When I performed the preliminary experiment I recorded my results in a table on the following page.
Preliminary Results Table
I have used the percentage change in mass to plot the graph on the following page.
On the graph point A is the point when the visking tubing has gained mass due to osmosis through the partially permeable membrane, showing a 9.6% increase in mass. The concentration in the visking tube between A and B is inversely proportional to the change in mass. The contents of the visking tubing at point B or isotonic to the sucrose solution therefore the concentration of the visking tubing is 0.24. At points B to C the visking tubing is hypertonic, as it is losing mass. From point C to B the visking tubing is flaccid, or plasmolysed, meaning the percentage decrease in mass is constant because the visking tubing has lost most of its water, therefore the is flaccid
Modifications of Preliminary Experiment
You cannot presume that visking tubing will act exactly the same as a partially permeable membrane in living cells, therefore for my main experiment I will have to change the visking tubing. I will use plant tissue instead of visking tubing, as it will be more accurate for showing the effects of osmosis. I will use potato cells as my source of plant tissue. I found that the range of sucrose solutions used in my preliminary experiment produced a large enough percentage change in mass to record. Therefore, in my main experiment, I will use the same range of solutions as I did in my preliminary: (0.0, 0.2, 0.4, 0.6. 0.8. 1.0 m/l). I will also use the same length of time in my main experiment as my preliminary, because this seemed a sufficient enough amount of time to produce significantly large and noticeable changes in the mass of the visking tubing, so that I could record the results in a graph and a table.
Main Experiment
Apparatus
- Potato cuttings
- Scalpel – to ensure that all edges are cut in a straight line
- Ruler – to get an accurate and manageable length for the potatoes
- 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 moles/litre of sucrose solution
- Boling Tubes
- Cling Film – to prevent any water from evaporating from the sucrose solution
- Top Pan Balance – to accurately weigh the potato cuttings
- Forceps
- Paper Towels – to dry the potato cuttings before weighing them
- Test Tube Rack
- Measuring Cylinder – to accurately measure the sucrose solution
- Cork Borer – to ensure that the diameter of the potato cuttings is kept constant.
Diagram of My Main Experiment
Method
To begin my experiment, I will cut 18 cylinders of potatoes using a cork borer. This will keep the diameter and shape of each cutting the same. Next, each potato cylinder will be measured out to 4cm long with a ruler, and then accurately cut with a scalpel. It is important that each cylinder is exactly the same length otherwise this could affect the surface area, which could affect accuracy of my results. After this, the potato cylinders are dried using paper towels and then weighed on a top pan balance. The original mass of the potato cylinders is recorded in my table of results. The potato cylinders are then placed in test tubes filled with different concentrations of the sucrose solution, as shown in the diagram on the previous page. Each test tube will then be labelled with the concentration of sucrose solution in them going from 0.0 to 1.0moles/litre. Cling film will then be placed over the top to stop any water escaping via evaporation through during the experiment. I will do this method for each of the sucrose solutions. I will then repeat this experiment three times so that an average result can be worked out, in order to help eliminate errors or anomalies in the results. The potato cylinders will be left for 24 hours and then each of them will be dried carefully, again with paper towels and re-weighed. However, it is very important to dry the potato cylinders before weighing them to ensure that any change in the mass of the potato is due to a change in the water content inside the potato cells, not due to any sucrose solution on the outside of the cylinder. When the potato cylinders are re-weighed I will record the final mass in my results table. Once I have got both sets of results, the weights of the potato cylinders both before and after the experiment, and I have found out the averages I will then find out the percentage change in mass. I will then plot a graph of percentage change in mass against the concentration of sucrose solution.
Variables I Would Keep The Same
- Length of the Potato Cylinder - If I altered the length of the potato cylinder it would change the surface area of potato exposed to sucrose solutions. This will affect the rate of osmosis. I will maintain the length of all the potato cylinders by using a scalpel and ruler to cut and measure lengths.
- Diameter – again, a change would alter the surface area of all the cell membranes exposed to sucrose solutions in turn affecting the rate of osmosis. I will make sure all the potatoes have equal diameter by using a cork borer.
- Time – the length of time will be a factor that will affect the amount of water molecules that can diffuse into or out of potato cell membranes.
- Temperature – the higher the temperature, the higher the kinetic energy of the water molecules, therefore the higher the rate of diffusion in or out of the potato cell membranes.
- Volume of Sucrose – If the potato cells are not fully covered in sucrose solution, it will affect the rate at which they can diffuse in or out of the potato cells.
Independent Variable
The variable that is changed in this experiment is the sucrose solution ranges. The range of solutions I used is as follows: 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 moles/litre. This helped me get more accurate and reliable results as I had a greater spread of data to work with.
Table of Results for My Main Experiment
I have used the percentage change in mass to plot the graph on the following page.
On the graph point A is the point when the potato cylinder has gained mass due to osmosis through the partially permeable membrane, showing an 8.5% increase in mass. The concentration in the potato cells between points A and B is inversely proportional to the change in mass. The potato cell vacuoles at point B are isotonic to the sucrose solution therefore there is equal movement in and out of the cell so no mass is lost or gained. The concentration when the of the potato cell is isotonic is roughly 0.2 moles/litre . At points B to C the potato cells are hypertonic, as they are losing mass. From point C to B the potato cell vacuoles are flaccid, or plasmolysed, meaning the percentage decrease in mass is constant because the potato cells have lost most of their water, therefore the cells have become is flaccid
Analysis
The potato cylinders, which were placed in a hypotonic solution, showed an percentage increase in mass, whilst those potato cylinders that were placed in the hypertonic solutions showed a percentage decrease in mass. The potato cylinders placed in the 0.0 moles/litre sucrose solution had an average percentage increase in mass of 8.5%. However, the potato cylinders placed in the sucrose solution of 1.0 moles/litre had an average percentage decrease in mass of 29.3%. The graph shows that the percentage change in mass was inversely proportional to the sucrose solution from 0.2 to 0.8moles/litre, because as one variable (the concentration of sucrose solution), went up, the other, (percentage change in mass), went down.
Scientific Reasons for My Conclusion
When the potato cells are placed in a hypotonic solution, water molecules will enter the plant cells by osmosis. Water molecules will move into the potato cells through the partially permeable membrane.
Diagram of Water Molecules Moving into a Partially Permeable Membrane by Osmosis
The entry of water molecules into the cell vacuole will cause the vacuole to swell and push on the cytoplasm and the cell membrane that will, in turn, push on the cellulose cell wall, which is elastic. However, once fully stretched it will resist the entry on any further water into the cell. The cell wall develops a pressure potential and the cell is said to be turgid. Since the cells have gained water by osmosis, they will gain in mass. The potato cells are turgid between 0.0 moles/litre and 0.2 moles/litre. When the potato cylinders are placed in hypotonic solution, water molecules will move out of the cell vacuoles and into the ceramic sucrose solution through the partially permeable membrane and cell wall by osmosis. The cells show a decrease in mass since they have lost water by osmosis.
Turgid Plant Cell
Flaccid Plant Cell
Isotonic Plant Cell
My prediction stated that the concentration of sucrose solution would be proportional to the percentage change in mass, This was only partly true because at sucrose solution…moles/litre the water stopped moving in due to the effect of the pressure potential of the cell wall. Also, at 0.2 moles/litre, the sucrose solution decreased due to the fact that the potato cells had become plasmolysed.
By looking at my results graph and my predicted graph, I can say that my conclusion does support my prediction to a certain extent. However, it does not fully support it as my graph shape is still slightly different than my predicted graph shape. Also, there is no alternative way of joining up my points.
Evaluation
My graph shows that from 0.0moles/litre of sucrose to 1.0moles/litre of sucrose that the concentration of sucrose is inversely proportional to the percentage change in mass. To make my results accurate and therefore reliable, I used a top pan balance to 100th of a gram to measure the length of the potato cylinders. I used a ruler to measure the length of potato cylinders. I repeated my experiment three times to enable an average change in mass to be worked out. Using an average change in mass helps to eliminate any errors. I used a cork borer to measure the diameter of the potato cylinders. To help ensure that the surface area of the cylinders remains constant, I made sure that they were fully emerged in sucrose solutions by measuring the same volume of solution with a measuring cylinder and placing the potato cylinders in boiling tubes with cling film over the top to prevent any water evaporating from the solution over a 24 hour period. I used a best-fit line when joining up my plotted points. This helped to ensure my conclusion was valid. I have one major anomaly on my graph at the sucrose solution of 0.4moles/litre, as the percentage change in mass is too low compared to the best-fit line.
Causes of Errors and Improvements in the Experiment
- Cutting the potato and measuring it with a ruler is only accurate to plus or minus one millimetre. This error in measuring the length of the potato cylinders could be prevented by using a pair of vernier callipers.
- The potato cylinders placed in the very concentrated sucrose solution of 1.0moles/litre, floated and therefore were not fully immersed in the solution, therefore altering the surface area of the potato cell membranes in contact with the solution. A way to improve this could be by tying a pin to the end of the potato cylinder so that the weight would drag it down. However, this would have to be done to all potato cylinders so that the results can remain accurate and fair. Also, the potato cylinders must be weighed with a pin tied to them because if they were weighed without it, the weight of the cylinders would be inaccurate as a pin adds to their overall weight.
- Some of the potato cylinders had potato peel left on them when they were being cut. Potato peel is waterproof and so would reduce the movement of water by osmosis in or out of the potato cells and would therefore lead to errors in the results. All of the potato peel should be carefully removed in a further investigation.
- The potato cylinders were cut from different potatoes, therefore the age and species of the potato could well have been different and this would alter the surface area of the cell membranes inside the potato cylinders or the concentration of water in the cell vacuoles. A different surface area of cell membranes and/or the concentration of water in the cell vacuoles would affect the amount of movement of water by osmosis. To prevent this from happening and causing anomalous results, the same type and age of potato should be used in the experiment.
- Some of the cells in the potato could become squashed and water removed from the potato cells during the drying of the potato cylinders. One way to improve this is to put the potato cylinders in a desiccator, as shown below, as this would dry them out without the risk of them getting squashed. Also, you could improve the experiment by just leaving the potato cylinders wrapped up in tissue paper to dry naturally. However, the downside of this is that it is very time consuming.
- A range of six solutions doesn’t provide sufficient evidence to make a firm conclusion. To provide more evidence to help make a firm, valid conclusion, a wider range of solutions with reduced intervals in between the sucrose solutions would need to be used. For example, a range of sucrose solutions of the following concentrations could be prepared: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 moles/litre.
- Measuring the change in the diameters of the potato cylinders, as well as the percentage change in mass of the potato cylinders could provide more evidence.
- I would repeat the experiment more than three times, as this would also provide me with additional evidence to make a firm conclusion.
- The shape of my graph is similar to my predicted graph and most of the plotted points are close, if not on the best-fit line, indicating accurate and therefore reliable results. However, I cannot make a firm conclusion about the affect of osmosis in plant cells since I have only investigated the affect of osmosis in potato cells. I cannot assume that all plant cells will behave in the same way as potato cells. To make a more valid conclusion, I need to repeat the experiment using other plant tissues e.g. carrot, lettuce or celery cells. I could also extend my investigation by looking at the degree of plasmolysis in plant epidermal cells such as beetroot, onion and rhubarb, in which the epidermis is stained with iodine. I could take the epidermis of these plant tissues and then place them in a range of sucrose solutions (0.1, 0.2, 0.2, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 moles/litre). After the tissues have been in the range of sucrose solutions and if necessary they have been stained, I would then observe and estimate the degree of plasmolysis on the cells by viewing them under a microscope.
