Answer: H+ ATPase pump exports H+ (protons), creating electrochemical gradient for K+ influx.
- A Ca<sup>2+</sup> influx into guard cells that triggers turgor loss and pore closure
- B H+ ATPase pump exports H+ (protons), creating electrochemical gradient for K+ influx
- C Cl- efflux alone, which lowers guard cell osmotic potential without affecting K+
- D Na+ influx through mechanosensitive channels on the guard cell membrane
Correct answer: B. H+ ATPase pump exports H+ (protons), creating electrochemical gradient for K+ influx
Explanation: Stomatal opening: light activates H+ ATPase in guard cells. H+ efflux hyperpolarizes membrane. K+ enters through channels. Water follows by osmosis, increasing turgor.
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.