Which of the following is not an accurate statement regarding the acid-growth hypothesis?
a) Expansin pushes the cellulose microfibrils apart
b) Expansin is activated by a low pH
c) Auxin activates the ATPase
d) Cell wall enzymes cleave the cellulose cross-linkages
e) The ATPase acidifies the cytoplasm
The correct answer and explanation is:
The correct answer is e) The ATPase acidifies the cytoplasm.
Explanation:
The acid-growth hypothesis explains how plant cells elongate in response to auxins, which are plant hormones. According to this hypothesis, the growth of cells is facilitated by changes in the pH of the cell wall, allowing it to loosen and stretch.
Here’s how the other options relate to the acid-growth hypothesis:
- a) Expansin pushes the cellulose microfibrils apart: Expansins are proteins that play a key role in the acid-growth hypothesis. They function by loosening the bonds between cellulose microfibrils, allowing them to slide apart. This process is crucial for cell wall expansion, as the wall needs to become more flexible for the cell to grow.
- b) Expansin is activated by a low pH: Expansin’s activity is pH-dependent. A lower pH, often achieved by proton pumping into the cell wall, activates expansins. This helps to loosen the cellulose network and facilitates the expansion of the cell wall.
- c) Auxin activates the ATPase: Auxin (specifically indole-3-acetic acid or IAA) stimulates the activity of a proton pump known as H+-ATPase in the plasma membrane. This pump expels protons (H+) into the surrounding cell wall, acidifying the wall and creating the conditions necessary for expansins to function.
- d) Cell wall enzymes cleave the cellulose cross-linkages: Acidification of the cell wall also activates enzymes that cleave the cross-linkages between cellulose microfibrils and other cell wall components, further facilitating wall loosening and cell expansion.
However, option e is incorrect because the ATPase does not acidify the cytoplasm; it acidifies the cell wall by pumping protons (H+) into the extracellular space, not the cytoplasm. The cytoplasm remains relatively neutral in pH during this process. Therefore, option e does not accurately describe the mechanism of the acid-growth hypothesis.