- I will fill two boiling tubes each with 1, 0.75, 0.5, 0.25 and 0mol concentrations of sucrose solution (30ml) using the syringes and the measuring cylinder. The boiling tubes should be labelled so that it is clear what concentration of sucrose is contained within them.
- 15 potato pieces and 15 swede pieces will be cut using the knife and cork borer. The borer will be able to strips of vegetable and the knife will be used to cut the strips into 2cm lengths.
- All the pieces will have their masses recorded and then three pieces of potato should be placed into one set of boiling tubes and three pieces of swede should be put into another set of boiling tubes. It should be noted which piece is at the bottom, middle or top of the tube so that there is no confusion when it comes to re-weighing the vegetable pieces.
- Leave the vegetable in their solutions for 40mins to allow for all the changes of osmosis to take place. They should then be dried slightly so that the mass of any excess solution isn’t recorded and the mass of the vegetable pieces should be recorded for a second time.
A percentage change in mass will be calculated for each potato piece. This will allow me to at least make an estimation of the concentration of the isotonic sucrose solution so that I can find the water potential of the two vegetables.
This is a table showing how to make up the sucrose solutions using the 1mol sucrose solution and distilled water.
I have done a similar investigation into the sucrose concentration of potato so I know that this method works and that the range of sucrose solutions being used should at least be wide enough to find the isotonic solution of the potato. Below is a table showing the results of the previous experiment. The percentage of sucrose in the potato was calculated as being 18%. I have extended the range of sucrose solution higher than this because I believe that the swede will contain more sucrose (as stated in my prediction below).
Diagram of Apparatus.
Fair Testing.
All the potato pieces and swede pieces will be in the solutions for the same amount of time so that it’s a fair test. The temperature of the solutions and the size of the vegetable pieces will also be kept the same, as will all other variables that may affect the percentage change in mass. By putting three vegetable pieces into each boiling tube I will be able to find an accurate average result for the percentage change in mass for each vegetable in each of the five different solutions. Having a range of five solutions I will be able to make sensible conclusions as to what is happening because I will be able to look at a suitable range of data.
Safety.
Care will be taken when using the knife and when using glass wear. The ceramic tile will be used as a cutting board so that the knife does not scratch the work surfaces.
Prediction.
I predict that the swede will have the highest concentration of sucrose and therefore the lower water potential of the two vegetables. The reason for this is that swede is a sweet root vegetable where as potato is not. Sucrose is a sugar present in root vegetables and has a sweet taste, so I think that the swede will contain the most sucrose. This will mean that more of the five sucrose solutions will be hypotonic towards the swede and that the isotonic solution will be of greater sucrose concentration than the isotonic solution of the potato.
Results.
A table showing the results for the potato pieces.
A table showing the results for the swede pieces.
I calculated the percentage change in mass for each piece so that I could find the average percentage change of the vegetable pieces in a particular concentration of sucrose solution. I plotted my results on a graph (overleaf) to find the concentration o the isotonic solution for each vegetable. I then found the water potential of the vegetables by referring to a graph showing water potential (kPa) at various sucrose solution concentrations.
Potato = -1200 kPa (0.425mol isotonic solution)
Swede = -2300 kPa (0.725mol isotonic solution)
Conclusions.
As I had predicted the swede had the lower water potential. This is because it contains more sucrose that the potato, which means that water is less likely to move out of the swede cells at a particular sucrose solution concentration than it would out of potato cells. The graph shows that the concentration of the isotonic solution for the potato is 0.425 mol and for the swede it is 0.725 mol, which the reference graph shows as being a water potential of –1200kPa for the potato and –2300kPa for the swede. Another point made in my prediction, which has been proved correct by my experiment, is that out of the five sucrose solutions, more were hypotonic solutions for the swede than for the potato. This means that the swede gained mass in more of the solutions because they were at a higher water potential than the potato. In osmosis water moves from an area of high water potential to an area of low water potential and because the swede has a lower water potential more of the solutions had a higher water potential compared to the swede (ie were hypotonic).
Evaluation.
The procedure that I used was suitable for the experiment. A good range of accurate results was obtained which allowed me to make sensible conclusions. All the pieces were cut from the same potato or swede so that they started with the same sucrose concentration within them.
However there were a few limits to the experimental method. The vegetable piece sizes were not all identical. Although I tried to get around this problem by calculating the percentage change in mass there were still some anomalous results possibly caused by the inaccuracies in size, the most obvious of which being the result for the 0mol concentrated sucrose solution for the swede. Another problem was that the vegetable pieces touched end to end which meant that some pieces had a higher exposed surface area than others. The time the pieces of vegetable were in the boiling tubes for was greater (30mins) than the method suggests they should be (though this shouldn’t matter too much as it applied to all the pieces so a fair test was still maintained).
The apparatus used gave accurate results. The measuring out of solutions could have caused some inaccuracy but the syringes were very precise so generally there was very little error arising from any measurements of solutions made. Cutting the vegetables would have been the major source of error in the procedure. The sizes of the vegetable pieces varied by 2mm either side of the 2cm length they were supposed to have. Weighing the pieces of vegetable was done accurately using electronic scales so this would have caused little error.
Although any sources of error are significant, the reliability of results is still high because I had three results for each concentration for each vegetable. This meant that I could calculate averages from results that were only very slightly inaccurate and still be able to make good conclusions from them.
The procedure could be improved in several ways. Each vegetable piece should have its own boiling tube so that each piece has the same exposed surface area. This would make the test a little fairer. The solutions should be measured out in large quantities and the boiling tubes should then be filled from the large amounts of solution made up. This would eliminate any slight error occurring from one concentration of solution for one vegetable being slightly different from the concentration of the same solution for the other vegetable. A final change to the procedure that I would make to solve the problems arising when cutting the vegetable pieces would be to use some sort of jig. This would mean that each piece is identical in size and is best described using a diagram:
The potato numbers refer to the order of the pieces in the boiling tube; 1=bottom piece, 2=middle piece and 3=top piece.
The swede numbers refer to the order of the pieces in the boiling tube; 1=bottom piece, 2=middle piece and 3=top piece.