Biology · Introductory biology · Worked example
Predict osmosis in three solutions
A red blood cell’s cytoplasm holds about 0.3 mol of dissolved particles per liter. The cell is placed in (a) pure water, (b) 0.15 M NaCl and (c) 0.5 M NaCl. NaCl splits into two ions, and neither crosses the membrane. Which way does water move in each, and what happens to the cell?
Count the dissolved particles
Each NaCl gives Na⁺ and Cl⁻, so a solution holds twice as many particles as its NaCl concentration. 0.15 M NaCl gives 0.3 mol of particles per liter, and 0.5 M gives 1.0.
Pure water
Outside has no solute and the cytoplasm has 0.3: the water is hypotonic. Water moves into the cell, which swells and bursts.
0.15 M NaCl
Outside and inside both hold 0.3 mol of particles per liter: the solution is isotonic. There is no net movement of water, and the cell keeps its shape.
0.5 M NaCl
Outside holds 1.0, more than the 0.3 inside: the solution is hypertonic. Water moves out of the cell, which shrivels.
Result
(a) Hypotonic: water moves in, and the cell swells and bursts. (b) Isotonic: no net movement, and the cell keeps its shape. (c) Hypertonic: water moves out, and the cell shrivels.
Your turn
A plant cell is placed in a solution more concentrated than its cytoplasm. What happens to its central vacuole and its plasma membrane?
Show the answer and explanation
Water leaves; the vacuole shrinks and the membrane pulls away from the cell wall.
The solution is hypertonic, so water leaves by osmosis. The wall keeps its shape, so the shrinking cell separates from it, which is called plasmolysis.
Keep exploring
The membrane opens in Membrane transport with the cell in 0.5 M NaCl; change the two concentrations and watch the water reverse.
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