Osmosis rate decreases with smaller water potential gradient

Steps:

• Observe data: As external water potential decreases from -100 kPa to -500 kPa, vacuole emptying rate drops from 20/min to 6/min, indicating less frequent emptying.
• Recall osmosis: Water moves from higher to lower water potential; cell cytoplasm has fixed, higher water potential than external solutions.
• Calculate gradient: More negative external potential narrows the difference (gradient) between cell and solution.
• Conclude: Smaller gradient means slower water entry, reducing vacuole filling and emptying frequency.

Why D is correct:

• Matches osmosis principle: Water potential gradient (ψ_cell - ψ_external) decreases as external ψ lowers, slowing net water influx into the cell.

Why the others are wrong:

• A: Incorrectly states gradient increases, opposite of data showing reduced emptying.
• B: Falsely predicts more rapid water entry and frequent emptying, contradicting observed decrease in rate.
• C: Describes decreased gradient accurately but omits effect on water movement and emptying frequency.

Final answer: D