To predict what happens to a cell, compare the solute concentration outside the cell with the concentration inside. Water moves by osmosis toward the side with more solute.
This lesson is part of movement across membranes in SPM Biology.
What is the three-step routine?
- Compare: is the outside solution hypotonic, isotonic or hypertonic to the cell?
- Direction: water moves toward the higher solute concentration.
- Result: name what happens to this type of cell, plant or animal.
| Solution | Water movement | Plant cell | Animal cell |
|---|---|---|---|
| Hypotonic | Into the cell | Turgid | Swells, may burst |
| Isotonic | No net movement | Stays as it is | Stays as it is |
| Hypertonic | Out of the cell | Plasmolysed | Shrinks |
Worked example: onion cells in three solutions
An original experiment. Onion epidermal cells have cell sap equal to 0.30 M sucrose. They are placed in 0.10 M, 0.30 M and 0.60 M sucrose solutions.
In 0.10 M, the solution is hypotonic because 0.10 is below 0.30. Water enters the cell, the vacuole expands and the cell becomes turgid.
In 0.30 M, the solution is isotonic. There is no net movement of water, so the cell looks unchanged.
In 0.60 M, the solution is hypertonic. Water leaves the cell, the membrane pulls away from the wall, and the cell is plasmolysed.
The mistake that reverses the answer
A common slip is to write “water moves from the higher concentration to the lower concentration” without saying concentration of what. A student then writes that water leaves the cell in 0.10 M because “0.10 is lower”.
Name the solute concentration every time. Water moves into the region of higher solute concentration, which is the cell in 0.10 M, since the cell sap is 0.30 M.
Why do plant and animal cells differ?
An animal cell has no cell wall. In a strongly hypotonic solution it keeps taking in water and may burst.
A plant cell has a wall, which stops it bursting. It becomes turgid instead, and turgor helps support non-woody plants.
A self-check question
A red blood cell with contents equal to 0.9% salt solution is placed in 2.0% salt solution. Predict and explain.
Answer
The outside solution has a higher solute concentration, so it is hypertonic to the cell. Water moves out of the cell by osmosis, and the cell shrinks. Compare: 2.0% is above 0.9%, so water follows the higher solute concentration.
Next, read explaining concentration gradients to link osmosis to the wider idea of gradients. The biological data interpretation trainer has data practice.
If your predictions still flip, a teacher in SPM Biology one-to-one tuition can ask you to explain each step aloud. You can start with the one-hour trial class (from RM50).