When plant cells are placed in concentrated salt solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall. They are said to be plasmolysed.
Prediction
From the above information I predict that the higher the concentration of salt solution, the more water will be lost from the potato tuber through osmosis which therefore means the mass of it will decrease. The lower the concentration of the salt solution, the more the potato tuber will gain water, which means an increase in mass. I think this because in osmosis water travels from a high concentration gradient to a lower one. If I put the potato tuber into an isotonic solution than it will neither gain nor lose mass. I believe that there will be a point where the tuber’s mass will not increase or decrease anymore as it is either in full turgor, can’t hold any more water or it is fully plasmolysed, it has lost all of its water.
I believe if I double the concentration of salt solution the potato tuber will loose more mass and may become fully plasmolysed.
Here is a sketch graph of what I think my actual graph will look like:
Precautions
During this experiment I will have to take the following precautions to conduct a safe and minimal risk experiment:
- Wear a lab coat-keeps your uniform clean
- Wear safety goggles-to protect your eyes
- Take care when using scalpel
- Move all bags and chairs out of the way-so you don’t fall over
- Tuck in all stools under the table
These precautions must be followed in order to avoid any injuries or accidents with equipment.
Method
To do this experiment I will use the following apparatus:
- Scalpel
- White tile
- Measuring cylinder
- Long tweezers
- Stopwatch
- Electronic scale
- Paper towels
- 7 test tubes
- petri dish
- Labels
- Thermometer
- Test tube rack
Here is a step-by-step guide of how I will conduct the experiment:
- I will take 7 potato tubers from the supply provided and cut them all using a scalpel, so that they all have dimensions of 2.5cm long x 1cm high x 1cm wide. I will make sure this is all done on a white tile, to protect the bench surface.
- Give each tuber a number 1-7.
- Put an empty petri dish on the scale and tear it so that the screen shows 0 grams. Then put each tuber into the petri dish in turn and weigh it.
- Record the data into a table. This will be the ‘starting mass’ and will be measured to the nearest hundredth of a gram.
- Set up 7 test tubes in a test tube rack and add 25ml of each concentration of salt solution into a different test tube using the measuring cylinder. There are 7 different concentrations of the solution, one for each test tube. These are the concentrations: 0M, 0.25M, 0.5M, 0.75M, 1.0M, 1.25M and 1.5M. As you can see they go up each time by 0.25M.
- Label each test tube with the concentration of the salt solution within it. Then add a thermometer to each test tube and check that they are all at room temperature.
- Put all the tubers in to the corresponding test tube at the same time, starting with the lowest number tuber going into the lowest concentration solution and so on. As soon as I put them into the test tube I will start the stopwatch.
- Leave all the tubers in the test tube for 5 minutes.
- Take all tubers out of the solution after 5 minutes using the long tweezers, wipe off the excess water using a paper towel and weigh it again in the same way as previously done. This data will then be recorded into the table under ‘mass after’.
- Look at the table you have filled in and work out the percentage mass change. Then fill the data into the table. To work out the percentage mass change I will use this simple formula:
Percentage mass change = Mass after-starting mass x 100
Starting mass
Here is a diagram of what my experiment will look like:
The readings I will be taking during the experiment will be the starting mass of the tubers and then the mass after 5 minutes of being in the salt solution. I will measure the masses using an electronic scale, which is very accurate and easy to use.
I will repeat the experiment 2 more times, exactly as before, so that I have 3 sets of results. This is so that I can improve the reliability and accuracy of my data, and I can see any anomalies. After getting all 3 sets of results I will take the average. This leaves me with a set of very reliable results that are conclusive.
Preliminary Results
Before doing the actual experiment we did a simulation practise experiment. Here are our results:
Repeat Experiment:
From this preliminary experiment we cannot see any obvious patterns in the results apart from the fact that most of the results are negative percentage mass changes. This may be due to the fact that we encountered many problems throughout the experiment, which have all stemmed from my method. Because of these faults I will have to change my method so that I don’t have the same problems in my actual experiment. These are the changes I will make to the method:
- Leave each tuber in the solution for 10 minutes instead of 5 minutes. This is so that the surface area of the tuber is increased, and more osmosis can take place and provide me with more reliable results.
- Cut the tubers so that they are 4cm long instead of 2.5cm long. This is so that there will be more of a mass change, making it easier for me to analyse my results.
- Place the first potato tuber in the solution at the same time as you start the stopwatch, i.e. 0 seconds, then carry on putting one into its corresponding test tube at minute intervals. This is so that I don’t have to try and put them all in at the same time, which is very difficult. This will also give me more time at the end to take them out, and it will not be such a rushed procedure.
Obtaining Evidence
After changing my method I did the experiment again. Here are my results for all 3 repeats I did:
Below are the final results I will be using for the investigation shown as a graph (average percentage mass change vs. concentration of salt solution):
Osmosis Via Potato Tubers
Analysis
The graph showing my results gives the line of best fit for the percentage change in mass of the potato tubers for the time they were in the salt solution (10 minutes). The line of best fit is a curve that slopes downwards and does not go through the origin. Because the line is not straight and does not pass through the origin, it means that the percentage gain/loss in mass and the 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/loss is inversely proportional to the concentration. The gradient does change in my graph, for example it gets slightly less steep as the concentration increases. This is because the potato tuber is increasingly losing water and becoming more flaccid, and so the change in mass of each molar concentration is becoming closer together. From the line of best fit that has been added in, it can be seen that all of my points were quite close to creating a smooth curve. This shows that my results are fairly reliable.
Osmosis is the diffusion of water through a selectively (partially) permeable membrane from a dilute solution to a concentrated solution along a diffusion gradient, until the two solutions become equal concentrations. My results prove this theory to be correct. When the potato tuber is put in a hypotonic solution (0 molars), water diffused into the cell as the solution was more dilute and contained more water, its mass increased shown as a positive percentage mass change on the graph. The cell membrane pushes the cell wall outwards and soon the tuber cells become turgid.
When the potato chip is placed in a hypertonic solution the cell’s water diffused into the solution, as it is more concentrated (in terms of solute) and has less water. This means the cell’s mass decreased, as water diffused out, shown as a negative percentage mass change on the graph. The cell membrane shrinks away from the cell wall.
The one thing that my graph doesn’t show is when the cell is fully turgid or fully plasmolysed. If it did you would see that the mass wouldn’t decrease any further after a certain point even though you carry on increasing the concentration of the salt solution.
My graph fits in with my prediction of the experiment graph because as I said the potato tuber would decrease in mass as the concentration of the salt solution is increased, and this is exactly what happened, but my sketch graph is a lot less steep compared to the actual graph.
Evaluation
On the whole my experiment was relatively easy to conduct, although there were some difficulties. For example it was difficult to cut the potato tuber exactly vertically as the scalpel had a tendency to cut slanted and at an angle, but this wouldn’t have affected the results too much as we calculated the percentage mass change. Another thing I found difficult was taking out the tubers from the thin test tubes they were in, using the tweezers. In the end I did it by just tipping the salt solution of the test tube out into the sink, leaving me with just the potato tuber in the test tube. I then gently shook the test tube to get the tuber out.
I changed my original method a few times so that I could conduct the experiment in a simpler and easier way and also to improve the reliability of my results. I increased the duration of time that the tubers were left in the solution for, from 5 minutes to 10 minutes. This was so that my results would become more conclusive. I then decided not to put all the tubers in the solutions at the same time, and decided to stager the times by a minute. This made it easier for me to take them out at the end and weigh them on the scale. The last change I made was to increase the length of all the tubers from 2.5cm to 4cm. This was also done to improve the results.
I repeated the experiment another 2 more times after the original result was taken. This left me with 3 sets of results in total, which is enough to take an average of and analyse. I used quite a large range of concentrations of salt solution, from 0M to 1.5M. This gave me sufficient results to study, but a wider range, maybe from 0M to 2.5M may have shown the potato tuber to be fully plasmolysed, which did not happen in my graph. Taking more results to fill the gaps in the graph would not be necessary in this investigation as there is a clear structure and pattern to all the points on the graph, which follow the curved best fit line.
The results on my graph seem to be quite accurate as they all lie near to the curved line of best fit, which I have drawn on. If I were to do the experiment again, I would find it very difficult to change the way I measured so that my results were more accurate. This is because the equipment I used for measuring things was very accurate. For example the electric scale, and the measuring cylinders. But I could improve the dimensions of each of the tubers using a ruler and making sure they are all the exact same in surface area.
In my results table I found an anomaly that doesn’t follow the trend of the other results. It is printed in red ink in the results table, and was not used when taking the average percentage mass change. I did not include this result because it is far too different from the other results, and if used would totally change the shape of the patter shown in my graph. It may have been caused by a number of different things:
- Me writing the mass wrongly into my table.
- The solution I was using may have got contaminated with tap water, or another concentration of the salt solution.
- I may have not wiped off all the excess water from the tuber before I weighed it.
- I might have mixed up the result with another result by accident.
- I might have accidentally used the same potato tuber again from a previous repeat.
My table of results shows that the readings did vary a bit, but this was consistent and followed a visible pattern. To make them more reliable I could take more readings of the same result and take an average. The only result that was too different was the anomaly highlighted, which was ignored and not included to make the graph.
To improve the reliability of my results further I could change my method so that the tubers are in the salt solution for longer period of time. This would allow more osmosis to take place. I also could use a larger volume of salt solution, and put the solution into a beaker instead of a test tube. This would mean that the tuber would not touch the side of the container it is in, unlike when they were put into test tubes.
This experiment can be extended or get additional evidence by changing the vegetable used, for example a carrot could be used instead, and then you could compare the 2 results and come to a conclusion about osmosis via vegetable tubers and see if osmosis is the same in all vegetables.