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C2.1.1 (HL)—Receptors as proteins

Receptors are proteins with complementary binding sites that allow only target cells carrying the matching receptor to respond to chemical signals.

Syllabus
First assessment 2025
Objective
C2.1.1
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks1–4

Common command terms

  • Describe
  • Explain

Recent exam appearances

November 2025Paper2 ["HL"] · TZ39(a)[ 4 ]C2.1.1 (HL)—Receptors as proteins
November 2023Paper3 ["HL"] · TZ223(b)[ 3 ]C2.1.1 (HL)—Receptors as proteins
November 2023Paper3 ["HL"] · TZ123(b)[ 3 ]C2.1.1 (HL)—Receptors as proteins
May 2022Paper1 ["HL"] · TZ125[ 1 ]C2.1.1 (HL)—Receptors as proteins
Practice this objective

Coverage 2022–2025 · Updated 15 Jul 2026

Receptors Detect Specific Signals

HL only

A receptor is a protein with a binding site specific to a signalling chemical, called a ligand.

Complementary shape and chemical interactions allow a ligand to bind selectively. Binding changes the receptor's state or conformation and initiates a sequence of responses inside the target cell.

Identify four linked parts: ligand → specific receptor binding site → receptor activation → intracellular response. A cell is a target only if it has the appropriate receptor.

Insulin can circulate past many cells, but only cells expressing an insulin receptor bind it and initiate the corresponding signal-transduction pathway.

Specific does not mean that a receptor completes the response by itself. It detects its ligand and starts a pathway; downstream cell machinery produces the response.

Receptors as proteins

HL only

Assessment in practice

1–4 marks
How it is assessed

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

Command terms

Describe / Explain

What earns marks

Build the answer around this relationship: Receptors are proteins with specific ligand-binding sites.

Representative question

Question 1

[Maximum number: 4]

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

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

  • Receptors are proteins with specific ligand-binding sites.
  • Only cells with the matching receptor respond to a signalling chemical.
  • Peptide hormones usually bind to receptors in the plasma membrane.
  • Ligand-receptor binding can initiate intracellular signal transduction.