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C3.1.20 (HL)—Auxin efflux carriers

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

Syllabus
First assessment 2025
Objective
C3.1.20
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–2

Common command terms

  • Analyse
  • Compare
  • Contrast
  • Deduce
  • Identify

Scoring notes

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

Recent exam appearances

November 2025Paper1A ["HL"] · TZ127[ 1 ]C3.1.20 (HL)—Auxin efflux carriers
May 2025Paper1A ["HL"] · TZ329[ 1 ]C3.1.20 (HL)—Auxin efflux carriers
May 2017Paper2 ["HL"] · TZ21(d)(ii)[ 2 ]C3.1.20 (HL)—Auxin efflux carriers
May 2017Paper2 ["HL"] · TZ21(d)(i)[ 2 ]C3.1.20 (HL)—Auxin efflux carriers
May 2017Paper2 ["HL"] · TZ21(c)[ 2 ]C3.1.20 (HL)—Auxin efflux carriers
Practice this objective

Coverage 2017–2025 · Updated 16 Jul 2026

Auxin efflux carriers create directional hormone flow

HL only

Coordinated auxin efflux carriers establish directional cell-to-cell transport and concentration gradients of auxin in plant tissue.

Auxin can diffuse into a plant cell but cannot freely diffuse out. Efflux carriers placed on one side of the plasma membrane export it; when neighbouring cells position carriers on the same side, auxin is actively transported in one tissue direction.

Auxin enters cell → asymmetrically positioned efflux carrier exports it → coordinated carrier polarity repeats across cells → directional transport concentrates auxin in a particular region.

If shoot cells relocate their auxin efflux carriers to the shaded side, coordinated export can establish the unequal auxin distribution needed for unequal elongation.

Diffusion alone does not explain tissue-scale direction. The location and shared polarity of membrane efflux carriers determine the gradient.

Auxin efflux carriers

HL only

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Analyse / Compare / Contrast.

Command terms

Analyse / Compare / Contrast / Deduce / Identify

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: 2]

Deduce the effect of NPA on auxin transport between L6 and the stem base.

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

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