Investigation To Find the concentration of sucrose solution that has the same "water potential" as potato tuber cells.

Aim: To Find the concentration of sucrose solution that has the same “water potential” as potato tuber cells.

Hypothesis: Osmosis is defined as the net movement of water or any other solutions molecules from a region in which they are highly concentrated to a region in which they are less concentrated. This movement must take place across a partially permeable membrane which lets smaller molecules such as water through but does not allow bigger molecules to pass through. The molecules will continue to diffuse until the area in which the molecules are found reaches a state of equilibrium, meaning that the molecules are randomly distributed throughout an object, with no area having a higher or lower concentration than any other.

For this particular investigation I think that the lower the concentration of the sugar solution in the test tube the larger the mass of the potato will be. This is because the water molecules pass from a high concentration, i.e. in the water itself, to a low concentration, i.e. in the potato chip. Therefore, the chips in higher water concentrations will have a larger mass than in higher sugar concentrations.

Earlier on, we performed a preliminary experiment where we tested osmosis in potatoes. We used three things to test it. Water, sugar solution and air in beakers. In this experiment we found that after about 24 hours, all the chips seemed different to the other. The chips kept in water had become turgid, as in they swelled up, the ones in sugar solution had become flaccid, they become thin and smaller. With the ones in air there was no change. This happened because of osmosis and that the water molecules moved from one place to the other but did not even out, which explains the turgid and flaccid ones. One had more water molecules inside the chip than in the beaker, the other had less. The mass and length of the chips had also changed meaning that the movement of molecules affect it quite a bit. What I intend to do is further the sugar solution experiment and find out if there is a point where the water does even out and that the mass of the chip remains the same even after being placed in sugar solution. Where the water molecules have an equal amount within the chip and ,outside in the beaker. What I intend to use for this is molarities, there must be a concentration of sugar solution where the mass of a chip remains the same after being placed in sugar solution for 24 hours. So I will place identical chips within different molarities (or concentrations) to see which one remains the same mass.

Molarity is

Plant cells always have a strong cell wall surrounding them. When they take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises and eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what makes the green parts of the plant "stand up" into the sunlight.

When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid." This is the exact opposite of "turgid". The contents of the potato cells shrinks and pulls away from the cell wall.

When plant cells are placed in a solution, which has exactly the same osmotic strength as the cells, they are in a state where there is no change in mass.

Variables Non-Variables to be considered

1) Solution concentration

2) Solution volume

3) Duration of experiment

4) Temperature

This is a preview of the whole essay

When plant cells are placed in a solution, which has exactly the same osmotic strength as the cells, they are in a state where there is no change in mass.

Variables Non-Variables to be considered

1) Solution concentration

2) Solution volume

3) Duration of experiment

4) Temperature

To create a fair test certain aspects of the experiment will have to be kept the same whilst one key variable is changed. I have chosen to vary the concentration of the sugar solution. This will hopefully give me a varied set of results from which I hope to make a decent conclusion. If any of the non-variables below are not kept constant it would mean it would not be a fair test. For instance if one of the potato chips was 1cm longer the surface area of the chip would be larger and there would therefore be more space for osmosis to occur. Doing all the tests at one temperature will control the temperature, so it cannot effect the results, so am going to carry out my experiment in a controlled environment. To keep the water potential of the potato initially will be kept the same by using the same type of potato, which have been treated in the same way, e.g. have all been cut without being washed and peeled. The mass of the potato is a dependent variable, and this means that it will be measured

throughout the experiment. I will measure the mass in grams. The potato chip will be measured before it is put in the solution, and after. This will allow us to see whether

osmosis has taken place, and to what extent.

The volume of the solution that the potato chips are kept in must be fair. The chips must be totally submerged in the solution, i will make sure the amount of solution will be kept the same to make it a fair test.

Planned method:

A range of sucrose sugar solutions will be prepared with concentrations 0 molar, 0.25 molar, 0.5 molar, 0.75 molar and 1 molar.

Sections of potato will be cut using a scalpel and will be measured using a ruler. This part of the preparation must be done very accurately as a change in the surface area may allow more or less osmosis to occur.

The mass of each chip will be measured as well so that more results can be obtained. Two chips will be placed in each test tube each time so that I can take an average for each tube.

I will use 20 ml of each concentration of sugar solution and once in the beakers and sealed they each will be labelled.

The potato pieces will then be placed in the different test tubes and then left for 24 hours. Before, I also recorded the time each set of potatoes were placed inside the beakers.

Then the potato pieces will be removed, the surface solution removed by gently blotting paper towels over surface at the same pressure to keep it a fair test and then they will be re-weighed. The time when taken out of the beaker will also be recorded for each set of potatoes.

Method:

1. Using a knife, I cut chips out of the potato

2. Using a scalpel and ruler I cut the chips down to roughly the same size (4cm). I then had 15 chips.

3. Taking a test tube rack I placed 5 test tubes and then labelled them 0 molar, 0.25 molar, 0.5 molar, 0.75 molar and 1 molar.

4. Using a pipette I put the same amount of each different concentration of solution and put each test tube in the right place in the rack. (20ml).

5. I then weighed every potato chip on a top pan balance and recorded the weights in a table.

6. I put 2 potato chips into each beaker and sealed them and placed the rack in a controlled environment. The 2 chips were used to create an average which gave me a better set of results and more accurate graphs. The time I placed each set of potato inside the beakers was also recorded down.

7. After 24 hours I drained out the solutions in the sink and placed all the chips on the paper towel in the order I had put them in the test tubes as to not confuse myself as to which chip came from which solution.

8. I dried each chip with the paper towel and then placed each one, one by one, on the top pan balance to weigh them and their weights were recorded in a table. The time of removal for each set of potato chips was also recorded down.

9.With these results averaged, I will not need to repeat the experiment again.

10. Once the table is full with these results, I will average them out to get better readings and see if there were any anomalies within my results as I will have to analyse why and how this could have happened.

11. With the averages, I will plot a graph showing mass by morality. With this graph I will have an easier time analysing my results and evaluating it overall. With the graph, I hope to find a point where the mass remained exactly the same. Thus finding a point in between where the chip is neither flaccid or turgid, a point where the amount of water is exactly the same outside the chip and inside and where the mass has remained the same.

Precautions:

>The measurements for the solutions had to be perfect as to not change the out come of the experiment.

>I had to ensure that every time I handled the potatoes my hands were clean and dry. This was to stop any kind of contamination and made sure that I did not pass on any extra water onto the potato.

This small graph below shows how I hope the graph I plot with the percentage averages will turn out. As the molarity increases the percentage change become less until it reaches a point where there is no change in weight.

Obtaining evidence

After the experiment was done the first two tables of results on the page before were collected.

After analysing them, we decided to repeat the experiment one more time for as the results for 0.5and 0.75 were not satisfactory and there were anomalies, we also decided to do and additional test with the molarity of 0.375, the results showed that the weight was nearest to the original between 0.25 and 0.5, so it would only be logical to try out 0.375, the exact middle of those two points. These results are shown on the last table.

On the page after that I have produced a single table with the results averaged. I have omitted the 0.75 and 0.5 from the first two graphs as they are anomalies and do not deserve to be incorporated into the results. I have replaced these results with the more recent ones I recorded including the 0.375 ones. From these averaged results , I will plot a graph from the percentage changes in weight.

On the next page is the graph plotted from the averages.

Analysis

The graph on the page before shows the averaged percentage changes in weight by the molarity and as you can see, there is something wrong. From the past results I knew something was wrong, but to repeat the experiment again and find another error is surprising. What is truly surprising through is the fact that the additional test we did turned out to be the results which changes the graph the most severely. Without plotting that results the graph would have been much straighter and look somewhat more familiar to my predicted graph. However I cannot change that now, it has affected my results quite severely and I will have to analyse why this could have happened.

Firstly though I will analyse the whole graph.

Because the line is not straight and does not pass through the origin, it means that the percentage gain and loss in mass and concentration are not directly proportional. However, there is a pattern on my graph, and this is, as the concentration of the solution increases, the percentage change in mass decreases. The graph shows that the percentage gain and loss is inversely proportional to the concentration This shows that my prediction was fairly reliable and that it fits with my analysis.

It shows that the potato cells increase in mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. From the line I have drawn on the graph, connecting 100% percentage change (no change) horizontally to the exact 90° vertically, I can see that the supposed isotonic point is 0.2125 molarity. The isotonic point is the point where the potato is not increasing or decreasing in mass and according to my graph, this is the point. I would also like to mention that this is before the dip in the graph at 0.375 molarity. At this point no osmosis is taking place, both the potato and the solution have an identical molar concentration. The next point, 0.25 M looses approximately 4.0 %. This shows that the water potential of the salt solution in the beaker is weaker than that of the potato chip. The next, 0.50 M, looses approximately 11.0 % in mass. This shows that the salt solution has an even weaker water potential than 0.25 M and that osmosis took place. This is why the potato lost even more mass, and it shows that the water potential in the beaker is less than that of the potato chip. This pattern carries on through the graph, and even more mass is lost, as more water moves out of the potato into the solution. My results also match with my initial predictions.

This graph of the change in mass could help prove the point of complete plasmolysis, whereby the potato cannot expand and take in any more water. As you can see as the molar concentration increases the change in mass decreases. What I would be able to do, with more graph paper is expand on this until the line crosses over the x axis, at this point the potato would be completely plasmolysed and extremely turgid.

The abnormal results which were supposed to enable me to advance in my study have now become a hindrance, they show something which shouldn’t be there. There is a dip in my results which then rises at the next result and continues normally. I can see no reason why this could have happened. The other results which were repeated a third time as the others were poor (0.50 and 0.75) turned out fine ,and they were all bored from the same potato, and kept in the same place as the 0.375 experiments. There seems to be no logical answer to these results, why or how it could have happened is beyond me, however, if I could have more time, I would repeat these tests again.

This graph shows a clear indication that there was an overall decrease in mass during the experiment.

Evaluation

The experiment was quite successful in my opinion. I obtained a large quantity of very accurate results from which I was able to create an informative graph. I think I took easily enough results for the amount of concentrations that I was using, and the time that I used for the experiment to last was enough to allow sufficient osmosis to occur. However if I was to repeat the experiment I might well increase the time of the result to allow more osmosis to happen and possibly find out the saturation point of the chips. The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results, i.e. 0.10m, 1.15m, 1.20m, and so on. This way would have allowed me to also find out the isotonic point far more accurately as the one that I estimated is very approximate. This would include 0.2125 which according to my graph is the isotonic point, I would like to test this theory and see if the graph remains true. This would also help me severely in discovering why the 0.375 results were so way off. There seems to be no logical explanation why is was so, and I would like to pursue it, to see whether they were not anomalies and were actually pure, correct results, this could disrupt my theory and prediction though but I wish to find out.

The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by mass, it could well have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I would have possibly found a machine to cut the potato as it would ensure that all potatoes would be the same weight and dimensions. As well as the potato I could have found a more accurate way to measure out the solutions and to determine the molar concentrations. Perhaps I could have used a burette. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a more accurate scale, e.g. not to 0.00g but to 0.0000g. Drying the potatoes way also an important factor, each chips saturation would have affected the results, affecting how much water it could hold, what I would like to be able to do is completely saturate them so they are all of equal saturation and there is no great factor there. Drying them with tissue was also time consuming.

Some of the anomalous results may have been caused by human intervention but that is why we repeated them, to reduce that factor. However with all this said I think that the experiment was quite successful and I was very pleased with the complete comparison of my results with my initial prediction, even with a slightly inaccurate graph.