Tonicity and Osmoregulation

AP Biology· difficulty 3/5

A red blood cell (cytoplasm ~0.9% NaCl) is placed in three solutions: distilled water, 0.9% saline, and 5% saline. Microscopy is used to record cell shapes after 5 minutes. The cells in distilled water rupture; cells in saline keep shape; cells in 5% saline crenate.

Hypotonic cell swells/lyses Isotonic no net change Hypertonic cell shrinks/crenates

Pure distilled water is BEST classified as which environment for the red blood cell?

  • A

    Acidic - protons enter and damage the cell

  • B

    Isotonic - no net water movement

  • C

    Hypertonic - water leaves, shrinking the cell

  • D

    Hypotonic - water flows in, swelling and lysing the cell

    check_circle

Explanation

Distilled water has nearly zero solute compared to the cytoplasm, so water enters the cell osmotically until the membrane bursts.

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