15.2.2—Auxin role in elongation growth

Auxin is a plant growth regulator that promotes elongation growth. Its effect is local and cell-specific: auxin binds to a receptor on the cell-surface membrane and activates changes that allow the cell to extend.

1. Auxin binds to a receptor protein on the cell-surface membrane.
2. Proton pumps move H+ into the cell wall, lowering its pH. This activates expansins, which loosen bonds between cellulose microfibrils.
3. Auxin also opens potassium ion channels. More K+ in the cytoplasm lowers its water potential, so water enters by osmosis through aquaporins.
4. Water entry increases internal pressure; with the loosened wall able to stretch, the cell elongates.

The result is auxin-controlled elongation growth in roots or shoots, contributing to plant growth and form. The reliable mechanism here is receptor activation → wall loosening plus water uptake → turgor-driven cell elongation; this card does not assert a particular shaded-side or phototropic redistribution unless separately supported by the matched objective source.

Auxin does not lengthen a cell by supplying water directly, and water entry alone is insufficient if the cellulose wall cannot stretch. The card stays with the general elongation mechanism and does not replace it with gibberellin signalling or an unsupported phototropism-specific explanation.