To investigate the effect of osmosis.

Charlie Wilson 10y 02/05/07

Osmosis Investigation

Planning Experimental Procedures

Aim

To investigate the effect of osmosis.

Background Research

In order to attain a working knowledge of osmosis and its effects, I looked through school textbooks together with some information I had obtained at the library.

Many substances pass in an out of cells by diffusion. Diffusion is the movement of particles from a region where they are at a higher concentration to a region where they are at a lower concentration. This process does not require energy from respiration. An example of this is the way cells take up oxygen and get rid of carbon dioxide:

The greater the difference in concentration betIen the two regions, the faster the rate of diffusion. Small particles also diffuse faster than larger particles. Water passes in and out of cells by a type of diffusion, called osmosis. Osmosis is the diffusion of water in plants, from a dilute solution to a more concentrated solution, across a partially permeable membrane; or from a high concentration to a lower concentration. This is shown below:

Put simply, osmosis is the passage of water across a partially permeable membrane from a dilute to a concentrated solution.

When a substance, such as sugar, dissolves in water, the sugar molecules attract water molecules and combine with them. The water molecules now combined with sugar molecules are no longer free to move and so the effective concentration of free water molecules in the solution has been reduced. The more sugar molecules there are in the solution, the more water molecules will be ‘tied up’. In other words, the more concentrated a solution is, the fewer free water molecules it will contain.

Pure Water Sugar Solution

Below is a dilute sugar solution separated from a concentrated sugar solution by a thin membrane. The membrane prevents the solutions mixing freely but allows individual water molecules to pass through. There are more free water molecules on the left than on the right and so water molecules will pass more rapidly through the membrane from left to right than from right to left.

In a similar way, there will be a net movement of sugar molecules from right to left across the membrane because there are more of them on the right. However, the sugar molecules are larger and move more slowly than the water molecules. Thus the most obvious effect is the flow of water molecules from the dilute to the concentrated sugar solution.

A membrane which stops solutions from mixing freely but allows molecules below a certain size to pass through it is described as partially permeable.

Preliminary Work

To demonstrate osmosis and the semi-permeable membrane, I decided to conduct a simple experiment using visking tubing. This is a form of processed cellophane which has pores in it through which small molecules can diffuse, so it is partially permeable. I filled the visking tube with a strong sugar solution, and making sure it was tied at both ends, submerged it in distilled water. Based on our background knowledge, I predicted that if left long enough, osmosis would take place, and I would be left with an iso-osmotic solution. This is when there is an equal concentration of water on both sides of the membrane. I left the experiment for three days, and when I returned the tubing had become turgid, showing that the process had taken place. I decided that, although this experiment clearly showed the effect of osmosis, it would be very difficult to measure, and it would not give accurate enough results.

I decided that to accurately measure osmosis, I would need to use plant tissue so that I would be able to easily measure the effect of osmosis. I decided to conduct another preliminary experiment to decide:

What plant tissue I would be using
The size of each specimen
Which solution I would use, and the concentration
How many different solutions I would use, and the range of concentrations
The temperature at which our experiments would take place
How I would accurately measure our results

I decided, for our preliminary experiment that I would use potato ...

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What plant tissue I would be using
The size of each specimen
Which solution I would use, and the concentration
How many different solutions I would use, and the range of concentrations
The temperature at which our experiments would take place
How I would accurately measure our results

I decided, for our preliminary experiment that I would use potato chips as our plant tissue, because they are cheap, readily available, simple to shape and easily observed. I was to leave the skin off the potatoes, as this may have an effect on our results. I would make sure that the potatoes were of equal length (using callipers), weigh them, and then I was to put each potato in a test tube. One test tube would have 30ml of distilled water in it, one would have 30ml of 1 molar sucrose solution, and one would have 30ml of 2 molar sucrose solution. I left each specimen in the test tubes for 10 minutes, and then I took them out of their liquids, dried them, and recorded our results. I used scales to take the mass, and callipers to take the length.

We decided that, although our preliminary experiment gave us the results we required, which matched our prediction, we would need to take more results with more solutions, improving accuracy. From these results it would be difficult to draw a meaningful graph. I decided that for our final experiments, I would use sodium chloride solution instead of sucrose solution, as I found that the former is sticky and more difficult to work with, and a salt solution would be non-sticky and the molecules are smaller, making the osmosis process quicker, and therefore more manageable in our one-hour lessons.

Equipment

For our experiment to be performed efficiently, accurately and safely, we will require:

Callipers
Ceramic Tile
Six test tubes
Test tube rack
Potato slicer
Water
Five different sodium chloride solutions
Stopwatch
Paper towels
Electronic Balance
Callipers
200ml3 Beaker
Scalpel

This is an illustration of how the experiment will be set up:

Safety

In order to ensure the safety of myself and my classmates, I took certain precautions. When I used the callipers to cut the potato chips to size, I used a ceramic tile to stop the table from being damaged; I wore safety goggles to ensure my eyes were not at risk from the blade, and took extreme caution whilst using the callipers. I also made sure that the substances that I worked with were not harmful to us.

Fair Test

To make sure that our experiment was fair, and the results that I took were accurate, I ensured that certain precautions were taken. I made sure that:

The key variable I have chosen to alter is the concentration of sodium chloride. This will give me a varied set of results from which I can draw a conclusion. It would not be a fair test if any other key variables were altered. If another variable; length for example, was not kept constant, then one potato chip may have a larger surface area than the others, giving more space for osmosis to occur.
I will keep the temperature the same, throughout the experiment process. For the purpose of convenience, I will be conducting them at room temperature, which, for our classroom was 23º Celsius.
I will use healthy potato chips, all taken from the same potato, and all the chips will be treated in the same way.

Each potato chip was nearly the same length, (within 2mm of each other in each experiment). I tried to ensure that they were all the same length, but this proved to be very difficult. So as not to let this affect our results, I Were to record our findings as ‘percentage change’.
I used the same electronic balance to weigh our potato chips both before and after our experiment, because measurements may vary slightly between scales.

Obtaining Evidence

Method

I would carry out this experiment by using all the safety issues and fair testing procedures to give me the most reliable and most accurate set of results.

Instead of the three solutions I used for the preliminary experiment, I used six, for improved accuracy. I used distilled water, and sodium chloride solutions of 0.1, 0.2, 0.5, 1 and 2 molar. We poured 25ml of each liquid into a test tube, and placed them in a test tube rack. Each test tube was labelled appropriately, to avoid confusion. The potato chips were accurately cut to size using the potato chip slicer and the scalpel, on the ceramic tile, so not to damage the table or myself. The potato chips were of equal length, so as to keep the size of the surface area constant. I then weighed the chips each using the electronic balance. All the results we took were recorded. We then put the first potato chip in the distilled water, and started the stopwatch. From our preliminary experiment, we decided that we should leave chip in for half an hour. We decided that we would stagger the times that the chips went in to the solutions to retain a high level of accuracy. Each consequent chip was placed in solution exactly one minute after the last, so when it came to taking the chips out of the solution, we could take out one chip at a time. This was beneficial, in that we could make sure each chip was in for half an hour, and that no chips spent any more or less time in the solutions, as this may have changed our results. As each chip was taken out of the test tube, the excess solution was removed using the paper towels. We had to be careful that each chip was treated in the same way, because this could also affect the outcome. Using callipers, the length of each chip was measured again with callipers, and weighed using the electronic balance. These results were recorded, and the change in length and mass was also noted, by subtracting the first measurements (before each chip was put in the solutions) from the last measurements (after each chip had been put in the solutions). This experiment was repeated three times, each time using clean equipment, new potato chips and different solutions, in order to achieve the same level of accuracy each time.

For a completely accurate experiment, the length and mass of each potato chip would be exactly the same. Due to equipment and time constraints, I found this difficult to achieve, and so I decided that I would record the percentage change in length and mass as well. This shows more accurately how the potato chip changed, based on the first recorded mass and length. This was done by taking the difference in mass or length, dividing it by the mass or length before the experiment, and then multiplying the answer by 100. I reduced all results to one or two decimal places, as I felt any more would be unnecessary.

After looking at my preliminary work it suggested that there wasn't enough concentrations of sugar to compare each result to each other, so their will be 8 different sugar concentrations to choose from for the actual experiment. The timing of the potatoes weren't left for enough time in the preliminary experiment, as the results didn't have much change, therefore I will leave them for an hour instead of half an hour to give a greater mass change. I will also be and measuring the mass change as a percentage as it will be more appropriate so that reasonable results could be obtained. I took an average of the percentage difference of each molarity for mass and length. I took an average by dividing the sum of the three experiments, and diving the total by three.

(See results table, separate sheet)

Analysing Evidence and Drawing Conclusions

Analysis

The results obtained from my experiements supports the prediction that I made, showing that the potato cells increase in mass in solutions with a high water concentration, and decrease in mass in solutions with a low water concentration. This smaller graph (not drawn accurately) shows how the potato chips in distilled water gained mass (at point A), and so did the potato in the 0.1M solution (point B). Just below point B, there becomes a state of iso-osmosis, where the chip neither gains or loses any mass. This is around the 0.2M solution. At point C, the potato chip is losing water, and at point D, the chip is fully plasmolysed. The chip has become wilted and will not lose any more water.

The gradient of the line is not constant, and, starting sharply, slopes down and becomes less steep, as the molarity increases. On my larger graph it is clear that all the results lie near to the ‘curve of best fit’, showing that I did not have many anomalies, showing that my results were reliable. These would have been rules out when I took an average of each set of results.

(See graph, separate sheet)

Conclusion

I have found out, based on background knowledge of the subject, and my experiment, that osmosis is a function of plant tissue. My investigation proved successful, as the results that I obtained and the graphs I drew from them were accurate, and fitted with the theory that I had researched. There were no anomalous results, and my results matched the prediction that I had made.

Evaluating Evidence

Evaluation

I believe that I conducted a fair, safe and accurate experiment, where the results shown in the tables and on the graphs were as I had expected, based on my background knowledge. My final results were reliable, due to the precautions I had taken to make the tests fair.

However, improvements to the experiments could have been made, if I was to repeat the experiment with more time. Firsly, I would have used more than six different solutions (in the 0 to 2 molar range) in which to put the potato chips, as, although six solutions provided enough evidence to prove my prediction, more could have provided a more accurate graph. I would also have repeated the experiment five times, to further reduce the chance of an anomalous result affecting my overall results. A potential source of error was the drying of each potato chip after it had come out of its solution. Due to human error, more liquid may have been taken from the surface of one chip than another, which would affect the mass of the chip. A more accurate method of drying each chip with equal consistency would have further improved my accuracy. I would also, as well as measuring the length of the chip, measure the width. This would help me to find out the volume of the potato chip, and aid me in explaining my results.

If I was to continue the investigation into osmosis, I would investigate other factors, such as changing the plant tissue used, or using a different substance in the solution.

Overall, I believe the experiment was successful, and I am pleased with the results.

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