3.1.4Movement into and out of Cells
Osmosis Experiments — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 8
Part 1 of 4Osmosis in brief
Definition
- Definition Osmosis is the net movement of water molecules through a partially permeable membrane,
- from a dilute solution to a more concentrated solution.
- Water molecules move in both directions all the time; osmosis describes the overall (net) direction.
Why plant tissue is used
- Why plant tissue is used Root vegetables such as potato are cheap, easy to cut into identical pieces,
- and their cells are surrounded by partially permeable membranes.
- Changes in mass are easy to measure with a balance, so they give a simple measure of water movement.
Part 2 of 4Method: potato cylinders in sucrose solutions
1
Use a cork borer to cut several cylinders from the same potato, and trim them to the same length so they are as similar as possible.
2
Prepare a range of sucrose solutions, for example 0.0 (distilled water), 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm3, and pour equal volumes into labelled boiling tubes or beakers.
3
Blot each cylinder gently, measure its starting mass on a balance and record it.
4
Place one cylinder in each solution and leave all of them for the same time, typically 20 to 30 minutes.
5
Remove each cylinder, blot it dry with paper towel to remove surface liquid, and measure its final mass.
6
Calculate the change in mass for each cylinder, and the percentage change if the starting masses differ.
Part 3 of 4Variables and safety
| Type | Detail |
|---|---|
| Independent variable | Concentration of the sucrose solution |
| Dependent variable | Change in mass of the potato cylinder (or percentage change) |
| Control variables | Length and diameter of cylinders, same potato, volume of solution, temperature, time in solution, drying method |
| Safety | Cork borers and knives are sharp: cut on a tile, away from your hand. Wipe up spills so the floor is not slippery. |
Part 4 of 4Interpreting the results
Change in mass of plant tissue
Mass increases
- Mass increases Water has entered the cells by osmosis, so the surrounding solution was more dilute than the cell contents.
Mass decreases
- Mass decreases Water has left the cells by osmosis, so the surrounding solution was more concentrated than the cell contents.
No change in mass
- No change in mass There is no net movement of water because the two concentrations are equal.
- Water molecules still move in and out, but at the same rate in each direction.
Finding the balance point
- Finding the balance point Plot change in mass against concentration.
- Where the line crosses zero, the solution has the same concentration as the potato cells.
Key terms
| Term | Meaning |
|---|---|
| Osmosis | The net movement of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane. |
| Partially permeable membrane | A membrane that lets some molecules, such as water, pass through but not others. |
| Net movement | The overall difference between the movement in one direction and the movement in the opposite direction. |
| Percentage change in mass | (final mass − initial mass) ÷ initial mass × 100. |
| Control variable | A factor kept the same so that only the independent variable affects the result. |
Avoid these mistakes
1
Saying that no change in mass means water has stopped moving — water still moves both ways, but there is no net movement.
2
Forgetting to blot the cylinders before weighing, so extra liquid on the surface makes the mass look too high.
3
Using cylinders of different lengths without calculating percentage change, which makes the comparison unfair.
4
Describing the sucrose solution as having water 'concentration' in a confusing way — say whether the solution is dilute or concentrated.
5
Leaving the cylinders for different lengths of time in different solutions.
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