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Question

Difficulty: HardHormonal Coordination in Plants and Animals

According to the acid growth hypothesis, plant stem elongation is mediated by indole-3-acetic acid (auxin) through a specific sequence of cellular actions. Arrange the following physiological steps in the correct chronological sequence, starting from the initial hormone perception to the final structural expansion of the cell.

  1. 1Auxin molecules bind to transmembrane receptor proteins on target cells in the elongation zone.
  2. 2Plasma membrane H+H^+-ATPase proton pumps are stimulated to actively extrude hydrogen ions into the primary cell wall matrix.
  3. 3Acidification of the cell wall apoplast activates pH-dependent wall-loosening enzymes called expansins.
  4. 4Activated expansins disrupt non-covalent hydrogen bonds linking cellulose microfibrils to cross-linking glycans.
  5. 5Osmotic water influx driven by turgor pressure causes the loosened cell wall to irreversibly stretch and elongate.

Answer

The correct sequence of auxin-mediated cell elongation is: (1) Auxin molecules bind to transmembrane receptor proteins on target cells -> (2) Plasma membrane H+H^+-ATPase proton pumps are stimulated to extrude hydrogen ions -> (3) Acidification of the apoplast activates expansins -> (4) Activated expansins disrupt hydrogen bonds between cellulose microfibrils and glycans -> (5) Osmotic water influx driven by turgor pressure causes cell wall expansion.
The acid growth hypothesis dictates that auxin first binds to cell membrane receptors, stimulating H+H^+-ATPase pumps to extrude hydrogen ions into the cell wall. The resulting apoplastic acidification activates expansin enzymes, which cleave hydrogen bonds binding cellulose microfibrils to cross-linking glycans. Finally, internal turgor pressure causes water to enter osmotically, expanding the weakened wall.

Step-by-Step Solution

1
Identify the initial trigger of the signaling cascade
Auxin binding to plasma membrane receptors on target cells initiates the acid growth pathway.
Hormones must first interact with specific receptors before downstream cellular responses can occur.
2
Determine the direct biochemical output of receptor stimulation
Proton pumps (H+H^+-ATPase) actively pump H+H^+ ions out of the cytoplasm into the cell wall matrix.
Receptor activation increases proton pump activity and gene expression.
3
Trace the environmental change in the cell wall matrix
The drop in cell wall pH (acidification) activates expansin proteins.
Expansins have an acidic pH optimum and remain inactive at neutral pH.
4
Identify the mechanical weakening mechanism
Expansins break hydrogen bonds between cellulose microfibrils and hemicellulosic glycans.
Releasing hydrogen bonds allows microfibrils to slide past one another under mechanical stress.
5
Identify the driving physical force for cell elongation
Internal turgor pressure forces water into the cell, physically stretching the weakened cell wall.
The loosened wall yields to hydrostatic pressure, resulting in cellular expansion.

Key Concept

Acid Growth Hypothesis of Auxin Action
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Topics

Hormonal Coordination in Plants and Animals

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