Aim
My aim is to work out the concentration in potato cell sap. I am to find the increase/decrease in weight of the potato after the experiment is complete using the knowledge I have of osmosis.
Osmosis
Osmosis is the passage of water molecules from a weaker solution to a stronger solution through a partially permeable membrane.
Before Osmosis After osmosis
Weak strong Water
solution solution molecules
Partially permeable Partially permeable
Membrane Membrane
The water diffuses from the weaker solution (high water concentration) into the stronger solution (low water concentration). Each cell is surrounded by a cell membrane, which has tiny holes in it, which gives access for small molecules to pass through. The large molecules cannot pass through because they are too big for the partially permeable membrane.
Equipment
- Potato
- Standard sugar solution = 1
- Sugar solutions of the following concentrations
1/32 1/16 1/8 1/4 1/2
- Water = concentration 0
- 7 test tubes and one test tube rack
- Cork borer – this enables you to cut samples of potato in the same diameter
- Scalpel
- Ruler
- White tile
-
25 cm3 measuring cylinder
- Top pan balance
- Paper towels
Diagram
Method
The equipment needed to carry out the experiment was set up according to the diagram. Seven test tubes were labelled with concentration levels and placed in order, in the test tube rack. 10ml of each sugar solution (1/32, 1/16, 1/8, 1/4, 1/2) was placed into the labelled test tubes; 10ml of 1/32 sugar solution into the tube labelled 1/32 and so on. After this was completed seven pieces of potato were cut out with a cork borer, and then cut to a length of 1.5 cm (15mm). The pieces of potato were then weighed separately, the weights were recorded and then the potatoes were placed into the test tubes. The time was noted and each potato piece was left in the solution for 30 minutes. After this time, the solutions were poured away and each potato piece was taken out of the tubes and weighed separately. The weights were recorded and were looked at and compared with the original weights of the potatoes. The experiment was repeated for a fair test.
Prediction
My prediction is that I know there are different strengths of solutions in the test tubes, so the potato weights will vary. The potato that is in the stronger solution will decrease in weight because the water has passed out into the strong solution (osmosis). This shows that the potato in the distilled water (weak solution) will increase in weight because the water enters the potato cell, as it is stronger than the distilled water.
Fair test
Accurate results can be obtained if a fair test is followed. Independent, dependant and control variables must be observed.
Independent variable
To ensure a fair test is followed equal amounts of solution must be used. The solution concentration levels that were used, started from a 1 molar concentration to a 0 molar concentration.
Dependant variable
The weights and measurements before and after will change. A percentage difference will be able to be calculated from the before and after results.
Control variable
The potato weights and measurements must be equal and the same potato must be used for both experiments, to make it fairer. Each piece of potato should be placed in the solution at the same time.
Secondary sources
Biology for you – textbook
From this textbook I researched information about osmosis and the model cell experiment.
Diagrams have also been taken from the same textbook.
Results
Experiment 1
Experiment 2
Using these results I am able to plot these values on a line graph, where the x-axis is concentration and the y-axis is percentage change in weight. A line of best fit is included in both graphs; the concentration of potato cell sap can be discovered from this.
