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C3.1.19 (HL)—Phytohormones

Phytohormones links named stimuli, coordinating structures, signalling routes and effectors so body systems or plant tissues produce an integrated biological response.

Syllabus
First assessment 2025
Objective
C3.1.19
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1–2

Common command terms

  • Identify
  • State
  • Calculate
  • Compare
  • Contrast

Scoring notes

Common mistake
Treating low oxygen as the only trigger and omitting carbon-dioxide-driven pH change.

Recent exam appearances

November 2025Paper2 ["HL"] · TZ34(b)(ii)[ 2 ]C3.1.19 (HL)—Phytohormones
May 2013Paper1 ["HL"] · TZ132[ 1 ]C3.1.19 (HL)—Phytohormones
November 2012Paper1 ["HL"] · TZ032[ 1 ]C3.1.19 (HL)—Phytohormones
Practice this objective

Coverage 2012–2025 · Updated 16 Jul 2026

Concept essentials

  • Phytohormones depends on linking the stimulus to the coordinating structure or signal.
  • The pathway for phytohormones must keep information flow and response direction clear.
  • Named tissues, hormones, neurons or effectors give phytohormones its biological specificity.
  • Evidence about phytohormones is strongest when structure, mechanism and outcome are connected.
ConceptIB Biology HL