Investigate the Effect of different Concentrations of Sucrose Solution on Potato Sticks

* scalpel

* 1 mole sucrose solution

* measuring cylinder

* distilled water

* boiling tube rack

Method

Using one molar sucrose solution make up different concentrations by pouring the relevant amount of distilled water into the mixture e.g. for 0.50 moles pour 50ml of one molar sucrose solution into measuring cylinder and make up to 100ml with distilled water. Pour 20ml of each solution into separate boiling tubes and stand in boiling tube rack. Prepare potato sticks by using cork borer. Make 15 potato sticks of similar lengths. Cut ends of each potato stick off with scalpel on tile, keeping them roughly the same length. Measure each potato stick with calipers and group into 5 sets of 3. For second potato stick in each group put a line on one end and for third mark a cross in order to distinguish them. Put bungs in boiling tubes when all potato sticks are in. After at least an hour remove potato sticks carefully from boiling tubes using tweezers. Don't mix the groups up. Place them on a paper towel and measure each one again with calipers and record results.

Caution:

* Be careful when using cork borer and always cut downwards towards a hard surface e.g. a tile

* Always cut on a hard surface when using a scalpel

Diagram

Preliminary work

Concentration of sucrose

solution (moles)

Length of potato

sticks (mm)

% Change

Before

After

0.00

40.95

42.00

2.5

0.25

40.50

41.00

.2

0.50

40.50

40.75

0.6

0.75

40.35

40.00

-0.9

.00

40.00

39.70

-0.8

By doing this experiment I have worked out the range of concentrations of sucrose solution I shall use. I have chosen to measure the lengths of the potato sticks instead of weighing them as it is easier and there would be problems with the water lost from the potato sticks if I were to weigh them. I have also decided to keep the potato sticks submerged for at least an hour as this will allow them to show a significant change in water loss or uptake. I will have three potato sticks for each concentration in the main experiment as this will allow me to obtain more reliable results because I will be able to find out the average percentage change. I am putting bungs into the boiling tubes whilst the potato sticks are submerged as this will prevent evaporation of the solution which is important as each boiling tube must have the same measurement of liquid in it to make it a fair test. Also another factor to control is the type of potato used and the cylinders must be from the same potato preferably as they may have different concentrations of solution in them.

Prediction

I predict that as the concentration of sucrose solution increases the length of the potato sticks will decrease. This is because the water in the potato sticks moves from a higher concentration to a lower one i.e. from the strong solution of water in the potato sticks to the weaker solution of water surrounding it. As osmosis occurs the potato stick will shrink in length as the water in the potato cells diffuses into the surrounding solution, making the potato stick flaccid. Similarly the potato sticks in the weaker solution will become turgid as water is taken in from the higher concentrated water solution to the less concentrated water solution inside the potato sticks, creating turgid cells as the cytoplasm becomes pressed against the cell walls and a slightly longer potato stick because of the saturated cells.

Concentration of

sucrose solution (moles)

Length

of

potato

sticks

(mm)

%

Change

Average % Change

Before

After

2

3

2

3

2

3

0.00

35.45

35.70

35.00

36.90

37.15

36.10

4.09

4.06

3.14

3.76

0.25

35.40

36.00

35.50

36.60

37.00

36.35

3.38

2.77

2.39

2.84

0.50

35.15

35.00

35.60

35.00

34.35

35.05

-0.42

-1.85

-1.54

-1.27

0.75

36.50

36.90

37.00

36.40

36.40

36.50

-0.27

-1.35

-1.35

-0.99

.00

37.00

37.15

37.10

35.00

34.85

34.50

-5.40

-6.19

-7.00

-6.19

Analysis

The graph shows that as the concentration of the sucrose solution increases the average length of the potato sticks decreases. This is because as the number of solute particles increases and the solution becomes more concentrated, there a fewer water molecules making it less concentrated in water molecules terms than inside the potato stick. Therefore as water moves from a higher concentration to a lower one through a partially permeable membrane, a process called osmosis, the water molecules in the potato stick cells are transferred into the solution. This loss of water makes the cells in the potato stick plasmolysed and leaves it flaccid and consequently shorter than it was originally.

This diagram demonstrates the idea of less water molecules being present in a concentrated solution, which makes the solution in the potato sticks more concentrated than that around it in terms of water molecules being present.

It is also true that as the concentration of the sucrose solution becomes more dilute, the potato sticks increase in length and become turgid. This is because in a dilute solution there are fewer solute particles but more water molecules, so the concentration of solution in the potato sticks in terms of water molecules is weaker than that surrounding it as there are less water molecules present.

This diagram shows what happens to the potato stick when it is placed in a dilute solution.

At 0.75moles of sucrose solution the change in length is very small. This may be because 0.75moles is similar to the concentration inside the potato stick cells so very little diffusion takes place and the potato stick stays the roughly the same length as it was before.

Conclusion

I therefore conclude that I was correct in predicting that as the concentration of sucrose solution increases the length of the potato sticks will decrease because water moves from a higher concentration to a lower one, a process called osmosis.

Evaluation

This experiment was relatively easy to undertake although I found using the calipers the most difficult process, however it allowed me to obtain very accurate and reliable results.

On the graph I have identified an anomalous result at 0.75moles. I believe that it has occurred at this point as I changed the potato I had because I could no longer make any more cylinders out of the original one. This change may have upset the trend of the results because this new potato may have had a different concentration of substance inside of it when compared to the original potato. Therefore I suggest that to make this experiment more reliable I could do the same experiment again using only one potato.

To investigate this topic further I could also test some more concentrations of sucrose solution, especially between 0.5moles and 0.75moles as I have already established that at around these concentrations the percentage change in the length of the potato sticks is small, indicating that this is near to the concentration of solution inside the potato stick cells. By looking further into this area I could find the exact concentration of the potato cells as this would be where it crosses the horizontal axis.

Lottie von Bertele