Tonicity and Osmoregulation

AP Biology· difficulty 3/5

Red blood cells were placed in three saline solutions and observed under a microscope. The table below records cell appearance after 5 minutes.

Solution % NaCl Cell shape A 0.9 normal B 0.2 lysed C 3.0 crenated

Which interpretation is supported?

  • A

    Solution B is hypotonic and Solution C is hypertonic to the cytoplasm.

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  • B

    Crenation in C indicates water entry.

  • C

    Solution A is hypertonic; cells lose water.

  • D

    All three solutions are isotonic to the cytoplasm.

Explanation

Cell lysis in dilute (0.2%) saline indicates net water entry (hypotonic external). Crenation in concentrated (3%) saline indicates net water loss (hypertonic external). 0.9% is isotonic.

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