Osmosis in potatoes
Cylinders or discs of fresh potato are often used to investigate The movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration. in living cells. To carry out this type of experiment, you need to:
- cut equal-sized pieces of potato
- blot with tissue paper and weigh
- put pieces into different concentrations of sucrose solution for a few hours
- remove, blot with tissue paper and reweigh
The percentage change in The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). can be calculated for each piece of potato.
\(\text{percentage change in mass} = \frac{\text{end mass - start mass}}{\text{start mass}} \times 100\)
Example one
A piece of potato has a mass of 2.5 g at the start and 3.0 g at the end.
percentage change in mass = (3.0 – 2.5) ÷ 2.5 × 100
= 0.5 ÷ 2.5 × 100
= +20%
The plus sign shows that it has gained mass. It will have gained water by osmosis.
Example two
A piece of potato has a mass of 2.5 g at the start and 2.0 g at the end.
percentage change in mass = (2.0 – 2.5) ÷ 2.5 × 100
= –0.5 ÷ 2.5 × 100
= –20%
The minus sign shows that it has lost mass. It will have lost water by osmosis.
Question
Rachel carried out an experiment to investigate osmosis in potatoes. She placed five 3 cm cylinders of potato in five test tubes which contain the same concentration of salt solution.
This is a section of the results table. Calculate the most accurate arithmetic mean from the results given.
Percentage change in potato mass.
| Test 1 | Test 2 | Test 3 | Test 4 | Test 5 |
| 16 | 16 | 15 | 34 | 19 |
| Test 1 | 16 |
|---|---|
| Test 2 | 16 |
| Test 3 | 15 |
| Test 4 | 34 |
| Test 5 | 19 |
The most accurate arithmetic mean can be calculated like this.
(16 + 16 + 15 + 19) ÷ 4 = 16.5
Accurate means close to the true value. The 34 per cent change in mass in Test 4 is an obvious A measurement that is very different from other measurements taken under the same circumstances in an experiment., and should not be included in the calculation of the mean to produce the most accurate result.
Example three
A graph of change in mass (vertical axis) against concentration of sucrose (horizontal axis) has been plotted and the results are shown here.
Where the line crosses the horizontal axis at 0% change in mass, the sucrose concentration is equal to the concentration of the contents of the potato cells. Therefore, the concentration of the water in the outer solution is equal to the concentration of the water in the cells. There is no net movement of water by osmosis.
Question
Describe and explain the relationship shown in the graph.
At low concentrations of sucrose the percentage change in mass increases as water has moved into the potato cells by osmosis from a region of high water concentration in the solution to lower water concentration in the potato cells, causing the gain in mass.
As the concentration of the sucrose solution increases, then the percentage change in mass becomes negative as water is leaving the cells by osmosis, from a region of higher water concentration in the potato cells to lower water concentration in the solution, causing the loss in mass.
