- The Suitable control in my actual experiment will be the temperature. The actual experiment will be conducted under room temperature. This is because if the temperature under which my experiment will take place in, is greater than room temperature, then, the rate of osmotic activity will be higher. If my experiment was conducted under a temperature that is lower than room temperature, then, the rate of osmotic activity would be at a slower rate. Therefore, constant room temperature is required as a suitable control.
Fair Test
To create a fair test certain aspects of the experiment will have to be controlled, whilst one key variable is changed. We have chosen to vary the concentration of the sucrose solution
Controlled Variables
-
Constant Temperature- otherwise the warmer the solution the quicker the rate of osmosis. This is because the water molecules will
be moving more quickly as the temperature increases.
-
Surface Area- this will speed up the rate of osmosis because there will be more space on the potato for osmosis to take place.
-
Type of Potato- the higher the concentration of water in the potato the quicker the osmosis will happen because less water will have to diffuse into the potato to even amounts of water inside and outside the potato.
-
Same volume of Sucrose Solution- in each test tube.
-
The potato chips- are the same size and mass.
-
Use an accurate balance- showing grams to 2 d.p, to ensure even small changes in mass are recorded. I will also try using this balance throughout the experiment so the measurements do not vary due to any changes that may be introduced by different scales.
Human Errors.
I will try to make my experiment as accurate as humanly possible – however in certain cases in the experiment, human error may occur:
- Whilst taking out the potato chips from the tubes and placing them on the paper towels, prior to weighing them, some potato chips may be allocated more time to dry than others.
- I must make sure that my hands are thoroughly dry before touching the potatoes otherwise, if wet, this would add moisture, and, the water from my hand may diffuse into the potato chip, and therefore increase the mass of the potato chip.
- To lower the margin of error, instead of using measuring cylinders I used a syringe that has a lower margin of error, to fill up the test tubes.
Safety
I will try to ensure safety around the lab:
- Wearing safety goggles
- Removing bags from the walking space
- Push the stools under the bench
- Stand up
- Walk carefully and don’t rush
- Handle the equipment and apparatus carefully
- Handle the scalpel with great care as it is very sharp and could be potentially dangerous.
My Prediction
I predict that the lower the concentration of sucrose solution, it will cause the chip to become larger in mass, and, the higher the concentration of the sucrose solution to become lower in mass. I predict that as I increase the concentration of sucrose then the amount of water moving in or out of the potato will change. Dilute sucrose or distilled water will make the make potato increase in mass. I can support this prediction by looking back at my introduction on osmosis. Water will move into the concentrated potato cells because of the high concentration gradient. In very concentrated sucrose solutions water will move out of the potato and into the sucrose solution. I predict that the mass of this potato will decrease.
Method
I plan to carry out this experiment by using all the safety issues and the fair testing procedures to give me the most reliable and most accurate set of results.
A range of sucrose solutions was prepared, with concentrations of 0.00 M, 0.25 M, 0.5 M, 0.75 M and 1.00 M. Adding varying amounts of distilled water to the sucrose solutions we made 0.25 M and 0.75 M sucrose solutions. Sections of the potato were cut and sliced with a scalpel, and were measured with a ruler. This specific part of the experiment was done very carefully because the slightest change in the surface area can change all the results by provoking more or less osmosis to occur. Then, the mass of each chip was measured as well, so more results were obtained. All the chips measuring 3 cm in length were placed into the test tube, containing the varying sugar solutions. I used 10 cm3 of each solution, and then I labelled the test tubes using sticky labels. The 15 test tubes containing a chip each were left overnight to allow osmotic activity to occur. Then the chips were removed and were re-weighed a day later.
A table to show my results obtained
Analysis.
Graph 1 shows the line of best fit for the percentage change in mass of the potato chips overnight. The graph 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 and loss in mass and 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 and loss is in inversely proportional to the concentration.
Graph 2 also supports the prediction that as the concentration of the sugar increases, the mass of the potato will decrease.
It can be seen from the graph that the gradient does change on the graph. It gets less steep as the concentration of sugar solution X axis gets bigger. This is because the potato chip is getting more and more flaccid. Also, from my graph it appears that I have not reached the point where there appears no further water loss, suggesting that the cell is fully plasmolysed. Maybe if I go up to a sugar concentration of 2.0M then it maybe achieved.
Scientific Knowledge
Osmosis is the passage of water molecules from a weaker solution into a stronger solution, through a partially permeable membrane. In this case, the tiny holes in the membrane of the potato will allow the water molecules to pass through in and out of the solution and the potato, depending on the concentration gradient of the two substances. The bigger the difference in the concentration of a substance on either side of the membrane, the faster it will tend to diffuse. This difference is called a concentration gradient.
With potato or plant cells, it swells up as water enters via osmosis, however, it won’t burst because the cell wall is tough, and it only stretches out slightly. When it can’t stretch anymore, it is called Turgid; this is what makes the plant firm and stand up. When the cell has very little water in the vacuole the potato cell is called flaccid, and Plasmolysis has occurred. Plasmolysis occurs when a plant/potato cell membrane shrinks away from its cell wall. This occurs when water is drawn out of the cell and into the extracellular (outside cell) fluid. The movement of water occurs across the membrane moving from an area of high water concentration to an area of lower water concentration outside the cell. However, there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred.
Using the scientific knowledge and the results from my graph I have come to the conclusion that, as the concentration of the solution increases, the percentage in the mass of the potato decreases. This is because, when there is a high concentration of the sugar solution, there is a low amount of water surrounding the potato. When there is a low amount of water surrounding the potato, osmosis, will tell you that the water will diffuse out, and cause the potato to become flaccid. However, when there is a low concentration of the sugar solution surrounding the potato, there is a lot of water outside. So, in this case osmosis will tell you that since there is more water outside, water will diffuse in, causing the potato to become turgid.
The potato chips placed in the distilled water are hyper-osmotic, therefore the water will diffuse by osmosis into the potato from an area of high concentration down the concentration gradient. This will result in a gain of mass for the potato. However, in the 1.0M solutions the opposite occurs as there will be a higher concentration of water within the potato than the solution, containing lots of solutes. This will make the potato hypo-osmotic and it will lose its water content as the water diffuses out.
My graph proves that my prediction was correct. This is because I made the assumption that, as the concentration of the solution goes up, the mass of the potato will decrease.
Evaluation
In my opinion, I think the experiment was successful as I obtained enough results, from which I was able to create informative graphs. However if I was to repeat the experiment, below are some of the aspects I would improve on.
- I found preparing the sugar solution the most difficult part of my experiment because I could never be 100% sure I was using accurate amounts. A more accurate pipette would have been useful. In future I could have used a burette, which would have ensured that the amount of solution in the tube was more accurate. If we were to be extremely accurate we could have used the process of titration whilst measuring the solutions.
- I could have also weighed each potato piece to a more accurate scale i.e to three of four decimal places; in this way I could notice even the slightest of change in mass that had taken place.
- I did not take into account the effect of fluctuations in the room temperature on my experiment, though I could not have controlled the temperature inside the laboratory. This would have an effect on the rate of osmosis because the molecules would move faster, and therefore increasing the rate of osmosis.
- If the sugar solution evaporates past the level of the potato, then the potato sample will have less surface area in the solution so this would make osmosis happen much slower. To stop any solution evaporating I could place a layer of cling film on top of the test tube.
- I was forced to rely on imprecise tools to cut the potatoes, such as rulers, scalpels and most of all my own judgment. Some kind of machine to cut the potato chips into exact pieces would have been much more accurate.
- I could also extend this experiment by repeating exactly as before. However this time I could take more results at the molarity levels eg 0.1M, 0.2M, 0.3M, 0.4M, 0.5M, 0.6M, 0.7M, 0.8M, 0.9M, and 1.00M. These solutions would produce more accurate results.
To extend the investigation I could use concentration s like 0.05M instead of 0.1M. This would increase the accuracy of the entire experiment including my graph.
- Another way to extend the investigation could be using different plant tissues. For example a carrot tissue or an apple tissue, or, I could keep my potato investigation and compare it to a carrot or an apple. I could then decipher which tissue is most liable for osmosis to occur.
-
Also, to extend the investigation I could vary the temperature and then put the potatoes in say 10 °C, 20 °C, and 30 °C water baths. The same investigation rules, like the variables and the time allocated for osmosis to take place, 24 hours, still apply.
But overall, given the apparatus that we got to carry out the test, I think this experiment turned out to be very successful, and I’m very pleased with my results because they backed up my predictions.