The osmosis trap: hypertonic vs hypotonic
A free TEAS® chemistry explainer with an original practice question
In osmosis, water crosses a membrane toward the side with more dissolved solute. The solute itself usually stays put; it is the water that moves.
Tonicity describes the solution outside the cell compared with the inside. In a hypertonic solution there is more solute outside, so water leaves and a red blood cell shrinks. In a hypotonic solution there is less solute outside, so water enters and the cell swells and can burst. In an isotonic solution there is no net movement of water, so the cell keeps its size.
A quick way to remember it: water follows solute. Find where there is more solute, and the water heads that way.
Try a sample question
A red blood cell is placed in a hypertonic salt solution. The solution has more dissolved salt than the inside of the cell. What happens to the cell?
- A.It swells, because water moves into the cell
- B.It swells, because salt moves into the cell
- C.It stays the same size, because water moves in and out equally
- D.It shrinks, because water moves out of the cell
Show the answer
D. It shrinks, because water moves out of the cell
It shrinks, because water moves out of the cell. In osmosis, water moves across the membrane toward the side with more dissolved solute. A hypertonic solution has more solute than the cell, so water flows out and the cell shrinks.
Why the other answers tempt:
- A. Water moves toward the side with more dissolved salt. Here that is outside the cell, so water leaves. A cell swells in a hypotonic solution, not a hypertonic one.
- B. Osmosis is the movement of water, not salt. The cell membrane keeps most of the salt out.
- C. Water moves in and out equally only in an isotonic solution, where the salt levels match. Here they don't.
Water follows salt. Hyper means more salt outside, so water leaves and the cell shrinks.
Checked against OpenStax Biology 2e, 5.2 Passive Transport. Original question, not from a real test.
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