Controlling Variables:
To manipulate the independent variable, I will concoct sucrose solutions of different concentrations (0.0mol.dm-3, 0.2mol.dm-3...).
To measure the dependent variable, I will weigh each piece of plant tissues before and after being immersed into the sucrose solutions. Weighing will take place on a balance, accurate to 2 decimal places in grams.
To keep each control variable constant, I will cut, measure and weigh the plant tissues to ensure that they are very similar (it would be impossible to have identical pieces of plant tissues, although this would be ideal) and use an 18mm cork borer to keep each piece of tissue the same shape and length. In order to control the volume of solution used, I will measure out 10cm-3 of the sucrose solutions into the test tubes before immersing the plant tissues. Furthermore, I shall bathe the test tubes in a water bath set to 20ºC (293K), using an independent thermometer to monitor the temperature (to ensure that the bath is reliable) and I will use a stop watch to aid me in keeping each sample immersed for precisely 40minutes (accurate to the second).
Apparatus:
- Distilled water: 180ml
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Sucrose solution (1mol.dm-3): 180ml
- 4 carrots (Touchon)
- 4 apples (Bramley)
- Cork borer (18mm)
- Balance, accurate to 2d.p. (grams)
- 36 test tubes
- 3 test tube racks
- 2 water baths (set to 20ºC)
- Timer (accurate to seconds)
- Knife
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Syringe (>= 10cm-3)
Method:
- Set up 36 test tubes into the test tube racks.
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Label 6 test tubes with each of the following: 0.0mol.dm-3, 0.2mol.dm-3, 0.4mol.dm-3, 0.6mol.dm-3, 0.8mol.dm-3 and
1.0mol.dm-3.
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Depending on the labels, fill each test tube with 10cm3 of the appropriate concentrations (of sucrose solution):
- Place the test tubes into the water baths (set to 20ºC/293K) and allow the sucrose solutions 10minutes to reach the desired temperature. Confirm with a thermometer.
- While the solution's temperature stabilises in the water baths, prepare the apple and carrot samples. Use a cork borer to extract 18 pieces of tissues from the apples and the carrots. Use a knife to cut each sample to the same length (3.0cm).
- Use a balance to measure the mass of all 36 samples of tissue and make sure that the masses are equal. Record the mass of each piece of tissue, including details of which samples will be used for each concentration.
- Whilst disturbing the water bath as little as possible, place all samples of plant tissue into the respective test tubes and start the timer.
- Once precisely 40 minutes have passed, remove the test tubes from the water baths and remove the tissues from the test tubes. Use paper towels to blot each piece of tissue to remove excess solution and measure the mass of each sample, recording the results in a table.
- Calculate the percentage change in mass for each sample (change in mass/original mass*100) and plot this against the concentration of solution used on a graph.
- On aforementioned graph, read off the concentration for which there was no change in mass and convert it into water potential – this is the water potential of the tissue.
Results:
Apple data:
Carrot data:
Scatter graph of processed data:
According to the graph, the solution for which there is no change in mass for the apple samples is approximately 0.43mol.dm-3 and 0.2mol.dm-3 for the carrot samples. This can now be converted to water potential by using the following table:
As such, the water potential of the apple samples appears to be -1415kPa, and the water potential for the carrot samples appears to be -540kPa.
Conclusion:
To conclude, I believe that my investigation supports my original hypothesis (“the plant tissues will change in mass as the concentrations of the sucrose solutions differ” and “apples will have a lower water potential than carrots. I believe this because apples are fruits, and as such contain more sugars and other solutes that carrots which make them sweet, which reduces the water potential”). As my results clearly show, the mass of each sample increases initially (at 0mol.dm-3 for the carrot samples and between 0-0.4mol.dm-3 for the apple samples), which is due to the sucrose solution having a higher water potential, which caused water to enter the samples to balance the water potentials inside and outside of the samples. Furthermore, as the concentration of the sucrose solution increased, its water potential was lowered until it was lower than that of the samples; which is when water started to pass out of the samples and into the sucrose solution, causing a decrease in mass.
The other part of my hypothesis, where I stated that the apples would have a lower water potential than carrots, appears to be supported. Since apples contain a greater number of solutes than the carrots (according to http://wiki.answers.com “all fruits contain natural sugars, especially fructose”), which lowers the water potential.
Evaluation:
This investigation could have been improved:
- Making sure that the tissue samples are identical is nearly impossible – size, mass and surface area all affect the rate of osmosis and I only concentrated on achieving similar mass and sized samples. A machine to slice/dice the tissue into chips of the same sizes would remove the human error component of preparing the samples.
- Weighing each sample after immersion allowed for different amounts of solution to remain on the samples – I patted each sample with a paper towel, but it's possible that in doing so, some water was squeezed out of the samples or that excess solution was still present on the samples. Being extremely careful with this step is very time consuming, but it would have produced slightly more valid results.
- The plant tissues used should be fresh, since older tissues will lose moisture quickly, or possibly even rot – affecting the results.
- The particular brand or carrots/apples used in the investigation are only representative of those brands, and not carrots/apples in general.
Bibliography:
- Class notes
- http://wiki.answers.com
- http://www.wikipedia.org
