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Acid-growth hypothesis
MCQ questions and answers.
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MCQ Questions
1. According to the acid-growth hypothesis, the earliest wall-level event after auxin treatment in a responsive shoot cell is most directly associated with:
A.
Increased proton extrusion into the apoplast, lowering cell-wall pH
B.
Immediate hydrolysis of cellulose microfibrils by auxin
C.
Direct conversion of starch into cell-wall pectins
D.
Inhibition of water uptake by the vacuole
Correct Answer:
Increased proton extrusion into the apoplast, lowering cell-wall pH
Explanation:
The acid-growth hypothesis proposes that auxin stimulates plasma-membrane H+-ATPases, causing proton extrusion and acidification of the apoplast. This acidic environment activates wall-loosening processes.
2. Which experimental result would provide the strongest support for the acid-growth hypothesis in an auxin-responsive coleoptile segment?
A.
Growth stimulation by an acidic buffer even when auxin is absent
B.
Growth inhibition by an acidic buffer despite auxin treatment
C.
No growth response to either auxin or acidic buffer
D.
Growth stimulation only when the cell-wall pH is maintained near neutrality
Correct Answer:
Growth stimulation by an acidic buffer even when auxin is absent
Explanation:
If external acidification can mimic auxin-induced elongation, it supports the idea that apoplastic acidification is a key intermediate in auxin-promoted cell expansion.
3. Why does acidification of the apoplast promote elongation according to the acid-growth hypothesis?
A.
It activates wall-loosening proteins that increase the extensibility of the cell wall
B.
It causes irreversible breakdown of the plasma membrane
C.
It prevents solute entry and thereby reduces turgor pressure
D.
It converts auxin into a stronger hormone inside the nucleus
Correct Answer:
It activates wall-loosening proteins that increase the extensibility of the cell wall
Explanation:
Low apoplastic pH activates or promotes the action of wall-loosening factors, including expansin-mediated weakening of noncovalent interactions in the wall. A more extensible wall can yield under turgor pressure.
4. A proton-pump inhibitor is applied to an auxin-treated elongating stem segment. Which observation would most directly indicate that proton extrusion is required for the acid-growth response?
A.
Auxin fails to lower apoplastic pH and fails to stimulate elongation
B.
Auxin lowers apoplastic pH but elongation increases sharply
C.
Apoplastic pH rises while elongation becomes independent of turgor
D.
The inhibitor increases proton extrusion and completely prevents wall loosening
Correct Answer:
Auxin fails to lower apoplastic pH and fails to stimulate elongation
Explanation:
If blocking H+-ATPase activity prevents both apoplastic acidification and auxin-induced elongation, it supports the proposed sequence in which proton extrusion precedes wall loosening and growth.
5. Which sequence best represents the causal order proposed by the acid-growth hypothesis?
A.
Auxin perception, activation of H+-ATPase, apoplastic acidification, wall loosening, water-driven expansion
B.
Water uptake, cellulose synthesis, apoplastic alkalinization, H+-ATPase inhibition, elongation
C.
Wall thickening, auxin degradation, proton uptake, cytoplasmic acidification, expansion
D.
Apoplastic acidification, auxin synthesis, membrane depolarization, wall rigidification, expansion
Explanation:
The hypothesis links auxin action to proton-pump activation, reduced apoplastic pH, activation of wall-loosening mechanisms, and subsequent expansion driven by cellular turgor.
6. An auxin-treated cell shows a rapid decrease in apoplastic pH, followed by delayed elongation. Which interpretation is most consistent with the acid-growth hypothesis?
A.
Apoplastic acidification is an upstream event that precedes mechanical wall loosening and expansion
B.
The pH decrease is a consequence of wall stretching and cannot contribute to growth
C.
The delay proves that auxin acts only by increasing cell division
D.
Rapid acidification indicates that the cell has lost all turgor pressure
Correct Answer:
Apoplastic acidification is an upstream event that precedes mechanical wall loosening and expansion
Explanation:
The hypothesis predicts that pH reduction occurs before measurable elongation because acidification must first activate wall-loosening processes before turgor can extend the wall.
7. Fusicoccin can stimulate acidification of the apoplast and cell elongation by promoting persistent activation of which membrane process?
Explanation:
Fusicoccin promotes sustained activation of plasma-membrane H+-ATPases. The resulting proton extrusion acidifies the apoplast and can induce acid-growth-like elongation.
8. Suppose an acidic buffer causes elongation in isolated stem segments, but the response disappears when the segments are placed in a strongly hypertonic solution. What does this demonstrate about acid growth?
A.
Acidification increases wall extensibility, but expansion still requires sufficient water-driven turgor
B.
Acidification directly supplies the water needed for cell expansion
C.
Acidification causes growth independently of wall mechanical properties
D.
Hypertonic conditions increase proton extrusion and therefore enhance acid growth
Correct Answer:
Acidification increases wall extensibility, but expansion still requires sufficient water-driven turgor
Explanation:
Acidification loosens the wall, but wall yielding produces elongation only when internal turgor supplies the mechanical force and water required for expansion.
9. Which finding would most strongly distinguish apoplastic acidification from cytoplasmic acidification as the relevant pH change in acid growth?
A.
Elongation occurs when the cell-wall space is acidified while cytoplasmic pH remains largely unchanged
B.
Elongation occurs only after the cytoplasm becomes strongly acidic
C.
Cell expansion is abolished whenever the apoplast becomes acidic
D.
Auxin-induced elongation occurs only when vacuolar pH increases
Correct Answer:
Elongation occurs when the cell-wall space is acidified while cytoplasmic pH remains largely unchanged
Explanation:
The acid-growth hypothesis specifically assigns the critical pH change to the apoplast or cell-wall space, where low pH activates wall-loosening mechanisms.
10. A mutant plant has normal auxin perception but a plasma-membrane H+-ATPase that cannot be activated by auxin. Which outcome is most likely in its elongating cells?
A.
Auxin-induced apoplastic acidification and rapid elongation will be greatly reduced
B.
Auxin will cause normal wall loosening by directly cleaving cellulose
C.
Auxin will acidify the apoplast through increased vacuolar proton release only
D.
Auxin-induced elongation will increase because the wall remains more rigid
Correct Answer:
Auxin-induced apoplastic acidification and rapid elongation will be greatly reduced
Explanation:
In the acid-growth model, auxin must activate plasma-membrane proton pumps to lower apoplastic pH. A defective pump therefore interrupts the pathway before wall loosening and elongation.
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