Tonicity and Osmoregulation

AP Biology· difficulty 3/5

Dialysis tubing bags filled with distilled water were placed into beakers containing 0, 5, 10, 15, and 20 percent sucrose solutions. Mass was recorded every 15 minutes for one hour.

Time (min) Mass change (g) 0% 10% 15% 20%

Which conclusion is best supported?

  • A

    Sucrose passes freely across the dialysis membrane.

  • B

    Mass change is independent of solute concentration.

  • C

    Water moves from regions of low solute to high solute against a gradient.

  • D

    The rate of water uptake increases with the sucrose concentration of the surrounding solution.

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Explanation

Bags in higher-percent sucrose gain more mass per unit time because the steeper water-potential gradient drives faster osmotic water entry into the bag.

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