Practical - The effect of salt or sugar concentration on plant tissue

Scientists investigate the effects of osmosis on living cells. They either:

  • observe, with a microscope, cells or tissues placed in solutions of different concentration
  • measure changes in mass or length of cylinders or discs of fresh potato or beetroot immersed in solutions of different concentrations.

The following experiment investigates the effect of different concentrations of sucrose solution on potato tissue. It could also be carried out using salt - sodium chloride solution - instead of sucrose.

Aims

  • To investigate the effect of a range of concentrations of sucrose solutions on the mass of potato cylinders.
  • To determine the concentration of the cell sap of potato cells.

Method

Prepare a range of sucrose solution concentrations. The concentration of a solution is measured in moles per cubic decimetre written as mol dm-3. For example, in this experiment your range could be from 0.2 mol dm-3 to 1.0 mol dm-3.

A 1.0 mol dm-3 solution of sucrose will contain up to 342 g of sucrose per dm-3. A 1.0 mol dm-3 solution of a substance contains one mole of the substance per dm3 of a solution, or one mole per litre of solution.

Set up a series of boiling tubes each containing a different concentration of sucrose solution. Also, set up one containing distilled water. This will have a concentration of sucrose of 0.0 mol dm-3 and will act as the control in the experiment.

Make sure each tube is labelled with the concentration.

Prepare a blank results table before you begin. Use a cork borer to cut a number of potato cylinders from the same tuber, and cut them to a similar length using a scapel. Make sure when weighing the potato cylinders, that their masses are not mixed up when recording them. Each cylinder will have a different mass before and after the investigation.

  1. Place each potato cylinder into a different sugar solution for 40 minutes.
  2. Remove the potato and record its new mass.
  3. Calculate percentage changes in mass for all potato cylinders.

For each sucrose solution concentration, repeat the investigation for several potato cylinders. This increases your confidence in the results - different potato cylinders might not behave in the same way. Making a series of repeat experiments means that any anomalous results can be identified and ignored when a mean is calculated.

Risks

  • Make sure that the potato is placed on a ceramic tile when using the cork borer - do not cut the potato cylinders towards your hand.
  • Care must be taken when using the scalpel.
  • Wear eye protection when using chemical solutions.

This experiment shows the effect of osmosis on plant tissue. The cylinders will decrease or increase in mass if they lose or gain water by osmosis. Plot a graph of your results, with the percentage change in mass on the vertical axis, and sugar solution concentration on the horizontal axis. Find the concentration of sucrose solution when the percentage change is zero. This will be the equivalent concentration of the cell sap of the potato cells.

The effects on plant tissues at a cellular level can be observed using a microscope.

Another way of looking at osmosis in plant cells is to mount a piece of onion skin, or beetroot on microscope slides in drops of different concentrations of sucrose or salt solutions. Observe the cells for a few minutes. It is easy to see plasmolysis in beetroot because the cell sap is red.