Evaluation
The experiment was easy to do , but all the results I had to take had to be accurate had to change my plan several times. Things like having the potato chip 5cm long changed to 3cm because it was it was hard to have all the chips 5cm long, and 3cm was sufficient. Also, I changed the amount of solution I would keep the potato chips in, because I only needed enough to cover the potato chip. These changes were easy to make, because I did a preliminary experiment, so I could iron out any errors that may have occurred in my actual experiment.
I think I took enough results for the amount of moralities that I was given, and the time restrictions that I had to follow, (1h20 mins). The range was big enough, but to fill in the gaps in my graph, I could have taken results of more moralities, i.e. 0.10m, 1.15m, 1.20m, etc. This way I could have found the isotonic point by there being no change in mass. This would show the water potential of the potato , and this result could then be used further. Also to make my experiment better I could have repeated it more, and possibly have worked with someone else, or with the whole class. I could have also used cloned potatoes, so that they would have all been the same. I could have also cut the potatoes into doughnut shapes, because the cells in the center of the potato may have a different capacity of water and the intake, or distribution of it, or different water potentials. If I had cut the potato into doughnuts, I would have only been using the out side of the potato, leaving out the middle. I could have also used a machine to cut the potato chips.
My results did lie close to the curve, and therefore were fairly accurate, but if I was doing the experiment again I could make changes, some as mentioned above. I could also use a burette measure out my solutions. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a digital and more accurate scale, e.g. not to 0.00 but to 0.0000g.
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, or on out of my six results could have been inaccurate, and changed the average drastically. Or perhaps the potato chip was not cut accurately, or that part of the potatoes, cells did not loose/gain mass well. My results did vary a lot for the same concentration. The fact that a certain part of the potato may not be the same as another was shown as the chips all of the same length were not the same weight, or even close. This is another reason to use cloned potatoes. My results were consistent, and there were no results that they were ignore .
I could extend my enquiry by testing the percentage change in mass with morality using a different substance. By this I mean using a different vegetable, perhaps celery or cucumber. Then I could find out whether osmosis occurs with the same patterns and trends with any vegetable.
Evaluation
I think that the method used was fairly accurate, as considering the equipment that I had used I had made the best possible outcome for my results obtained. I think that my results are quite accurate, however there are always ways in which to improve the reliability of my results and make them more accurate. Ways in which I could change it is; that I could have made the potato pieces all the same weight to begin with making the percentage increase and decrease a lot more accurate. My method was good, it was easy to carry out and causes no hassles or complications with the investigation. When looking at my results I can conclude that they are good enough to support a firm conclusion, as my results were as what I had predicted. My results were also very similar to what scientific information from expert sources predicted it would be. This would basically be the osmosis theory that water moves from a high concentration to a low concentration. This would be that way petri dish 1's potato piece had taken on weight and the rest of the petri dishes potato pieces had gained smaller amounts of weight to where it started loosing its weight.
Improvements for this investigation would be that I should have repeated the experiment more times, making the results more accurate. Further work that could be done to give more evidence towards the conclusion would be to find out the effect of temperature on turgidity and flaccidity. Whether it would speed up or slow down the osmotic process.
Hypothesis:
The palisade cell is surrounded by a cell wall that is mainly made of fibres of cellulose and contains other substances. It is permeable to water and allows dissolved substances to pass through. It is not selective unlike the cell membrane. The cell membrane is a thin layer of cytoplasm around the outside of the cell which separates the cell and its outer surroundings. It is selectively permeable, so it controls which substances are allowed to leave and enter the cell e.g. water. It also maintains the structure of the cell. The cytoplasm is a watery jelly-like solution. This is where many chemical reactions (metabolism) take place which keeps the cell alive by providing energy and making substances that the cell needs. In the cell, there is a nucleus which controls the chemical reactions in the cell. The plant cell also contains many chloroplasts and a sap vacuole. Chloroplasts contain chlorophyll which is necessary for photosynthesis. The sap vacuole is a solution of water, salts and sugars.
Osmosis is the movement of water from an area of high water potential to an area of low water potential through a selectively permeable membrane. Water potential is the measure of the tendency of a solution to lose water. In a solution, the number of free water molecules is less than in pure water. This is because, the molecules of the solvent attract some of the water molecules and hence stop them from moving freely. This reduces the number of free water molecules. An area of high water potential has a higher concentration of free water molecules than the area of low water potential. The semi-permeable membrane has pores along it. Water molecules can easily pass through these pores. Depending on the size of the pores, heavier molecules in the solutions can also pass through but at a much slower speed. In osmosis, there is an area of high water potential (more free water molecules) and an area of low water potential (less free water molecules). Because the area of high water potential has more free water molecules, it moves into the area of low water potential. Although the area of low water potential has got water molecules moving into the area of high water potential, it is not as much as the number entering. This means that the net movement is moving into the area of lower water potential.
As the water potential outside the potato increases, the percentage change in mass of the potato will increase as well. Osmosis occurs in potato as the cell wall allows water to pass and the cell membrane is the selectively permeable membrane that allows water to pass. The cytoplasm contains salt and sugar dissolved in water. The sap vacuole contains cell sap, which is also a solution of sugar and salt in water. When the potato is in water, the water potential outside the potato is higher than the water potential in the plant cell of the potato. The net movement of the water is then into the cells. The water enters the cytoplasm. The cytoplasm then has a higher water potential than the sap vacuole so the net movement of the water is into the sap vacuole. This causes the vacuole to expand as there is water entering. This pushes the cytoplasm against the cell wall causing the cell to be turgid and to increase in mass as there is more water entering the cell.
If the potato is put in a concentrated sugar solution, the water potential outside the cell will be lower than the cell inside. The water from the cytoplasm then leaves the cell. The vacuole then has a higher water potential than the cytoplasm and water leaves the vacuole. This causes the vacuole to contract and causes the cytoplasm to contract as well. The cell membrane is drawn in as a result of this. The cell has now become flaccid and has decreased in mass as water is leaving the cell.
Factors that affect the change in mass of potato is the water potential of the potato and of the solution outside the potato, the surface area of the potato and time. If the potato has a large surface area there are more pores in the membrane of the potato for the water to enter. Time is a factor as it affects the amount of water molecules that has left or entered in the end result.
I believe that the graph will look like this because I predict that the higher the sugar concentration, the lower the percentage change. The line of the graph decrease quickly and then its descent slows down. To begin with when the water potential of the outside the potato is higher than that the potato, the percentage change in mass would increase as the net movement of water molecules would enter the potato. When the water potential of the sucrose solution is the same as the potato, there would be no change in mass as the number of water molecules entering the potato is the same as the number leaving the potato. When the water potential of the sucrose solution is lower than potato, the percentage change would decrease because the net movement of the water molecules is leaving the potato.
Method
The dependent variable that we changed was the water potential outside the potato chip, The pilot study showed that a range of 0% to 70% was suitable because after a period of time, two potato chips had sunk to the bottom and the other three were rising to the top. They showed a change in density of the potato chips. The potato chips that were floating was doing so because the proportion of air in the potato was increasing as the water was moving out of the potato. We decided to measure mass as our independent variable. We didn't measure length although the length did decrease as the concentration of the sucrose increased, the change in length was not as sensitive as the change in mass.
The water potential of the potato is a factor of the change in mass, so all the potato chips were bored from the same potato to keep the experiment fair. Another variable that affected change in mass was the surface area of the potato chip. We solved this by standardising the chips. We standardised the chips by using the same cork borer. We pushed the cork borers on a white tile rather than our hands for safety reasons. We standardised them again by cutting them all the same length using the Vernier calipers.
We changed the concentration of sucrose by using different amounts of sucrose and dilute water. We kept the total amount the same at 20cm³. We measured the amounts by using a 10ml syringe and a measuring cylinder. We chose 5 different concentrations so that the concentrations had the same deviation so that they were regular. We chose 0%, 17.5%, 35%, 52.5% and 70%. We also use replicates. So that we had fair results, the replicates was in the same solution at the same time so there was no slight difference of the water potential outside the potato. Due to the size of the boiling tube, we could only have two potato chips at one time so we only had one set of replicates.
Before we put the potato chips in the solutions, we measured the mass of the potato chips using a top pan balance. We identified which potato was which by observing the position of the potato chips. We timed how long the potato chips were in the solutions using a stopwatch. We tried to keep them all in the solutions using a stopwatch. We tried to keep them all in the solution for the same amount of time to keep the experiment fair as time is also a factor of the change in mass. We dried the potato chip and then measured their mass with a top pan balance.
Conclusion
Throughout the experiment, when the sucrose concentration increased, the percentage change in mass decreased i.e. when the sucrose concentration was at 0%, the percentage change was +5.9%. When the sucrose concentration increased to 35%, the percentage change decreased to 11.7%. And when the sucrose concentration increased to 70%, the percentage change decreased to -20.9%. This fully supports my prediction that as the sucrose concentration increased, the percentage change in mass would decrease. The increase in sucrose concentration means that there is a decrease in the concentration of free water molecules. Because of osmosis, these free water molecules would enter the potato. As there is less free water molecules, the amount that enters the potato chip is less. At a point, the amount of water molecules that enters the potato chip would be less than the amount that leaves the potato. At this point, the net movement of the water molecules is leaving the potato so the mass of the potato is less. So the less water molecules that enters the potato, the less the change in mass of the potato. The graph shows the water potential of the potato to be the same as the water potential of a sucrose solution that is 3%.
The graph shows a large decrease in percentage change but then the decrease slowed down. There was a large decrease at first when the sucrose concentration was 0% to 17.5%. The decrease was from +5.9% to -9.9%, a difference of 15.8%. The rate at which the change of mass decreased slowed down so when it was from 52.5% to 70%, it went down from -14.3% to 20.9%. The difference there was 5.6%. This supports my prediction that the change of the percentage change of mass would decrease as the sucrose concentration increased.
Evaluation
Most of the results were reliable as the replicates were close to each other. For example when the sucrose concentration was at 0%, the results were +5.6% and 6.1% and when the sucrose concentration was at 52.5%, the results were 14.1% and 14.5%.
There was an anomalous result when the sucrose concentration was at 70%. The results were -24.3% and 17.5%. I believe that the anomalous result was the -24.3%. There should be another replicate so that it is easier to see which is the anomalous result. I think that it is anomalous because it had small slice in it from when we were removing it from the borer. This led to the chip having a larger surface area in proportion to the mass compared to the chips. Because it had a larger surface area, it would have a larger change in mass compared to the other chip.
I am satisfied with the range as I was looking for the effect of osmosis in plants. There is enough reliable evidence here to show the effects. But as I was also looking for the water potential of the potato, the range was too large. The water potential of the potato is between 0% and 17.5% but there was a large difference between the percentage change in mass of these concentrations. The graph was too vague about where the water potential is so it was not really sufficient or accurate. The water potential that was given was too approximate.
The Vernier calipers were more accurate than using a ruler and the top pan balances were very accurate. Although the cork borers helped standardised the potato chips, occasionally when we were removing the chips out of the cork borers, we made a small cut. This only slightly increased the surface area in proportion with the mass and slightly decreased the weight. But as the changes in mass were small themselves, this could have led to overestimation.
Another problem we encountered was keeping the potato chips in the boiling tubes for the same amount of time. They were not because there was a delay between each reading so that we were sure which replicate was which. In some readings this could have lead to underestimating and in other readings it could have lead to overestimating.
I would have improved the experiment by timing each reading individually so it would have been more accurate. I would improve the experiment to find the water potential of the potato by having a smaller range of percentages e.g. 0% to 15%. I would also have more replicates perhaps by using a larger boiling tube or maybe beaker.
Planning
I am investigating the effect of osmosis on a potato. Osmosis is the diffusion of water down a concentration gradient through a semi permeable membrane.
I am investigating the effect of water concentration on the potato chips. This means that I will put potato chips into salt solutions of different concentrations. There are also other factors involved, these are:
- Temperature of water
- Size of chip
- Light
- Mass of potato
- Volume of solution potato is in
- Type of potato
- Time left in solution
- Surface area of potato chip
- Using the same balance to measure chip
If I am going to make this a fair test, then I will have to control these other factors. Doing the tests at one fixed temperature will control the temperature of the water. For the purpose of my experiment I am going to do all the experiments at room temperature.
The size of the potato is very important. This is because if the size of the potato varies, so will the amount of mass it can loose or gain. I am going to try to cut all the pieces of potatoes, to the same size. Because of the ‘chipper’ I will have chips with the same width and height, but the length of the chips will differ. I am going to cut the chips to 3cm in length.
The light intensity cannot really me measured or kept the same, as the chips will be moved from one room to another, for storage. However I will try to keep the chips in the same place when I am storing them.
The mass of the potato is a variable, and this means that it will be measured throughout the experiment. I will measure the mass in grams. The potato chip will be measured before it is put in the solution, and after. This will allow us to see whether osmosis has taken place, and how much osmosis has occurred.
The volume of the solution that the potato chips are kept in must be the same. The chip must be totally covered in the solution, and the amount of solution will be kept the same because all the chips are the same size. The amount of solution I cover each chip in will be 25ml.
The type of potatoes I will use is going to be kept the same, because different potatoes may absorb at different rates. For this experiment I am going to use the same potato.
The time the potato chip is left in the solution must be kept the same for each chip. This is because more or less solution may be absorbed depending on time. I am going to keep each potato chip in each solution for nine days.
The surface area of the potato chips will be kept the same by having all the chips the same size. This must be kept the same because the amount of surface area exposed to the solution may effect the rate of osmosis. The thickness of the chip will already be the same, so I will cut the length of each chip to 3cm long.
To make the mass readings more fair, I will take each chip, roll the chip gently on a paper towel, to remove all excess solution, and I must not squeeze the chip.
I am also going to use the same balance to weigh my chips. This is because the measurements may vary slightly between scales.
I predict that the higher the concentration of salt in the solution, the more water will move out of the potato chip. We know that osmosis is the flow of water in a solution through a membrane while the other molecules are unable to pass through the membrane. Experimentation is needed to discover which membranes allow osmosis, as not all membranes act in this way. Some membranes may allow all or none of the molecules of a solution to pass through, only a few may allow a selective flow. The cell membrane of the potato is partially permeable (it lets some substances in but not all substances). This means that water particles can diffuse into the cells, via osmosis. This occurs if the cells are surrounded by a weak solution e.g. Vimto. If the cells are surrounded by a stronger solution, e.g. very salty water, the cells may loose water by osmosis.
To ensure safety, I will keep my boiling tubes in a large glass beaker so they cannot fall over. The potato chips will be carefully cut on a white tile using a scalpel, so that no one will be cut.
My experiment is not testing how permeable the chip’s membrane is, but it is testing the rate of osmosis of the salt solution. I am predicting that the higher the concentration that the chip is exposed to, the more water will move out of the chip; making it flaccid.
I am going to set up the experiment so I have three results per concentration, I will then take an average to make the test fairer. One result may not be accurate because of human error, so to get rid of the errors, I will repeat it. I will also do at least 3 types of concentrations, to have enough results and to see if my prediction is correct. I am probably going to do 4, these being;
- Water
- 3 x 2g salt
- 3 x 3g salt
- 3 x 4g salt
The equipment I am going to use will be:
- A chipper
- A white tile
- A scalpel
- 10 boiling tubes
- A large beaker
- A measuring cylinder
- A large potato
- Salt
- Water
- Weighing scales
Analysis
My first graph is just there to show comparisons between the mass before and the mass after. This is quite useful, as it shows that the chips with 2g salt lost around the same mass, whereas with the chips in 3g salt G and F lost more mass than E.
The second graph shows the change in mass of the chips. In theory this graph should be quite useful as I can guess the change in mass of the chip that has 5g salt in the water. However this will probably not be happening with mine, as it is quite wrong. I don’t know if my results are wrong or not, but the graph looks very wrong. From the graph I have noticed that I have one anomalous result per gram of salt. B, E and I are all anomalous, and this changes the bottom line of the graph, but the top two still look dodgy. If J had a .87g change in mass (or a figure around .9g) then the top two lines of the graph would have been quite good.
Evaluation
The experiment was easy to do, but all the results I took had to be accurate. Unfortunately I had to change my plan several times. There are several reasons for this, these being that someone stole my boiling tubes, which I had spent 2 weeks getting exactly the right amount of salt into. There were also other things, like having the chips 5cm long. I changed this to 3cm because I couldn’t find 10 5cm long chips. Also, I changed the amount of solution I would keep the potato chips in, because otherwise the water spilled out of the tube, and I only needed enough to cover the chip (it was 100ml). These changes were easy to make, as they were only minor, however the case of the stolen boiling tubes meant that I had to spend several lunchtimes taking my results again.
I think I took enough results for the coursework. The range was big enough, but to fill in the gaps in my graph I could have taken different results, i.e. 1g, 1.2g, 1.4g etc. or even smaller. This way I could have found more accurate results. Also to make my experiment better I could have repeated it more, and possibly have worked with someone, as I was working on my own, or even with the whole class. I could have also cut the potatoes into doughnut shapes, because the cells in the middle of the potato might have different properties, making them react differently to osmosis. I could have also used a machine to cut the potato chips. I could also weigh each chip on a digital and more accurate scale, e.g. not to 0.00 but to 0.0000g.
There was one main anomalous result, this is highlighted in red and is two sizes bigger (tube I), but there is one more, but it is not that far out (tube B). This may have been caused by human error, or one out of my ten results might have been inaccurate, and changed the average. Or perhaps the chip was not cut accurately, or I added the wrong amount of salt. concentration.
I could extend my coursework by testing the same brand of chip using a different substance. By this I mean using a different thing instead of salt, i.e. sugar or coffee or even vimto. Then I could find out whether osmosis occurs differently with different things diluted in the water. I could even use different vegetables.