An experiment to work out the concentration of plant cell sap using potato tuber cells.

Useful Science involved –

Osmosis

Osmosis is the diffusion of water from a dilute solution to a more concentrated solution through a semi-permeable membrane. The water moves from where it’s a high concentration to where it’s a low concentration. Concentration of the water is the only thing that matters; the solute molecules (e.g. sugar) can’t pass through the membrane. The effect of all of this is to gradually dilute the sugar solution

Example -

Prediction – After taking a close look at my preliminary experiment, I predict that the molarity of the potato will be 0.3M and the mass will increase if the solution is less than 0.3M and decrease if it is more. If the solution is exactly 0.3M then there will be no change in mass, this is an isotonic solution. A below 0.3M would mean that water would move in by osmosis and a solution above 0.3M would mean that water would move out by osmosis from higher to lower concentration. A solution of 0.3M (solution that crosses the point a zero) would mean that water wouldn’t move in or out at all (isotonic solution).

Method -  

1. Firstly we cut out 6 potato chips all of them the same size (roughly an inch long) this was done using a cork borer.
2. We dried each piece and weighed them separately recording each weight in a results table.
3. We then placed each chip in the different concentrations
4. Next we labelled each glass tube with our initials and the correct molarity (eg.0.1M)
5. We left them to settle for a day
6. Then we dried them and weighed them once again
7. Finally we recorded the final weight in the results table and worked out the change in weight and the percentage change.
8. We did this experiment twice to get more reliable and better results that will in affect improve my conclusion.

Conclusion

Both of the graphs have a negative correlation, which we would expect. My prediction was that the molarity of the potato would be 0.3M. If we look at where the line of best fit passes at the point zero, we can see that it is approximately 2.5M, which slightly disagrees with my prediction although it is fairly close. Both graphs show that when the potato was in a solution less than approx 2.5M, then water moved in and therefore the weight of the potato increased and a solution over approx 2.5M, then water was lost and therefore the weight of the potato decreased.

The water in each case has moved from where it is in high concentration to where it’s in low concentration along a concentration gradient. The semi permeable membrane in the potato allows the water to enter and leave. The sugar molecules can’t fit through this and therefore stays put. The point in which both lines cross at point zero which is about 2.5M, here water doesn’t enter or leave, this is because the concentration is equal outside and inside the potato. This agrees with my prediction.

Evaluation

The results which I ended up with were fairly good and backed up my prediction. Looking at the graphs, the pattern wasn’t perfect; there were one or two anomalies. The anomaly that stood out the most was on graph 2 on 0.5M; it is slightly out off the pattern. This may have been because the weight of that particular potato chip was 2g lighter than the chip used in the first experiment. However, the mass of these chips was the same. This may have occurred because the second solution may have been left slightly longer or that it had been dried more thoroughly than the chip in experiment 1. I think the method I used was fairly reliable. There are a few things that I could have changed – I think we could have had a better method for drying the chips because if a chip isn’t dried properly then there is much more water on the chip at the start than another chip. This may affect the results and therefore may end up affecting the conclusion. Another thing we could have improved was the to try and do each individual solution at the very same time. The problem with this is that we simply didn’t have enough people to do this. We could have even looked at different variables such as trying different temperatures or stirring the solution, this would increase the rate and would be quicker, give us a better, wider range of results. The temperature at the time we did each set of experiments may have been different and may have affected the results. This also was a problem, keeping the temperature constant. Nevertheless, although there was the odd anomaly the pattern on the graphs was clear and stood out, therefore I think that they were still a good set of results.