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C2.1.3 (HL)—Functional categories in animals

Animal signalling includes hormones and nervous communication, with chemical messengers acting through target cells, receptors and feedback mechanisms, linking signal detection to a specific target-cell response.

Syllabus
First assessment 2025
Objective
C2.1.3
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper2
Typical marks1–7

Common command terms

  • State
  • Explain
  • Compare
  • Contrast
  • Outline

Recent exam appearances

November 2024Paper2 ["HL"] · TZ02(e)[ 1 ]C2.1.3 (HL)—Functional categories in animals
May 2021Paper2 ["HL"] · TZ16(c)[ 7 ]C2.1.3 (HL)—Functional categories in animals
November 2019Paper1 ["HL"] · TZ025[ 1 ]C2.1.3 (HL)—Functional categories in animals
May 2019Paper2 ["HL"] · TZ26(c)[ 8 ]C2.1.3 (HL)—Functional categories in animals
May 2018Paper1 ["HL"] · TZ125[ 1 ]C2.1.3 (HL)—Functional categories in animals
Practice this objective

Coverage 2012–2024 · Updated 15 Jul 2026

Compare animal signal types

HL only
Compact comparison table showing hormones, neurotransmitters, cytokines, and Ca2+ ions across rows and source, distance, speed, and target-cell response across columns, with side labels for hormone classes and neurotransmitter classes.

Animal chemical signals differ in source, distance, speed and target-cell effect. Hormones are released by endocrine cells into blood and act at distant targets, usually more slowly. Neurotransmitters are released by neurons across synapses and act rapidly on nearby target cells. Cytokines are released by immune and other cells and coordinate local or systemic responses. Ca²⁺ ions act as short-lived intracellular signals that regulate target proteins.

  • Hormone: endocrine source; bloodstream; distant target; relatively slow and longer-lasting.
  • Neurotransmitter: neuron; synaptic cleft; adjacent target; rapid response.
  • Cytokine: immune or other cell; local or body-fluid signalling; coordinates immune responses.
  • Ca²⁺: intracellular messenger; changes protein activity inside the responding cell.
  • Hormones can be protein/peptide, steroid or amine; neurotransmitters include acetylcholine, amino acids, peptides, amines and nitric oxide.

Functional categories in animals

HL only

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using State / Explain / Compare.

Command terms

State / Explain / Compare / Contrast / Outline

What earns marks

Build the answer around this relationship: Hormones are chemical messengers secreted by endocrine cells or glands.

Representative question

Question 1

[Maximum number: 7]

Compare and contrast hormonal and nervous communication.

Chemical Signalling

HL only

A ligand only affects target cells with the matching receptor. Quorum sensing uses autoinducers and thresholds for group behaviour. Animal signals differ by source, distance, speed, target-cell response, and chemical class. Hydrophilic ligands use transmembrane receptors and relays; steroid and thyroid hormones use intracellular receptors that affect transcription. Named pathways then show the logic: acetylcholine opens sodium channels, GPCRs switch G proteins through GDP-GTP exchange, epinephrine uses cAMP and kinase cascades, insulin RTKs use dimerisation/autophosphorylation to move GLUT4 and promote glycogenesis, steroid hormones change gene expression, and feedback either amplifies or restores stability.

  • Start every signalling answer with ligand specificity and target-cell receptor matching.
  • Then choose the route: local synapse, blood-borne hormone, transmembrane receptor, or intracellular receptor.
  • For named mechanisms, give the sequence, not just the pathway name.
  • Finish feedback comparisons with amplification versus stability.

Concept essentials

  • Hormones are chemical messengers secreted by endocrine cells or glands.
  • Leptin is secreted by adipose tissue and acts on the hypothalamus to inhibit appetite.
  • Hormonal communication is usually blood-borne, while nervous communication is neuron-based and faster.
  • Feedback mechanisms help regulate hormone-controlled physiological variables.