Practical skill 2: Plant tissue in solutions of different concentration

Aim

To investigate the effect of the concentration of a sucrose solution on the mass of potato cylinders, and to find the concentration of solution that has the same concentration as the potato cell contents.

Background

Water diffuses through partially permeable membranes by osmosis, and water moves into and out of cells by osmosis through the cell membrane. In Extended terms, osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. The cell membrane of a plant cell is partially permeable.

When potato tissue is placed in a solution that is more dilute than the cell contents, there is a net movement of water into the cells by osmosis, so the cells swell, become firm (turgid) and the tissue gains mass. When it is placed in a solution that is more concentrated than the cell contents, water leaves the cells, so the cells become soft (flaccid) and the tissue loses mass.

There is a concentration of solution at which the mass does not change. At this concentration there is no net movement of water, so the solution has the same concentration (and water potential) as the cell contents. You can find it from a graph where the line crosses the x-axis.

Hypothesis

Potato cylinders will gain mass in dilute sucrose solutions and lose mass in concentrated solutions, because water moves into the cells from a dilute solution and out of the cells into a concentrated solution by osmosis.

Variables

Equipment

• Large potato; cork borer (about 1 cm diameter); white tile; scalpel or sharp knife; ruler graduated in mm
• Sucrose solutions of 0.0 (distilled water), 0.2, 0.4, 0.6 and 0.8 mol/dm³, 20 cm³ of each
• 5 large test-tubes (150 mm × 25 mm), labelled, in a rack; measuring cylinder, 25 cm³
• Balance reading to 0.1 g or better; paper towels; forceps
• Stop-clock; marker pen

Risk assessment

Method

1. Label five large test-tubes 0.0, 0.2, 0.4, 0.6 and 0.8 and use a measuring cylinder to put 20 cm³ of the matching sucrose solution in each.
2. Use a cork borer to cut 15 cylinders from the same potato. Push the borer into the potato on a tile, and push the cylinder out with a clean rod.
3. Using a scalpel and ruler, trim all the cylinders to the same length, for example 4.0 cm, and remove any potato skin.
4. Divide the cylinders into five groups of three. Blot each group dry with a paper towel.
5. Measure the mass of each group of three cylinders to 0.1 g and record it as the initial mass.
6. Put one group into each test-tube, making sure the cylinders are covered by the solution. Start the stop-clock.
7. Leave the tubes in a rack at room temperature for 60 minutes.
8. After 60 minutes, remove the cylinders from one tube, blot them dry in the same way, and measure their mass. Record it as the final mass. Repeat for each tube in the order they were started.
9. Feel the cylinders and bend one gently. Record whether they are firm, slightly soft or floppy.
10. Calculate the change in mass and the percentage change in mass: (final mass − initial mass) ÷ initial mass × 100.
11. Plot a graph of percentage change in mass against concentration. Use the graph to find the concentration at which there is no change in mass.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Plot change in mass / % (y-axis, with positive and negative values) against concentration of sucrose solution / mol/dm³ (x-axis). Mark points with crosses (×) and draw a single smooth best-fit curve. Read the value where the curve crosses the x-axis (0% change): this is the concentration of the solution with the same concentration as the potato cell contents. Read the value to the nearest half small square.