Osmosis in potato cylinders
Change the sucrose concentration outside the cells and watch water move in or out. Find the concentration at which nothing changes.
- Osmosis
- Partially permeable membranes
- Turgid and flaccid cells
Change in mass
+21.0%
Net water movement
into cells
Cell state
turgid
Predict
At which external concentration will the cylinder show no change in mass?
Experiment
- 1Set the external concentration to 0.00 and note the direction of the arrows and the change in mass.
- 2Raise the concentration slowly and watch the mass change fall towards zero.
- 3Find the concentration at which the mass change is zero, and read it off.
- 4Go past that point and confirm the arrows reverse and the cylinder becomes flaccid.
Explain
Osmosis is the net movement of water molecules from a region of higher water concentration to a region of lower water concentration, through a partially permeable membrane. The membrane lets water through but not sucrose.
In dilute solution the water concentration outside is higher, so water enters, the vacuole swells and the cell becomes turgid. In concentrated solution the water concentration outside is lower, water leaves, and the cell becomes flaccid. If water keeps leaving, the membrane pulls away from the cell wall and the cell is plasmolysed.
The point of zero change is the useful one: it tells you the concentration inside the cells without ever measuring it directly.