Factors affecting Osmosis in Potato Tissue.

Safety Notes

Must be careful when using sharp knives and potato cutters, also basic lab rules must be followed to ensure a safe experiment.

Prediction

I predict that the pieces of potato in higher salt concentrations will lose weight and that the pieces of potato in no salt and small amounts of salt solution will gain weight. See the below for explanations.

Turgidity in Plant Cells

The osmotic potential of the cell sap is greater than that of the surrounding solution. There is a net movement of molecules into the cell.

Plasmolysis

The osmotic potential of the cell sap is less than that

of the surrounding solution. There is a net movement

of water molecules out of the cell.

The cell becomes limp or flaccid. As more water leaves

the cell the cytoplasm peels away from the cell wall.

The cell is plasmolysed.

As you can see above, salt is a bigger molecule than water, salt is to big a molecule to pass through the membrane of potato cells, where as water molecules are able to pass through. Based on this knowledge the pieces of potato in distilled water are going to have more water passing in than out but potato in high concentration salt is going to have more water going out than in, hence losing weight. This process of water molecular transfer is known as osmosis.

Results

concentration of salt(molar)

Original Mass(g)

End Mass(g)

Change in Weight(g)

% mass change

0

3.06

3.18

0.12

9.98

0

3.05

3.54

0.49

0.2

3.05

3.21

0.16

4.58

0.2

3.07

3.19

0.12

0.4

3.01

2.62

-0.39

-14.1

0.4

2.96

2.51

-0.45

0.6

3.04

2.59

-0.45

-14.36

0.6

3.09

2.66

-0.43

0.8

3.07

2.56

-0.51

-16.19

0.8

2.98

2.51

-0.47

3.01

2.31

-0.7

-21.31

2.95

2.38

-0.57

For Graph see previous page.

Conclusion

From the graph and results we can see that my prediction was correct items placed in higher concentrations of salt water lost weight where as lower concentrations gained weight. As you can see I have one anomalous result which is circled. From my results we can see that the potato placed in the lower concentrations of salt water gained weight, there is a higher concentration of salt in the potato and so water moves in by osmosis, as stated in my prediction. The potato placed in the higher concentrations of water lost weight as the water had a greater concentration of salt than the potato so water moved out by osmosis. This is what I predicted what would happen; there is a net flow of water either in or out of the potato depending on the concentration of the water on the outside. For lower concentrations the net flow was into the potato and for higher concentrations the net flow was out of the potato.

In the lower concentrations of salt water the osmotic potential of the potato is greater than that of the surrounding salt solution. Meaning that there was a net movement of molecules into the potato, just as I had predicted. From the graph we can see that it is just levelling off at top, this shows that the cells were just becoming turgid, had they been left for a longer time the cells would have become turgid, holding as much water as it possibly could. In the higher concentrations of the salt water the osmotic potential of the salt water was less than that of the surrounding salt solution. This meant that there was a net movement of water out of the potato into the salt solution, as stated in my prediction. This is known as Plasmolysis, the cell of the potato becomes flaccid. From the graph we can see that the concentration the salt water would have to be for the osmotic potential to be even would be 0.26M. We can tell this as it is where the line of best fit crosses the zero line in percentage change in mass.

I believe there is sufficient evidence for us to come to a firm conclusion and accept the prediction. We have a graph which shows us exactly what we predicted would happen. The evidence that we have is clear showing everything predicted to do with osmosis and the osmotic potential of the potato pieces. However saying this we could have provided a better conclusion had we obtained more results, instead of testing 0.2M then 0.4M etc. we could have tested 0.1M, 0.2M and so on. This would have given us more detailed results.

Evaluation

My graph shows an excellent set of results except for one, which is anomalous; this could have been caused by a number of things. The most reasonable of which is that the salt water solution was not measured out properly and so was not of the expected salt concentration. The results gathered were what we expected to see, osmosis was definitely evident.

There are many problems with this experiment, or at least many ways to improve it. For a start more accurate equipment could be used to make sure the concentrations were exactly right, the way we were doing it with measuring cylinders was very in accurate. Temperature is a problem that could have occurred with our experiment as our test tubes were carried across to the opposite building. The test tubes in the middle may have stayed warm where as the ones on the outside may have lost heat. A way to overcome this would be to use an incubator to keep test tubes at a constant temperature. The potato pieces were generally the same size in my experiment but it could have been made more accurate by having more accurate size potatoes, this could be achieved with the use of better scales.

Within our experiment we could have provided further evidence by testing the potatoes at different concentrations. For example instead of testing at 0.2M then 0.4M etc we could have tested at 0.1M then 0.2M and so on, this would have given us much more accurate results.

The experiment could have been extended in many different ways some of which may have been to test the effect of temperature in osmosis or the effect on osmotic potential using sugar concentrations rather than salt concentrations. Also we could have looked at the osmotic potential of different vegetables and compared them all, this would have given us a much better understanding of osmosis.

Robert Bentham

Candidate No. 9318 Centre No. 16327