An investigation into the isotonic point of Potatoes

Introduction:

In this experiment I will be trying to find out the isotonic point of potatoes. This is a point where the concentrations of water inside and outside the potato cells are equal. Water gets into or out of various cells by Osmosis.

(Hypothesis) Osmosis is 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 such as a cell wall, 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.

Water potential is a measure of whether a solution is likely to gain or lose water molecules from another solution. A dilute solution, with its high proportion of free water molecules is said to have a higher water potential than a concentrated solution, because water will flow from the dilute to the concentrated solution (high water potential to a low water potential).

Safety;

The only safety precaution needed in this experiment is to be careful with various apparatus such as the tool used to get potato chips out of a full potato.

Apparatus:

Method:

I set up the experiment as shown above. I cut out eight pieces of potato using the tool that cuts cylinders (chips) out from a potato. Then I weighed each potato chip and noted it down. I weighed it using a very accurate weighing scale. I then labelled eight test tubes so I know which experiment is which and then I put a potato into each test tube and I noted down which potato mass went into which test tube so I know exactly which potato lost the most mass after the experiment. I then measured out different measurements of sugar and water solution to put into each test tube. I did this using equipment that helped giving accurate results. We left he whole experiment over night to take its course of action. After I reweighed each chip of potato and noted down its weight making sure that the correct chip of potato belonged to the right test tube. I repeated the experiment four times and took an average of the percentage change.

Prediction:
In a high concentration of water the amount of solute (e.g. sucrose) is low. This could be called a weak or dilute solution. In a low concentration of water the amount of solute (e.g. sucrose) is high. This could be called a strong or concentrated solution. When two such solutions are divided by a semi-permeable membrane, the water will move from the area of high concentration to the area of low concentration, until both sides are equal (have reached equilibrium).

Knowing that osmosis will occur across a semi-permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane, and knowing that when this happens to cells they will either become turgid if water flows into them, or plasmolysed if water flows out of them, and thus change their volume, we want to test the hypothesis that:

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         If the concentration of a solution into which a chip of potato is placed is greater than a certain level the chip will contract, and if the concentration is less than that level it will expand. This can be seen in living cells. The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration) water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid. If done with potato cells the cells would increase in length volume and mass because of the extra water. If these potato cells were placed in a solution with a low water concentration, then the opposite would happen. Water would move out of the cell into the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmolysed. The potato cells will have decreased in length, volume and mass.

        The greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater the amount of water that leaves the cell.

        However, there will be a point where the concentrations of water inside and outside the potato cells are equal. This is the isotonic point. At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred.

Fairness:
We will make this a fair test by considering various factors:

1 - Temperature
2 - Water potential of potato initially
3 - SIZE of potato
4 - Light intensity
5 - Mass of potato
6 - Volume of solution potato chip is in
7 - Type of potato
8 - Time left in solution
9 - Surface area of potato chip

10 - Use the same balance to measure chip

11 – the molarity

Various things we do not have control over such as factors 1 (the room temperature may be different for different experiments), and 2 (it may be hard to measure the water potential of each potato chip).

Things we do not need to consider in great detail are factors such as 3 (different size potatoes can be used so long as the mass is roughly the same), 5 (as long as the masses are roughly the same it is ok, the percentage change in mass will have to be taken), and 10 (the balance used must give readings accurate to at least one decimal place, if this is the case any balance should be suitable).

Factors that will have a noticeable effect on the accuracy of the results are 6,8 and 11.

Factors that we will change, are the strengths of the solution for each experiment (the molarity). After each use the potato must be changed as well.

Range:

I have decided that I will take the whole range of results at least 3 times and take an average. This will make the end set of results a lot more accurate. I will take readings with molarity strengths from 0.1 molar to 0.8 molar. Each time 0.8 molar equals 8 millilitres and 0.1 molar equals 1 millilitre. Therefore giving a total volume of 8 millimetres each time. If the molarity of the solution is 0.2, I will use 2 millilitres of sucrose solution and 6 millilitres of distilled water in order to give a molarity of 0.2 and also a total volume of 8 millilitres. I will obtain some results and then I will plot a graph finding the isotonic point. Once the isotonic point is found I will do more experiments within a region where the isotonic point was found but with more decimal places e.g. if the isotonic point is found at about 0.5 molar, I will take more readings with the molarity 0.46, 0.47, 0.48, 0.49, 0.50, 0.51, 0.52, 0.53 and 0.54. This will ensure that the most accurate isotonic point can be found.

Reliability:

The results will be made accurate by a repetition of results. This will definitely make the results more accurate and so therefore more reliable. I will repeat the whole experiment at least three times and then I will obtain more results after the isotonic point is found. I will also repeat any anomalous results. The end set of results will be reliable enough to draw up a good enough conclusion.

Preliminary work:

Experiments that I did before this current experiment included the experiment to illustrate turgor in a plant cell. This involved using a visking tube. Osmosis occurred as the visking tube acted as a semi permeable membrane. The point of doing this experiment is to show that there is a resemblance to the way a potato works and a visking tube.

O

Equipment:

The equipment needed to make my measurements were measuring cylinders, a burette and accurate weighing scales.

The weighing scale made my results that extra little bit more accurate as the scales were very, very sensitive and gave mass to at least one decimal place. The burette made sure that the correct volume was being used each time.

Results:

Average results table

Isotonic point = 0.0 = molarity of 0.482926829

I have worked out the molarity that gives an isotonic point. I worked this out by adding 0.7 to 3.4 giving 4.1(I added 0.7 to 3.4 because that was the range between 0.4 and 0.5. The molarity must be less than 5 but more than 4 to give 0.0, so I divided 0.1 (the difference between 0.4 and 0.5) by 4.1 and I got 0.024390243, then I multiplied that by 0.7 (the difference between 0 and itself) and I got 0.01707317, after that I took that number away from 0.5 to get 0.482926829 (The isotonic point). I will still do more experiments between 0.4 and 0.5 for a precise isotonic point.

I have left all results that have been found out to one decimal place. Doing this makes it more accurate and as you can see above, the average results table show very accurate percentage changes. This is why I used specific equipment in order to enhance the results.

A

As you can see from my results there is a clear pattern. The greater the molarity of the solution, the more mass the potato loses. This is very clear in the results. What has just been proven agrees with my theories in my prediction. There are no anomalous results and the reason for this is because of the repetition of the results.

Below is a line graph representing the results I have obtained.

As you can see from the above graph the line of results is in a straight line. No results are clearly anomalous. If there are some anomalous results it would be very hard to get them any more accurate. The isotonic point is roughly at 5.0 but I will do more readings in this area to obtain a much more accurate isotonic point. The molarity clearly has a large affect on the percentage change.

This proves my point about the fact that molecules will move from a highly concentrated area to a less concentrated area. If the solution had a low molarity then its concentration would be high. The high concentrated area moved to the low concentrated area (being the potato) and son therefore resulted in its mass increasing.

Attached to this write up there is a graph at the back which has a line of best fit on it. The line will be able to predict a percentage change for any molarity between 0.1 and 0.8.

Conclusions:

This graph of the change in mass helps 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. From left to right the first two points on the graph are very spread out indicating that there was a large change in the mass. This decreases throughout the increasing molar concentration until the change is minuscule (about 0.02g).

The results that I obtained are very similar to the trend in which I had predicted before (refer to my prediction for a more scientific understanding).

E

Evaluation:

The experiment was easy to do , but all the results I had to take had to be accurate. Things like having the potato chip 5cm long changed to 3cm because it was it was hard to have all the chips 5cm long, and 3cm was sufficient. Also, I changed the amount of solution I would keep the potato chips in, because I only needed enough to cover the potato chip. These changes were easy to make, because I did a preliminary experiment, so I could iron out any errors that may have occurred in my actual experiment.

I think I took enough results for the amount of molarities that I was given, and the time restrictions that I had to follow, The range was big enough, but to fill in the gaps in my graph, I could have taken results of more molarities, i.e. 0.10m, 1.15m, 1.20m, etc. This way I could have found the isotonic point by there being no change in mass. This would show the water potential of the potato, and this result could then be used further. Also to make my experiment better I could have repeated it more, and possibly have worked with someone else, or with the whole class. I could have also used cloned potatoes, so that they would have all been the same. I could have also cut the potatoes into doughnut shapes, because the cells in the centre of the potato may have a different capacity of water and the intake, or distribution of it, or different water potentials. If I had cut the potato into doughnuts, I would have only been using the out side of the potato, leaving out the middle. I could have also used a machine to cut the potato chips.

My results did lie close to the line, and therefore were fairly accurate, but if I was doing the experiment again I could make changes, some as mentioned above. I also used a burette to measure out my solutions. This ensured that I had an accurate amount of fluid in each test tube. I could also weigh each chip on another digital but more accurate scale, e.g. not to 0.00 but to 0.0000g.

There were not any anomalous results, but some results were not as close to the line as others. This may have been caused by human error, or one out of my results could have been inaccurate, and changed the average drastically. Or perhaps the potato chip was not cut accurately, or that part of the potatoes, cells did not loose/gain mass well. My results did vary a lot for the same concentration. The fact that a certain part of the potato may not be the same as another was shown as the chips all of the same length were not all the same weight. This is another reason to use cloned potatoes. My results were consistent, and there were no results that they were ignored.

I could extend my enquiry by testing the percentage change in mass with morality using a different substance. By this I mean using a different vegetable, perhaps celery or cucumber. Then I could find out whether osmosis occurs with the same patterns and trends with any vegetable.