Auxin promotes cell growth links named stimuli, coordinating structures and effectors so body systems or plant tissues produce an integrated biological response.
Coverage 2015–2023 · Updated 16 Jul 2026
Auxin promotes cell elongation by stimulating H⁺ secretion into the apoplast, acidifying and loosening the cell wall.
Lower wall pH promotes loosening of cross-links between cellulose-containing wall components. With the wall more extensible, water-driven turgor pressure can lengthen the cell.
Auxin gradient → different H⁺ secretion rates → different apoplast pH → unequal wall loosening → unequal cell elongation → curvature in phototropism.
Higher auxin on the shaded side of a shoot increases wall loosening and elongation there, so the shoot bends toward the lateral light.
Auxin loosens the wall but does not supply the water or turgor needed for expansion. Its effect depends on concentration and tissue context.
This objective is assessed through structured response, commonly using Explain / Identify.
Explain / Identify
Mark schemes reward named structures, correct direction of information flow and explanations that keep the sequence causal.
Saying light directly makes cells grow instead of tracing auxin redistribution and cell elongation.
Representative question
When a plant cell grows, the cell wall must expand. Explain the role of auxin in cell wall expansion.
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.
