Answer: Lowers water potential so water enters.
- A Raises guard-cell water potential
- B Removes all guard-cell water
- C Lowers water potential so water enters
- D Bursts the guard cells open
Correct answer: C. Lowers water potential so water enters
Explanation: K<sup>+</sup> entering guard cells lowers their water potential; water then flows in, the cells swell and the pore opens.
Water can travel through cell walls (apoplast) or cell-to-cell through the cytoplasm (symplast) - but the Casparian strip at the endodermis blocks the apoplast route entirely, forcing everything through a selectively permeable membrane (symplast) before it can enter the xylem.
Concept context
Water and mineral absorption, the ascent of sap, transpiration, and phloem transport via the pressure flow hypothesis.