Required practical 2: Osmosis in potato cylinders

Aim

To find how the concentration of a sugar solution affects the mass of potato tissue, and to estimate the concentration inside the potato cells.

Background

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. The cell membrane of a plant cell is partially permeable. It lets small water molecules through but not larger sugar molecules.

If a plant cell is in a solution that is more dilute than its cytoplasm, water moves into the cell by osmosis and the tissue gains mass. If the solution is more concentrated than the cytoplasm, water moves out of the cell and the tissue loses mass. If the solution is the same concentration as the cytoplasm, there is no net movement of water and the mass does not change.

Because the starting masses of the cylinders are never exactly the same, the change in mass is given as a percentage change. The concentration of the sugar solution is measured in mol/dm³.

Hypothesis

As the concentration of the sugar solution increases, the percentage change in mass of the potato cylinders will decrease, because less water moves into the cells by osmosis and more water moves out.

Variables

Equipment

• Large potato
• Cork borer (about 8 mm across), white tile and ruler
• Knife or scalpel (used by the teacher or with care on the tile)
• 6 boiling tubes (or large test tubes) and a rack
• 1.0 mol/dm³ sucrose solution and distilled water
• 2 measuring cylinders or syringes (10 cm³ and 25 cm³)
• Top-pan balance (accurate to 0.01 g)
• Paper towels, forceps and a marker pen
• Stopwatch or clock
• Eye protection

Risk assessment

Method

1. Label six boiling tubes 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 (the concentration in mol/dm³).
2. Use the 1.0 mol/dm³ sucrose solution and distilled water to make 20 cm³ of each concentration (for example 0.4 mol/dm³ is 8 cm³ of sucrose solution and 12 cm³ of distilled water). Put each into the correct tube.
3. Push the cork borer through the potato to cut six cylinders. Cut the skin off the ends.
4. Cut each cylinder to the same length (for example 3.0 cm) using a ruler and a knife on the tile.
5. Blot each cylinder gently with a paper towel to remove the surface liquid. Weigh each cylinder one at a time and record its initial mass in the table.
6. Put one cylinder into each tube so that it is completely covered by the solution. Note the time.
7. Leave the tubes in a rack at room temperature for the same length of time, ideally overnight (about 24 hours). If the practical must fit in one lesson, leave them for at least 30 minutes; the changes in mass will be smaller but the pattern is the same.
8. Use forceps to take out each cylinder. Blot each one dry in the same way as before and weigh it again. Record the final mass.
9. Calculate the change in mass for each cylinder: final mass − initial mass.
10. Calculate the percentage change in mass for each cylinder: (change in mass ÷ initial mass) × 100. A loss in mass has a negative sign.
11. Repeat the whole experiment at least twice more (or use three cylinders for each concentration) and calculate a mean percentage change for each concentration.
12. Plot a graph of mean percentage change in mass against the concentration of sucrose solution.
13. Draw a line of best fit through the points. Find the concentration where the line crosses the x-axis, because this is where the mass does not change.

Results

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

Drawing the graph

Plot a line graph. Put the concentration of sucrose solution (mol/dm³) on the x-axis and the percentage change in mass (%) on the y-axis. The y-axis must go below zero to include negative values, and the x-axis crosses at 0 on the y-axis. Plot each point with a small cross and draw a smooth line of best fit. Read off the concentration where the line crosses the x-axis: here there is no change in mass.