Graph:
The preliminary graph shows me that as the concentration of sucrose solution increases, the percentage change in mass decreases.
Conclusion:
I am trying to prove that the concentration inside the potato is lower than the concentration of the water molecules, outside the potato. I decided to do a preliminary experiment, to see if my hypothesis seemed correct and sensible to carry on, to a further, main experiment. In my preliminary experiment I decided to use distilled water, and 1.0M of sucrose solution, but in my main experiment I have decided to use distilled water, and then eight different concentrations of sucrose solution, evenly spread out. By looking at my graph and results from the preliminary experiment I can see that as the concentration of sucrose solution increases, the percentage change in mass decreases.
Main Experiment
Method
Cut the potato cylinders to the correct sizes (e.g. 5cm in length and 0.6cm in width), so that every potato cylinder is the same size and this will help me to create a very fair experiment. Then I will dry them out using paper towels so that it doesn’t affect the weight with surplus water. Record their masses and lengths on a sensitive weighing scale. Place them into the seven different solutions and leave them for twenty minutes, so that enough time is allowed for the water molecules to diffuse into the potato tissue. Then remove them and dry them out a second time to remove any excess water, so it doesn’t effect weight, therefore creating a fair experiment. Record their masses and lengths again. The concentrations of sucrose solution used are 0.1M, 0.2M, 0.3M, 0.4M, 0.5M, 0.6M, 0.7M and 1M. Distilled water (0.0M) is used in the last test tube, because it doesn’t contain any sucrose at all and therefore it cannot get a faster rate of osmosis, it is pure. For safety reasons I will be very careful when using the scalpel and potato corer because these instruments have very sharp edges and cannot break the skin. I will wear safety goggles and a lab coat to prevent any chemicals damaging my eyes, or any chemicals reacting with my skin or clothes.
Equipment
- One boiling tube with 30cm cubed of distilled water
- Six boiling tubes with 30cm cubed of different concentrations of sucrose solutions
- Seven potato cylinders, approximately the same mass and length
(E.g. 5cm)
- Measuring cylinder/Syringe
- Scalpel
- Weighing scales
-
Test tube rack
- Seeker
- Cutting tile Potato Extracts!
- Cork borer
- Paper towels
To make it a fair test:
- The potato cylinders must be of the same width (about 0.6cm) and length, about 5cm, to create a similar mass and the mass depends on the accuracy of the other two. This is because if there’s more potato tissue in one test tube it will lose or gain more of the solution surrounding it, as there will be more of the tissue, so there is more chance.
- The temperature of the solutions must remain constant e.g. 22 degrees Celsius so that a hotter temperature doesn’t speed up the rate of osmosis. This is because enzymes work faster, at a higher temperature, excluding very hot temperatures, when the enzymes would be denatured. Also if the reaction were happening at a colder temperature, the reaction would be slower because enzymes work slower, at a lower temperature as they have less energy.
- Each boiling tube must contain the same volume of solution so that they have the chance to intake and release the same amount of water content.
- All of the potato cylinders must come from the same potato or from one of similar conditions, because different potatoes are stored in different atmospheres or temperatures and so could have more or less water content.
Prediction
I know from my preliminary experiment that when a potato cylinder was placed into a test tube filled with 30cm cubed of water the potato cylinder would increase in mass but the potato cylinder that was placed in 30 cm cubed of 1.0M of sucrose solution would be the only one to decrease in mass. The sucrose molecules in the sucrose solution are too big to be able to diffuse through the potatoes permeable membrane. In my main experiment I can predict that with a lower concentration of sucrose solution there will be a bigger increase in mass size.
Results
Mass:
Length:
Width:
Percentage Changes in potato mass:
Graph:
The graph shows me that as the concentration of sucrose solution increases, the percentage change in mass decreases. The graph shows a correlation.
Analysis:
In my experiment I predicted that when a potato cylinder was placed into a test tube filled with 30cm cubed of water the potato cylinder would increase in mass but some of the potato cylinders that were placed in 30 cm cubed of different concentrations of sucrose solution would only increase in mass by a minute amount. Diffusion is the exchange of things dissolved in fluid (solutes) across the membrane due to differences in the amounts of the solutes on the two sides (concentration gradient). If there is a higher concentration of a given solute on one side of the membrane than on the other, then diffusion will occur to try to make the concentrations on both sides of the membrane the same. Osmosis is the movement of water across a selectively permeable membrane driven by a difference in the amounts of water on the two sides of the membrane, and osmosis can only occur with water.
In my experiment the water particles diffused into or out of the potato tissue by osmosis. In my preliminary experiment I saw that the molecules of the sucrose solution were too big to fit through the membrane wall of the potato cylinder and so could not diffuse through, therefore causing the potato tissue to change weight. However in my main experiment I saw that if the sucrose molecules are of a lower concentration then a small number of water particles will be allowed to pass through the potato wall. As the concentration of sucrose molecules gets lower more and more water particles can diffuse through the potato cylinder cell membrane. This is because with a lower concentration there is more water and so the water particles can diffuse through even though the some other molecules cannot. Even so the mass of the potato cylinder in the test tube filled with water increased much more because the water particles can diffuse through because these were simply water molecules whereas the sucrose solutions are based on sucrose (glucose molecules), which cannot. The potato increases in mass because when plant cells gain water by osmosis, they swell. The cell wall prevents them from bursting under the pressure, however the cytoplasm pushes against the cell wall, making the cell turgid and therefore heavier in mass.
I think that the majority of my results are sound but one of them has to be an anomaly because when a potato cylinder was added to 30cm cubed of 0.2M sucrose solution there was a 5.28% decrease.
This anomaly could have occurred, as it was one of the last to be put into a test tube and so it was probably taken out a few seconds after its pre-de-cesses, but this would cause it to allow less water molecules to pass through the membrane as it had less time to act. I noticed that as the concentration of the sucrose solution decreased, there was also a percentage decrease, apart from in water where there was a 5.18% increase. This is because the water molecules can easily fit through the cell wall of the potato cylinder and increase its mass, but as soon as some of the solution is made up of sucrose particles, they cannot pass through the cell wall, and therefore there is a percentage decrease.
After carrying out this experiment I have realised that it is very important to represent the data in a very clear, and ordered table, chart or graph so that the numerical data is easy to handle, and therefore ready to analyse and understand from a glance rather than having to spend a long time just trying to understand what figures belong to which beaker.
Evaluation:
I think that I can draw fairly reliable conclusions from my experiment. The equipment I used was fairly scientific, and I used the best equipment that I could access. The scales I used calculated up to two decimal places and this showed clear differences to others results that only recorded one decimal place. I could see this when I was calculating the percentage changes within the potato cylinders later on. I think that my results were very precise for a school experiment, but obviously if I was doing this experiment with more developed equipment my results would be even more accurate. If I had used scales to say, four significant figures, this would of course have been much more accurate and my conclusions would have been more reliable. Also in my experiment I only used nine different moles of sucrose solution to note down results, draw a graph, and finally write conclusions. But, if I was doing this experiment on a more scientific level I would probably use about twenty different solutions and this would help my conclusions to be more precise.
However I only had one anomaly in my experiment and my conclusions drawn from my experiment were correct, meaning that my actions carried out during the experiment must have been done accurately and carefully. I trust my results because I know that I carried out the experiment by following all of the correct steps, and not just recording my results, but also developing my answers by thinking how and why I ended up with those answers. One method I could review would be when the potato cylinders were dried on the paper towel before they were placed into the solutions, because one cylinder may have been dried for longer which could have created a lower mass, which could have affected my overall results. Also cutting the potato cylinders to the correct lengths, widths and masses and having to measure them with a ruler, was very time consuming and not as precise as I would have liked. Therefore in the future I could use a more scientific way of carrying out this part of the experiment. If I ever carry out this experiment again, I will use a scientific piece of apparatus to record the temperature of the different solutions so that it is fair. Also some parts of the experiment were a bit limiting. For instance, I would have liked to have carried out the experiment a few more times using various types of potatoes, to see if the type of potato makes a difference to the conclusions which can be drawn from the experiment.
