Two potato cylinders shall be placed into two separate test tubes per concentration. There shall be two different types of distilled water and seven types of sucrose solution (5%, 10%, 15%, 20%, 25%, 30% and 35%). The tissues shall remain suspended in their separate test tubes for six hours in total.
The potato chips shall be weighed before being submerged into their separate solutions. I shall also measure them at one hour checkpoints for six hours. Hence the percentage change in mass can be calculated.
Change in mass ÷ Original mass × 100 = Percentage change of mass
Hypothesis:
Osmosis is defined as the net movement of water or any other solutions molecules from a region in which they are highly concentrated to a region in which they are less concentrated. This movement must take place across a partially permeable membrane such as a cell wall, which lets smaller molecules such as water through but does not allow bigger molecules to pass through; in this case the partially permeable membrane is the potato.
The molecules will continue to diffuse until the area in which the molecules are found reaches a state of equilibrium, meaning that the molecules are randomly distributed throughout an object, with no area having a higher or lower concentration than any other.
I predict that the potato tissue, which has been submerged into the distilled water solution, shall increase in mass as the solution is moving from a high level of concentration to a low level of concentration in this case the potato chip. The potato tissue that has been submerged in the sucrose solution, at a concentration of 20% will decrease in mass and shorten in length as the substance in this case moves from a high concentration (potato tissue) to a lower level of concentration (sucrose solution) through the semi-permeable membrane which is in this case the potato.
For this particular investigation I think that the lower the concentration of the sugar solution in the test tube the larger the mass of the potato will be. This is because the water molecules pass from a high concentration, i.e. in the water itself, to a low concentration, i.e. in the potato chip. Therefore, the chips in higher water concentrations will have a larger mass than in higher sugar concentrations.
Plant cells always have a strong cell wall. When they take up water by osmosis they start to swell, but the cell wall prevents the cell from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means they become swollen and hard. The pressure inside the cell rises and eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what make the green parts of the plant "stand up" into the sunlight instead of being weak and unable to get sunlight.
When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid." This is the exact opposite of "turgid". The contents of the potato cells shrink and pull away from the cell wall. These cells are said to be plasmolysed.
When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means "about to be".
Earlier work that allowed me to improve my planning was the trial experiment which ensured that I did the actual experiment more accurately. For example in the trial experiment our group did not measure the starting mass which we later discovered is a key factor in the process of osmosis. Other information that I have obtained is from revision guides and science text such as GCSE Biology.
Safety Precautions:
For safety precautions, I shall place a white square tile on our work table, once this has been done I shall put the potato on the white tile and insert the cork-bearer, a cork-bearer allows me to remove a cylinder of potato tissue, I must not put the potato in my hand and then insert the cork-bearer as it may pierce through my skin.
The precautions we took were: Whilst cutting the potato, extreme care and precision had to be taken with the scalpel as it is very sharp and could easily cause a serious wound, we also had to repeatedly measure the length using the steel ruler. The measurements for the solutions had to equal, for the experiment to be fair. We had to ensure that every time we handled the potato cylinders that our hands were dry and clean so that the moisture or perspiration from our hands would not affect the potatoes moisture.
Equipment
- The equipment that shall be used in this experiment are:
- Volumetric Flask (500cm capacity)
- Burette (for precision)
- Balance (2, 3d.p. for accuracy)
- Cork Bearer (to extract potato tissue)
- Beaker
- Tile (to rest the potato on)
- Test Tube (to house the solution and potato tissue)
- Forceps (to remove potato tissue from the solution)
- Clock (to allow notification of estimated time of checkpoint)
- Measuring Cylinder (to measure solution)
- Callipers (to measure potato length and diameter)
- Paper tissues (to dry excess moisture of potato)
- Varying concentrations (for experiment)
- Potatoes ( for experiment)
- Labels (to label the test tubes)
Variables
Variable are factors, which may alter the reaction of an experiment. The variables that we shall need to focus on for our experiment are heat and concentration. To allow a fair test we must put all the potato tissue in their separate test tubes at the same time so that the room temperature will not be different for each test tube. The surface area of the potato tissue must be the same for all of them, they cannot be smaller or larger as the smaller the surface area is the faster the reaction will take place, and the larger the surface area the slower the reaction is. We can make sure that the surface area is the same by measuring the cross-sectional area of the potato tissue.
To create a fair test certain aspects of the experiment will have to be kept the same. As I shall be varying the concentration of the sucrose solution it will allow me to attain a set of varied results from which I may understand the theory of osmosis and therefore create a conclusion to my experiment. If any of the variables below are not kept constant it would mean that the experiment is unfair. For instance, if one of the potato chips was 5mm longer the surface area of the chip would be larger and there would therefore be more space for osmosis to take place.
To keep the water potential of the potato equal I have used the same cork bearer to remove all the potato chips and they have all been weighed at 4.00 grams (2d.p.) and measured at 60mm. The same balance is used to weigh my potato chips. This is because the measurements may slightly vary between scales. The volume of the solution that the potato chips are kept in must be fair. This must be 10 ml. Room temperature was used to conduct the experiment which was 27 degrees centigrade.
Method:
1. I took two average sized ground potatoes and checked that they were both healthy and hard.
2. Using a cork bearer with a diameter of 9mm I extracted potato chips from the potatoes. Therefore each potato chip had a diameter of 9mm.
3. Using a scalpel and steel ruler (for greater precision) I trimmed each potato cylinder to 60mm but I had to be very careful whilst cutting the potato as the scalpel is very sharp. In total I had 16 potato chips. Therefore each potato chip had a diameter of 9mm and a length of 60mm
4. Taking two test tube racks we placed eight test tubes on each rack each accompanied with a label of the solution that the potato cylinder were suspended in, this varied from distilled water to sucrose solution with a range of 5%-35%.
5. Using a measuring cylinder I measured out different amounts of sucrose solution and distilled water which was 10 ml for each.
6. I then weighed every potato chip on an electronic balance accurate to 2d.p. and trimmed the potatoes until all had an equal mass of 4.00 grams.
7. After putting the potato cylinders into their separate test tubes we took note of the time and came back at one hour checkpoints for six hours.
8. At every checkpoint the potato cylinders were removed from their test tubes using forceps and dried off lightly using a paper towel, their masses recorded and were then replaced into the test tubes using the forceps. As was stated in my planning section of the coursework, I had to keep all of the different non-variables the same, to ensure that none of them affected the results of the experiment in any way.
Trial Experiment
Firstly we got a potato to experiment on. We put our potato on a white tile after which we inserted a cork borer into it extracting two cylinders of potato tissue. We removed the potato skin and measured the two pieces of potato tissue each at 50mm in length. We then measured two equal amounts of liquid, one, which was sucrose solution at a concentration of 20% and the other, distilled water. We inserted one of the two pieces of potato tissue into sucrose solution at 20% concentration, and the other was submerged into distilled water. The potato tissues were inserted into separate test tubes. We labelled them test tube A and B. We inserted the potato tissues into their separate test tubes at the same time. We did this so that the room temperature would be the same for both of them making it a fair test. We put the test tubes into a test tube holder and left them in the lab for approximately 24 hours after which we removed the potato tissues and measured the mass and length of the two tissues. In test tube A the length was 51 mm and therefore had increased by 1 mm and the new mass was 4.3 grams. In test tube B the new mass was 3.2 grams and the new length was 45 mm a decrease of 5 mm.
The tissue in Test Tube A felt harder than it did before we inserted it into the test tube.
The tissue in Test Tube B actually felt soggier and more fragile than it did before we inserted it into the test tube at the beginning.
Analysing and Drawing Conclusions
From the investigation I have found that the theory of osmosis is true. The results show that, in accordance with our hypothesis, the cylinders will expand when external solute concentration is low (high water concentration), and contract in strong solutions (low water concentration). This is due to osmosis, where water passes from weak solutions to strong solutions across a semi-permeable membrane, such as a potato cell membrane. The graph of average percentage change against solution strength shows that the results trend follows the theory.
Overall, the results seemed to match my prediction apart from the 5% sucrose solution concentration, which gained mass.
In my graphs the average percentage change line kept on decreasing as the concentration increased. When the lower concentrations of sucrose such as 5 % and distilled water are in contact with the potato tissue there is more water concentrated inside in the solution therefore water osmoses into the potato tissue and becomes turgid (stiff) as its full of water. At the concentrations of 10 – 35 % mass is lost by the potato chip, as the concentration of the water inside the cell is greater than that which is outside the cell therefore the water osmoses outside of the cell into the surrounding solution and becomes plasmolysed (soggy). The trend of my graph is that firstly mass is gained but sharply decreases continuously, this is due to the varying concentrations of sucrose.
Evaluation
I think that the experiment was successful. I had a large amount of accurate data from which I was able to draw graphs and conclusions. The range of concentrations was adequate but if I repeated the experiment with a larger range I could have possibly gained more accurate results. This would have allowed me to also find out the isotonic point far more accurately then the one estimated on the graph. Two solutions with equal solute concentrations are said to be Isotonic, therefore the isotonic point is the equilibrium of the two concentrations which in our experiment was 9 % concentration of sucrose. Osmotic potential refers to the potential for water to move into a region. Pure water has an osmotic potential of 0 (zero) and the greater the solute concentration the more negative the osmotic potential. Osmosis, then, could be defined as the movement of water from a region of less negative osmotic potential to a region of more negative osmotic potential. A region with higher solute concentration (more negative osmotic potential) is said to be Hypertonic.
I think that the weighing and cutting of the potatoes was the most difficult and lengthy part.
I believe that the experiment worked out quite well, as the results we obtained seemed to follow a trend apart from one anomalous result, which will be discussed later.
I think that the results were accurate enough for the investigation as we measured the mass and length in millimetres and grams to two decimal places. We also had two separate results for each solution, which allowed us to calculate an average value.
There was an anomalous result for the 5% sucrose concentration, which at every checkpoint increased in mass instead of decreasing. The event occurred in both test tubes of 5% sucrose concentration. Another incident that occurred is that the 10% sucrose also started to increase in mass at the first two checkpoints, but later decreased, this was discussed in the planning that weaker concentrations may increase in mass at first. The anomalous result may have been gained due to excessive drying of the potato tissue however I doubt this as the tissue gained mass at every checkpoint.
Another method which I would like to have introduced to make the results more accurate is to have potato tissues with different cross-sectional areas and masses; I would do this so that average values could be calculated depending on the potato and to observe if the desired events took place due to the law of osmosis.
I think that the method that we carried out to do the experiment was the best we could have done with the equipment available as we measured all potatoes equally and accurately along with the solution in which there were to be suspended in. We also prepared and submerged the potatoes all at the same time, which allowed it to be fair as well. A further improvement is that we may have tried to use the burette which ensures greater precision in the measuring of solutions. The mass of the potatoes may have also been measured more accurately by using scales with greater precision that may read up to three or four decimal places.
Our method gave results that are reliable except the anomalous result that occurred. I would have like to have repeated the experiment to see if this still happened.
I believe that we have enough correct, accurate results to allow us to obtain a reasonable conclusion. This is because all our results apart from the 5% of sucrose seemed to follow a trend for instance the distilled water increased in mass whereas all the other sucrose concentrations decreased in mass by the end.
