Practical - Effect of concentration on the mass of 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 cylinders or discs of fresh potato or beetroot. Cylinders will have a larger mass than discs, so scientists will have larger measurements to work with.

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

Aims of the experiment:

  • to investigate the effect of a range 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 (a sugar) solutions. 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 with each of these solutions. 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.

Carry out the investigation as shown below. Prepare a blank results table before you begin. 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.

For each sucrose concentration, repeat the investigation for several potato cylinders. This allows you to make the experiment more repeatable – 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.

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 sugar or salt. Observe the cells for a few minutes. It is easy to see the process of plasmolysis in beetroot because the cell sap is red.