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C3.1.18 (HL)—Positive phototropism

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

Syllabus
First assessment 2025
Objective
C3.1.18
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1–4

Common command terms

  • Identify
  • Outline
  • Explain
  • Describe

Scoring notes

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

Recent exam appearances

November 2024Paper2 ["HL"] · TZ08(a)[ 4 ]C3.1.18 (HL)—Positive phototropism
May 2022Paper1 ["HL"] · TZ233[ 1 ]C3.1.18 (HL)—Positive phototropism
May 2021Paper2 ["HL"] · TZ16(b)[ 5 ]C3.1.18 (HL)—Positive phototropism
May 2018Paper1 ["HL"] · TZ230[ 1 ]C3.1.18 (HL)—Positive phototropism
May 2017Paper1 ["HL"] · TZ134[ 1 ]C3.1.18 (HL)—Positive phototropism
Practice this objective

Coverage 2014–2024 · Updated 16 Jul 2026

Shoots show positive phototropism

HL only

Shoots show positive phototropism.

Light direction is detected at the shoot tip, while auxin redistribution causes greater elongation on the shaded side. The shoot therefore curves toward the light.

light sensed at tip → auxin moves to shaded side → shaded cells elongate → shoot bends toward light.

Auxin concentration rises on the shaded side, so those cells extend more and the shoot exposes its leaves to light.

The curvature is caused by unequal growth, not by the tip physically pulling the rest of the shoot.

Positive phototropism

HL only

Assessment in practice

1–4 marks
How it is assessed

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

Command terms

Identify / Outline / Explain / Describe

What earns marks

Mark schemes reward named structures, correct direction of information flow and explanations that keep the sequence causal.

Watch for

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

Representative question

Question 1

[Maximum number: 5]

Outline how the hormone auxin controls phototropism in plant shoots.

Plant Signalling Integration

HL only

Plant signalling integrates directional growth, phytohormones, and responses to stimuli. Tropisms are directional growth responses in seedlings; experiments compare shoot or root growth under lateral light or gravity stimuli. Shoots show positive phototropism toward lateral light; auxin redistribution causes greater elongation on the shaded side. Phytohormones are plant signalling chemicals controlling growth and development; auxin, cytokinin, and ethylene coordinate responses to stimuli and internal state. PIN auxin efflux carriers actively move auxin between plant cells; asymmetric carrier placement maintains auxin gradients during tropic responses. Auxin activates proton pumping into the apoplast; acidified cell walls loosen, allowing water uptake, elongation, and bending. Auxin generally promotes root formation; cytokinin promotes shoot formation and division; their ratio coordinates root-shoot growth and plant tissue culture outcomes. Ethylene is a gaseous phytohormone that promotes fruit ripening; ripening fruit releases more ethylene, creating positive feedback in nearby fruit.

Concept essentials

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