CORE PRACTICAL 4: Investigating osmosis in potatoes

Question 1

Aims of this practical:

Answer 1

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.)

Question 2

Method:

Answer 2

1- Prepare a range of sucrose solutions eg 0%, 20%, 40% and 100%.
2- Set up a series of boiling tubes with each of these solutions. The 0% sucrose solution will act as the control in the experiment. Make sure each tube is labelled with the concentration.
3- 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.
4- Dry a potato strip using a paper towel. Measure the mass of the potato cylinder.
5- Place the potato strip into the 0% solution for 20 minutes.
6- Remove the potato strip, dry it carefully using paper towel. Measure and record the mass of the potato strip.
7- Repeat for each sucrose concentration.
8- For each sucrose concentration, repeat the investigation for several potato cylinders.

Question 3

why should you repeat the investigation for several potato cylinders for each sucrose concentration? (2)

Answer 3

• this allows you to check the precision of your results
• results that are close together for the same concentration suggest that the results are precise
• the potato cylinders might not all 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.

Question 4

why does repeating the experiment aid in identifying anomalous results? (2)

Answer 4

• the potato cylinders might not all 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.

Question 5

Risks: (3)

Answer 5

• make sure that the potato is placed on a ceramic tile when using the cork borer – do not cut the potato cylinders.
• care must be taken when using the scalpel.
• wear eye protection when using chemical solutions.

Question 6

results of the experiment: (2)

Answer 6

• 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.

Question 7

equation for measuring the rate of water intake:

Answer 7

water intake in 1 hour = change in mass x (60 mins/ length of the experiment)

Question 8

what can you deduce from where the plotted line crosses the horizontal axis at 0 per cent change in mass?

how could this be identified on the graph?

Answer 8

the sucrose concentration of the solution is equal to the concentration of the contents of the potato cells.

This can be identified on the graph as the point which shows no net movement of water by osmosis, which would be represented by a change in mass.