Investigating the Difference in Isotonic Point in Sweet and White Potato Essay

To observe whether equal sized white potato or sweet potato cores reached the isotonic point in the same concentration of sucrose solution.


Osmosis is diffusion of water from areas of high water potential to areas of low water potential. It does not require an input of energy. Plants use osmosis to transport minerals from their roots to their leaves, and to take in water in the soil. Because the plant cell is taking in water and minerals, its mass increases.

The Maryland Department of Natural Resources states that “plants use water to carry moisture and nutrients from the roots to he leaves and food from the leaves back down to the roots.” (

An isotonic point is the moment when the solutions inside the cell and outside the cell have the same water potential since the two have an equal concentration of water molecules. This is when the rate of water leaving the cell is the same as the rate of water entering the cell.

Therefore, equilibrium is reached. This means that the plant cell will not increase any more in mass, as there will be no net movement of water.

In my experiment, I will be using two different types of potato – the sweet potato and the white potato. I think that it will be interesting to see whether the Sweet Potato is actually sweeter than the White potato (as the name suggests). The sugar content in sweet potatoes is 4.2 grams for every 100 grams you ingest. The sugar content for white potatoes is also 4.2 grams (per 100g). Hypothesis:

The isotonic point of sweet potato will be at a higher sucrose concentration than white potato because I think that it actually does contain more sugar than white potato. Null Hypothesis: There will be no difference in the isotonic points. Method:

* Sweet potatoes
* White potatoes
* White tile for cutting on
* 5 Stop Watches, each set for 30 minutes +/- 0.05 seconds + reaction time (215 milliseconds) + time taken for us to start removing the potato
cores from the sucrose solution (10 seconds)
* Balance +/- 0.005g

* Paper towels
* 10 50ml glass beakers
* Core borer (diameter=1cm)
* Ruler +/- 0.05cm
* Pins for colour coding
* Scalpel
* A flat surface
* 100ml Measuring Cylinder +/- 0.5ml
* 100ml Distilled Water (0.00mol/dm³)
* 100ml 0.25mol/dm³ sucrose solution
* 100ml 0.5mol/dm³ sucrose solution
* 100ml 0.75mol/dm³ sucrose solution
* 100ml 1mol/dm³ sucrose solution
* Thermometer +/- 0.5°C


* Cut out 25 cylinders from each type of potato using the core borer. * Using a scalpel, plate to cut on and a ruler cut each of these down to 2cm each (+/- 0.05cm). * Using the measuring cylinder, put 50ml distilled water in 2 different beakers, and then do the same with each concentration of sucrose solution. * Measure the mass of each potato core using the balance, noting it down and colour coding with a coloured pin. * Put 5 white potato cores into the first beaker (with the distilled water) and do the same with 5 sweet potato cores simultaneously. As you do this, start the stop watch (that should have been set for 30 minutes). * 5 minutes later, add the next 10 potato cores to the next two beakers (the 0.25mol/dm³ sucrose solution) and start the next stop watch.

* Repeat this every 5 minutes, until all of the potato cores have been added to their beakers. * When the first stop watch gets to 0 (i.e., when the potato cores have been in for 30 minutes), take out the potato cores from the distilled water. * For each one, dry it using a paper towel with 2 full rolls and a blot on each end. * Measure the mass of the potato cores, noting the change in mass for each one. * Repeat this with every concentration of sucrose solution, when their stop watches go off. * Find the average change in mass for each potato type for each concentration of sucrose solution. * Compare the results for the potato types.

Variable:| How it will be controlled:|
Control Variables|

Size of potato core| The same potato corer will be used to cut out the cores, and then each will be cut using a ruler and scalpel to 2cm long. The ruler is accurate to +/- 0.05cm.| Time the potato cores will be left in the solution for| Each solution will have a stop watch (+/- 0.05 seconds), ensuring that the cores will only be in the solution for 30 minutes. We also need to allow for human reaction time (215 milliseconds) and the time taken for us to start removing the potato cores from the sucrose solution (10 seconds).

| Temperature of the solutions| The temperature of each solution will be taken at the start with a thermometer accurate to +/- 0.5°C, and they should all be the same temperature and will then be kept in the same room. Rather than actually controlling the temperature, the temperature was just monitored. | Volume of solutions| Using a 100ml measuring cylinder (+/- 0.5ml) and by getting down to eye level, the 50ml of solution for each beaker will be carefully poured out and put into the beaker.| Independent Variable|

Concentration of sucrose solution| Different concentrations of sucrose solution (specifically, 0.25mol/dm³, 0.5mol/dm³, 0.75mol/dm³and 1.00ml/dm³) and distilled water (0.00mol/dm³) will be used.| Type of potato| Two types of potato (sweet potato and white potato).| Dependent Variable|

The change in mass of the potato cores| The mass of the potato cores will be measured using a balance (to an accuracy of +/-0.05g) before and after being in the solution and the change in mass will be calculated.|

Qualitative Results

For the distilled water, both types of potato sank to the bottom. For the 0.25mol/dm³ sucrose solution, the white potato sank and some of the sweet potato floated half way up the solution. For the 0.5mol/dm³ sucrose solution, all of the white sank and 3 of the sweet potatoes floated on the surface of the solution and 2 floated half way. In the 0.75 mol/dm³ and 1 mol/dm³ sucrose solutions, all of the white potato sank and all of the sweet potato floated.

The temperature of the room was 21°C. We did not check whether the temperature changed throughout the experiment.

Conclusion and Evaluation:

For the white potato, at sucrose solution less than 0.38mol/dm³, the potato cores gained mass. For example, at a sucrose solution concentration of 0.00mol/dm³, the average change in mass was at +0.04. This shows that there is higher water potential in the solution than in the potato. At sucrose concentration more than 0.38mol/dm³, the cores lose mass. For example, at a sucrose solution concentration of 0.75 mol/dm³, the average change in mass was -0.06g. This shows that there is more water potential inside the potato cells than in the solution.. For the sweet potato, the potato gained mass in all of the concentrations of sucrose solutions. For example, at the lowest sucrose solution concentration of 0.00mol/dm³, the average change in mass was +0.05g.

At the highest sucrose solution concentration (1.00mol/dm³), the average change in mass was +0.02g. This shows that there was higher water potential in the solution than in the sweet potato for every concentration I used. In order to estimate the isotonic point, the line of best fit was extrapolated. I would estimate that the isotonic point is 1.15 mol/dm³. The gradient of the white potato is much steeper than the gradient of the sweet potato. The white potato intersects the axis at a much lower concentration, which shows that its isotonic point is lower than that of the sweet potato.

The change in mass is due to the movement of water, entering the potato cells through osmosis (due to the difference in sucrose between the potato cell and the surrounding solution). This means that, when the change in mass is zero, the concentration of sucrose is the same inside and outside the cell (i.e., it has reached it isotonic point). The sweet potato reached its isotonic point at a higher concentration than the white potato did. Therefore, sweet potato does actually have a higher concentration of sugar than white potato does. This confirms my hypothesis, as this is what I predicted would happen.


As can be seen on my graph, my error bars are all quite small. I feel that this proves that my data is fairly accurate. Improvements to my method:

* The changes in mass were so small that the scales used, although they were to 2 decimal places, may have affected the validity of our results. It would have been better to us if we’d have used scales to, for instance, 3 decimal places. The measuring cylinder used was only to an accuracy of +/- 0.5ml. We could have used a more accurate measuring cylinder as well. Because the change in mass was so small, we could have improved the method by using much higher concentrations of sucrose solutions (for instance 1.00, 2.00, 3.00, 4.00 and 5.00 mol/dm³). This would have increased the change in mass.

* We didn’t actually find the isotonic point of sweet potato. From my graph, I can look at the trend in change in mass and estimate an isotonic point for it, but the validity of our experiment is affected by this, as this is obviously only an estimate. This could suggest that we didn’t go up to a high enough sucrose solution concentration. We should have gone up to a higher sucrose solution concentration, as this would mean we could have got a more accurate estimate for the isotonic point of sweet potato. * The solutions may have all been at different temperatures. The more heat energy, the faster osmosis can happen. This means that our results may have been affected by this. To improve this, I could have taken the temperature of each solution, to ensure that they are all the same. This would have excluded the possible variable of temperature for different solutions.

* Each potato core was in the solution for a different amount of time. If we’d have taken all of the potato cores for one solution at the same time, then each one would have a different amount of time exposed to the air and we could only weigh one at a time. Therefore, we decided to take them out one by one. This means that the potatoes left in the solution longer will have had a greater change in mass as they were exposed to the sucrose solution for longer. An improvement to my method could be to take them all out at exactly the same time. The improvement for this would be to use a fresh kitchen towel each time. * Also, this means that each potato core would have been left in the air for a different amount of time before finding its mass. Liquid could have evaporated and this would have affected results, especially as this means that some potato cores were left drying longer than others. To improve this, I would have to have the same amount of balances as balances, which would not be practical.

* The method we used to dry the potato cores allowed human error (i.e. how forceful each dab was, how slow each roll was, etc ), although our drying technique was quite consistent (we had the same person doing it each time, with two full rolls and a dab on each end). We could have used a machine to dry each one. Also, we didn’t use a new paper towel each time, which means that some of the cores were being dried on a slightly damp paper towel. * When we were cutting out the potato cores, we could have been slightly more accurate. The ruler we used was just a standard classroom ruler, i.e. to an accuracy of +/- 0.05cm.

We had three people cutting the cores, so we each might have cut them slightly differently (for example, one of us could have cut to the middle of the 2cm line on the ruler, another to the end of it). One way in which this area could have been made more accurate is we could have used a ruler which had smaller increments. Although this did not have much impact on the results because we measured the mass of each core individually, the slight difference in surface area could have had some effect.*********LITERARY REFERENCE****** * In between when the potato cores were cut out and when they were added to the solutions, each one was left out in the open air for a different amount of time, meaning that each had dried a different amount before being added to the sucrose solutions.

Ideally, we could have used a cutting machine to cut them all out at exactly the same time. Putting them in an airtight container or wrapping them in, for example, cling film as soon as each one was cut may have helped here. * In stopping and starting the stop watches there was also some human error (e.g. human reaction time for when the stop watch went off to when we actually took out the potato cores). The only way to improve this would be to use machines, which would only be suitable for mass production.


8 December 2011, Isotonic. Available at [Accessed 26 February 2013] Copyright © 2000-2011 Internet Brands, Do Sweet Potatoes Contain
Sugar?. Available at [Accessed 26 February 2013] 21 March 2012, Potatoes, white, flesh and skin, baked. Available at [Accessed 26 February 2013] Copyright 2006-2012 , Human Reaction Statistics. Available at [Accessed 26 February 2013] Copyright 1999-2013, How to calculate error bars. Available at [Accessed 26 February 2013]

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