Investigation On Osmosis

Method

The method I actually carried out is as follows:

Cut 2 cylindrical pieces of potato of diameter 1cm

and of length 4cm for each solution (10 in total).

Label the petri dishes so as to avoid any mix-ups.

Using a measuring cylinder mix the correct amount of distilled water with the sucrose solution to make 100ml of the varied concentrations of sucrose solution (50ml for each test). E.g. mixing 75ml of sucrose solution with 25ml of distilled water will give me a 0.75 molar sucrose solution.

Weigh the potato chips on an electronic scale and

take an average weight. Record these on a table.

Average weight = Weight

Number of items

Place the potato chips in their labelled petri dishes and started the stop clock.

After 24 hours I removed the chips and dried the surface with a paper towel.

Weigh the chips on the same electronic scale again and find another average.

Record them neatly on a table so that the results

do not get mixed up or lost.

Finally I repeated the experiment again so I could have 2 sets of results to compare.

Apparatus

I was provided with the following equipment to help me with my investigation:

* 5 petri dishes

* 2 measuring cylinders of different sizes

* An electronic scale

* A knife

* A potato corer (to obtain the cylindrical pieces)

* A stop clock

* A ceramic tile for cutting the potato on

* A potato

* Measuring beakers, to hold the solution until use

* Tin foil, to cover the solution until use.

* Distilled water.

All the above equipment was used in the experiment. (See method)

Safety

During the experiment I had to follow safety measures to avoid accidents. Whilst cutting using the knife I had t be very careful and concentrate fully as not to be distracted which may of led to a slip of the knife. Glass measuring cylinders had to be carried carefully as not to drop them and have glass all over the floor for someone to step on or fall on.

Fair Test

Making sure that our experiment was a fair test was very important. If by accident we had made a mistake then the experiment may have been ruined. An electronic scale was used because it is very accurate and leaves no room for human error, and we always used the same electronic scale so that the measurements where always measured the same. We took an average of the potato chips so that if there was any irregularities they would not be so bad as the other results would point it out and hopefully correct it at least al little bit. We repeated the test 2 times so that we would have 2 sets of results to compare when I plot the graph and will be able to see if any thing wet wrong. Drying the surface of the potato chips is very important because if you do not do it the weight of the water that is there will be added and if you dry it too much then weight will be lost and the results will be wrong. The stop clock ensured that each experiment was on for more than 24 hours to obtain the maximum amount of osmosis. The same measuring cylinder was used for every solution and it was washed and dried to avoid any added sucrose or distilled water, it was dried out to make sure no irregularities where made.

There where however some things that we can to control that may affect the result. We could not use the same potato for all the chips because it was not big enough and we also could not ensure that the next potato was similar. We could however make sure the potatoes cam from the same place and this would therefore help make the experiment fair. We tried to control the temperature as best we could but there is no way we can make it exact and constant in such a large room, so the rate of the osmosis may have been affected by temperature. We did try and minimize all the above by leaving the petri dishes out for over 24 hours.

Results

I collected results from the first experiment and recorded them in the table below. I then worked out the percentage change of the potato chips.

(Percentage change = Difference between before and after/Original)

Solution

Weight before

Weight after

% Change

experiment (g)

experiment (g)

0 molar

3.34

4.105

23

.25 molar

3.43

3.52

2.60

.50 molar

3.37

2.825

-15

.75 molar

3.31

2.39

-28

molar

3.43

2.21

-36

When I repeated the experiment in recorded the results in a graph like the one above and again worked out the percentage change.

Solution

Weight before

Weight after

% Change

experiment (g)

experiment (g)

0 molar

3.11

.975

4.4

.25 molar

3.26

3.24

-0.6

.50 molar

3.24

2.64

-18.5

.75 molar

3.11

.975

-36.5

molar

3.36

2.32

-31.0

In order to make it a fair test I took the percentage change for each solution in each experiment and worked out an average of all the results. This would give me one set of solid results to plot on my graph and to analyze hopefully without any irregular results.

Solution

% Change in

% Change in

Average

First Experiment

Second Experiment

% Change

0 molar

23%

4.4

8.70

.25 molar

2.60%

-0.6

.00

.50 molar

-15%

-18.5

6.75

.75 molar

-28%

-36.5

-32.25

molar

-36%

-31.0

-33.50

Analysis

The graph included shows a line of best fit of the average percentage change for both the experiments. By looking at the line you can see it obviously not straight, which tells us that the concentration of the sucrose solution and the percentage change is not directly proportional. The only pattern that can be seen on my graph is that, as the concentration of the sucrose solution gets stronger the more the average mass of the potato chips decreases. I can see on my graph that the gradient of the line decreases, as you look from left to right, telling me that as the concentration of the sucrose solution increases the more the potato is becoming 'flaccid'. When a plant cell is flaccid the contents of the potato cells shrink and pull away from the cell wall. These cells are said to be plasmolysed. The evidence is that the points are becoming closer and closer together. My results are not that close together but this may be because I used a large scale so the points will be spread further apart, so I am confident that my results are reliable to make good a analysis of.

The main aim of my graph is to show that the average mass of the potato chip will increase with a low sucrose concentrate solution and decrease with a high sucrose concentrate solution. I did not carry on past 1 molar of the sucrose solution but if I had I think that the gradient of the line would have gotten less and less until it was flat, at this point the potato chip would be completely plasmolysed. The isotonic point is the point at which there will be no water lost or gained, no osmosis is taking place. This point can be found when the line crosses the X-axis. My isotonic point is labeled at 0.2625 molar. At 0.50 molar the average potato chip lost about 17.5% of its original mass. This tells us that at this concentration that the water potential in the sucrose solution surrounding the potato chip is less than the water potential inside the potato chip. At 0.75 molar the average potato chip lost about 30% of its original mass. This tells us that as this concentration the water potential outside the potato chip is even less than at the concentration of 0.50 molar. This pattern continues thought the graph and supports my earlier prediction.

From the graph I think the point of complete polymosis can be understood. If you look from left to right, the first two points are far apart showing a large decrease in mass but as you move further along as the concentration increases the points become closer together. I think that if the graph continued we would find that there would very little difference between the next few points.

All of the above results facts match my predictions made earlier before the experiment began.

Evaluation

In my opinion the experiment and the way in which it was conducted was very successful. I managed to collect a good number of quality results that I was able to easily analyze. I think I repeated the experiment enough times with enough chips to draw a firm conclusion, however if I was to repeat the experiment again with no time limit I might as well repeat the experiment many more times. I left the chips there for over 24 hours so I felt as if the maximum amount of osmosis had taken place, if I had done the experiment in one lesson then I don't feel the maximum amount of osmosis would have taken place. The one thing I would like to change if I repeated the experiment would be the number of sucrose solutions I prepared. I was only given 1 molar solution so my maximum was 1 molar but I would like to go up to 2 molar of sucrose in the solution when the experiment would be repeated.

The most difficult variable to control would have been the cutting of the potato. As we only had a ruler to measure with it was really down to the human eye to make the chips as even as possible, which left lots of room for error. By cutting the potato chip longer than the rest or fatter it would give a bigger surface area and therefore more area for osmosis to take place on. By leaving the chips in the solution for over 24 hours I feel that I have countered this problem and it did not really affect my experiment. The other variable that was hard to control was the amount of solution in each petri dish. We wanted 50ml in each one but by using a measuring cylinder some may have been left inside a not gone into the petri dish. To make it more accurate next time I would use a burette.

There where no results that where completely off the line but there is one that I would like to discuss. I tried to rule out abnormal results by taking an average of them all but it seems that I should have repeated the experiment more to rule out this abnormal result. It is the result for .75 molar that is the odd one out, it measured at a 32.25% loss but the best fit line shows it should have been about a 29% loss. This result on the best fit line would have been closer to the first result telling me that my two chips taken for 0.75 molar where from a bad part of the potato. This result rules out the possibility that the first experiment chips where dried too much with the paper towel because if it was then the best fit result would have been closer to my second experiment result. I admit that some of the chips may have been dried too much but not as much to change the result by 7%. However even with this result I am satisfied that my experiment was very successful and that my results are accurate enough to have been analyzed.