In the 0.25 molar solution, the mass of the chip will slightly decrease as both the concentration gradients are quite similar, but in the 0.5 and 0.75 molar solutions there will be a noticeable mass decrease in the potato chip because the concentration gradients are very different. The water concentration in the potato chip is higher than in the 1 molar sucrose solution, so the water will leave the chip through a partially permeable membrane and go to the sucrose solution. The mass of the chip will decrease, thus the chip becomes flaccid.
The graph below shows a simple curve when the concentration of the solution is plotted against the percentage change in mass.
At point A the mass of the chip has slightly increased as there was a lower water concentration in the chip and the water entered the chip from a region of high water concentration.
At point B the cell sap is equal. Therefore there is no difference between the two concentrations so it is balanced. No water will enter or leave the chip that is why there is no change in mass.
As the concentration of sucrose increases, the mass of the chip decreases as seen at point C, this is because the sucrose solution has a low water concentration and the water leaves the chip because it has a higher water concentration, thus the chip loses mass.
The following apparatus was used to perform the experiment:
-
Scalpel x 1
-
Potato borer x 1
-
Test tube x 5
-
Test tube rack x 1
-
Measuring cylinder x 1
-
Stopwatch x 1
-
Weighing scale x 1
-
White tile x 1
- Paper towel
Method:
First I got a potato borer and pushed the potato borer through the potato, cutting it into long cylinders with a diameter of 5 mm. I had 5 potato cylinders. This was done on a white tile. I then used a scalpel and cut the chips to a length of 6 cm by measuring with a ruler.
Taking a test tube rack I placed 5 test tubes and then labeled them 0.00M, 0.25M, 0.5M, 0.75M and 1M. Using a measuring cylinder of 20 ml, I measured the different concentrations of sucrose solutions. I weighed the potato chips on weighing scales and recorded the weight. I put the potato chips in each test tube and immediately started the stopwatch for 20 minutes.
After 20 minutes I took the chips out of the solutions and dried them with a paper towel so that the water outside the potato tissue would not alter the weight it is supposed to be. The mass of the chips was measured and recorded in the table below:
I repeated the experiment for reliability. This time I left the potato chip in the solution for 10 minutes but the results were not as good as the previous experiment’s. Not enough time was given for osmosis to take place. So 20 minutes is a good time for the experiment.
Fair Test
The experiment must be a fair test, if it is not we will be obtaining the wrong results. To make sure it is a fair test all aspects of the experiment must be the same except for the concentration of the solutions which will give us a varied set of results.
The measurement s of the solutions has to be exact. The length and width of the potato chips must be as accurate as possible as this could affect the rate of osmosis. The temperature must be constant through out the experiment that is why I did the experiment at room temperature. The same weighing scales will be used throughout the experiment, this is because the measurements may vary slightly between scales.
The stopwatch should be started as soon as the potato chips are put inside the test tubes, and should be stopped as soon the 20 minutes have passed. Taking the chips out of the solution, drying them and measuring the mass should all be done as quickly as possible.
Lastly and most importantly the chips should be in compete contact with the solution if not, osmosis may not occur to its fullest potential.
Safety
Safety is extremely important and must be taken into consideration.
Whilst using the scalpel to cut the potato, extreme care must be taken as the scalpel is sharp and it could easily harm you or someone else.
Caution must be taken when using the potato borer. You must not put your hand underneath the potato when pushing the potato borer as you can stab your hand and cause a serious injury.
The Actual Experiment
The same concentrations of sucrose were used, the length of the potato chips was 6 centimeters and the width was 6 mm. I changed the diameter from 5 mm to 6 mm because 5 mm was not wide enough and more osmosis could take place with a larger surface area, thus the greater change in mass. The solutions were measured at 20 ml and the time was 20 minutes.
I followed the same procedure as before.
Average Change in Mass
The results show that the potato cylinders expand when the external solution concentration is low (high water concentration) and contract when it is in a high molar solution (low water concentration).
I have plotted a graph from my results.
The graph shows that as the concentration of sucrose increases the mass of the chip decreases, this is due to osmosis when water molecules pass from a region of high water concentration to a region of low water concentration through a partially permeable membrane.
The point where the line crosses the x-axis is the isotonic point. This is where no osmosis is taking place, i.e. no water is moving in or out of the potato, both the potato and the solution have an identical molar concentration. At 0.25 Molar a slight loss in mass has taken place as the solution has a low water concentration and water has been lost due to osmosis. An even greater mass loss has occurred at 0.50 Molar as the solution has an even lower water concentration. When the concentration reaches above 0.75 M, there appears to be no further water loss, suggesting that the cell is fully plasmolysed. This shows that the sucrose solution has a very low water concentration. This pattern carries on through out the graph, and even more mass is lost, as more water moves out of the potato chip into the solution.
The line in the graph is straight so the gradient does not change which means that the average change in mass is directly proportional to the concentrations.
From the line of best fit, it can be seen that all of my points are close. This shows that my results are fairly reliable.
My results are very similar to my prediction. This was that osmosis would take place and there would be a loss in mass as the molar concentrations increased. Even the graph is very similar to my predicted graph.
Evaluation
In my opinion the experiment went well. I obtained a good set of results from which I was able to plot informative graphs. My graphs have a strong resemblance to the one in the prediction.
The time, 20 minutes was enough for osmosis to occur and the range of concentrations was adequate but if I was to repeat the experiment I would have had more types of molar solutions, i.e. 0.10 M, 1.15 M, 1.20 M, and so on. This would have allowed me to find the isotonic point more accurately as the one that I estimated was an approximation. I could have also obtained more accurate results.
Making sure the potato cylinders were exact was difficult as it could have affected the surface area and overall the rate of osmosis. If I were to repeat the experiment I could have used cloned potatoes and a machine that would cut out the chips with the exact dimensions and weight. I could have also weighed each chip on a more accurate scale, e.g. 0.0000g rather than 0.00g. When the potato chips were removed from the test tube and dried I may have dried some chips more than others and so some would have had more water than others which would affect the mass. If the experiment was repeated I would have found another way to dry the potatoes that would ensure that all were dried in the same way for the same time
My results did lie close to the line so they were fairly accurate but to be completely accurate I could have had made a few changes which are mentioned above. There were not any anomalous results, but some results were not as close to the line as others. This may have been caused by human error.
For reliability, I have plotted a graph showing the changes in mass from both attempts made against the difference in concentrations of sucrose solution.
Most of my results are close to the line of best fit but there are a few further away from the line, this could have been caused by human error, or perhaps the potato chip was not cut accurately, or cells did not lose or gain mass from that part of the potato.
To make this experiment perfect I could have used more molar solutions, the temperature in the room could have been accurate as the temperature may change due to weather conditions or if the experiment was done near a window, longer time could have been given for more osmotic activity to occur and the potato chips could have all been identical in mass, surface area, length and width.
Over all I thoroughly enjoyed the experiment and I am very pleased with my results.
