.

Exam points

  • Identify negative feedback when a response opposes an initial change and returns a regulated condition toward normal.
  • Recognize hormonal negative feedback when high estrogen inhibits FSH secretion.

Question 1

[Maximum number: 1]

Average human internal (core) body temperature is 36.4∘C36.4^{\circ} \mathrm{C}, with slight variations between day and night. The average surface temperature of the Earth in the 20th century was 13.9∘C13.9^{\circ} \mathrm{C}. It has now risen to 14.8∘C14.8^{\circ} \mathrm{C}.

Identify with a reason whether this response is positive or negative feedback.