Therefore, potatoes that are left for a very long time would lose a lot of water, making the cells in the potato flaccid, which would cause them to look soggy or floppy. The longer they are left, the more opportunity the water has to leave the potato cell. However, after a certain amount of time, I think that the water potential in the cell and in the air would be the same. This would mean that there would be no more water leaving (or entering) the cell, as the two would be in equilibrium. I predict that this would happen after about 4 or 5 days of storage. Though, if they were moved and stored somewhere else, there could be a slight change in water potential once again.
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Preliminary Investigation
During my preliminary investigations I found out that the best way to measure water potential would be to measure the change in mass of a potato slice. Measuring the potato as a slice rather than as a whole potato would be easier because it is more efficient to work without the skin, so osmosis would take place faster. Also, as a practicality issue, it is easier to manage a slice of potato than the whole thing.
Water is quite heavy, so you can tell when it has moved out of the potato by a change in mass. Using this theory, you can also tell when water has moved into a potato slice. Osmosis is not a spontaneous occurrence, so you need to leave the potato in a solution for a couple of minutes for any reasonable results to be gathered.
Variables
Input Variable: The age of the potato
Output Variable: The change in mass (as a measure of water potential)
Control Variables: - The size of the slice of potato
- The concentration of solution used
-Time potato is left in solution for
Safety
- Normal lab precautions (tying hair back, stools under desks etc.) should be observed
- The experiment would involve water, which is easily spilt. Therefore, you should be careful when moving around the lab as there could be spillages in the floor which you could slip on.
- You should be careful when using a scalpel to slice the potato, as they are sharp and you could cut yourself with it.
- We’ll be using glass beakers which could be smashed, so you should be careful not to cut yourself on shards of class if this happens.
Apparatus
- Distilled water
- Six large test tubes
- A test tube holder
- A ruler
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A 10cm2 syringe – it is easier to use one syringe to measure out the 10cm, than it is to use a 5cm syringe twice
- A scalpel
- An electronic balance – the readings are more reliable than an analogue balance
- Small pieces of paper
- Potatoes of different ages
- A stopwatch
Method
- Get six samples of potatoes, one should have been stored for each of the following times: 1 day, 2 days, 5 days, 7 days, 10 days and 14 days.
- Cut each with a scalpel so that you have three slices (to allow for repeats) with dimensions of approximately 8cm x 1cm x 1cm. This is so that each slice has approximately the same surface area, as this could have an effect on the rate of osmosis.
- Take your first slice and weigh it on a piece of paper. Record this mass.
- Place the slice in a test tube and place it in the test tube holder.
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Using a syringe, measure out 10cm2 of distilled water.
- Add this to the test tube with the potato and start to time with the stop watch
- Leave it in there for one minute, to allow osmosis to take place.
- After one minute, place the slice of potato on another small piece of paper and weigh it. Record this mass in the table.
- Throw the piece of potato away.
- Do this experiment for one other slice of the other five types of potato.
- Repeat the whole experiment twice to make sure it is reliable and increase accuracy.
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Results Table
Analysis
From my results you will be able to see if there is any correlation between the amount of time potato is stored for, and the change in mass when places in distilled water. A large increase in mass means that the potato had a low water potential, and a large decrease in mass means that the potato had a higher water potential. I would expect the ‘younger’ potatoes to have a decrease in mass and the ‘older’ potatoes to have an increase in average mass.
The graph would be able to show us that there is a point at which the mass does not change any more. I think this is because the water and the potato will be at the same water potential, so no water will leave or enter the potato, causing a change in the mass. The two will be in equilibrium.
From my results, many calculations would be able to be carried out. You could work out the approximate change in mass per day, and see if this is a linear change or not. You could also work out where the equilibrium is.
Evaluation
Overall, I think my experiment is quite precise. The apparatus is class B, and although you could get more precise measurements from class A apparatus, I think the results that you could get are good enough for this experiment. I have chosen to use electronic equipment in a couple of places (balance, stopwatch) as these are more accurate. Although, the stopwatch could be a problem as it is impossible to add the potato and start the stopwatch at the same time which could cause a slight delay and leave the potato in the water for slightly longer than one minute.
The experiment is valid, as it measures change in mass which directly correlates to change in water potential, which is what I said I would investigate in this experiment.
To make the results reliable you could repeat the experiment more than twice. Overall however, I think my experiment is reliable enough and quite fair. There are variables beyond our control, of course (such as air temperature or pressure) which could affect the rate of osmosis, and the experiment would have to be carried out in a sealed environment to rectify this.
References
[1] Wikipedia article of ‘Osmosis’
http://en.wikipedia.org/wiki/Osmosis
19:26, 16th February 2007
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