When my solutions were mixed I collected 1 large potato. Placing it on a cutting tile (which provided a stable surface to cut on). I cut 12, sized 7, cylinders of potato, being careful when using the core borer and directing it away from myself, to avoid cutting myself.
Using a scalpel, I carefully removed any peel from the edges of potato (as it could lead to unfair testing due to the different plant cells in peel and in the actual potato – see table of variables). Once I had 12 cylinders of the same length, 3cm, I began weighing them using top balance to a 2 decimal place, as this was the most accurate equipment available to me.
I decided to use the potatoes as they were and not try to make them all exactly the same (which was a mistake, although the change in mass can be found, the surface area of each potato will be unequal which will effect the overall rate of osmosis). I recorded the mass of each cutting into a coherent table were I was to later add more results.
I placed 2 potato cuttings (this was to make my results more reliable and fairer, in case I got any odd results they would even themselves out when an average was calculated) into each of the 6 beakers and was to leave them in for a set time (I decided the set time by using the time in my preliminary work were the greatest change in mass occurred) 24 hours in my case. I covered each beaker with cling-film to avoid any of the water leaving the beaker through evaporation (as this would make the solution more concentrated, through water loss) and to avoid anything entering the beaker and ruining the experiment.
I chose 24hours as the set time because although it didn’t fit with my results from my preliminary work, it was the only time suitable for me to return and I checked with other people that this was a suitable length of time. Carefully I removed the cling-film then removed the cuttings, with forceps (to avoid contamination of my fingers) from the beakers and placed them on some paper towel. To make the experiment fair I gently rolled the cuttings on a paper towel to remove any excess liquid. I individually weighed each cutting. I recorded the mass and found the averages of the two cuttings. I found the difference in the 6 potatoes samples within the solutions, as this would tell me whether osmosis had occurred or not. I then calculated the percentage of the change in mass and plotted an accurate graph with the results I gained.
I then cleaned my hands and the surfaces used to prevent the spread of reproduction of any bacteria.
Fair Test:
This experiment will be a fair test because:
- I have used the same potato for all the cuttings.
- I have repeated the experiment two times (three would be an improvement) and found an average to make my results more reliable and more accurate.
- I kept the six beakers in the same room at the same temperature for the 24hours they were left to eliminate the ‘temperature of the room’ as a variable.
- I kept the cuttings in for a set time (24hours) because the amount of osmosis which occurs depends on the time the cuttings are left.
- I used distilled water to avoid any sort of variation in the water.
- All the equipment had been cleaned before hand and I used the equipment wisely to avoid contamination.
- I used the same weighing scale when weighing the mass of the cuttings before and after to insure the weighing was as accurate as possible as the mass may vary from scale to scale.
- I used the same sugar solution to avoid any variation.
- I removed any excess solution to make the mass of the cuttings as accurate as possible.
Prediction:
I predict that the more dilute the solution (the higher the concentration of water) is, the greater the mass of the potato cutting will be after the set time. The higher the concentration of sugar in the solution is, the more water loss there will be and therefore the lighter the cutting will become. I believe osmosis will occur at a greater rate within the cutting, when there is a greater amount of water than sugar in the solution. This means the cuttings that are put into a higher concentration of water will end up with a larger mass than those put into a higher concentration of sugar.
Osmosis is the movement of water from a weaker solution into a stronger solution through a semi-permeable membrane while the other solutions are blocked and unable to pass through the membrane. The water will move from a higher concentration of water to a lower concentration of water through a semi-permeable membrane. For example if there was one side of a semi-permeable membrane with 70% water and another side with 40% water, the side with the greater percentage of water, the 70% side would flow from its higher concentration to the lower concentration to help even out the concentrations to an isotonic point, were both sides will be of equal strength. The water molecules are far smaller than the sugar molecules are therefore they are able to fit through the membrane to the lower concentration.
Each potato cutting is made up of many cells, each of which are surrounded by a cell membrane which separates its’ contents from the contents of the surrounding solution. The cell membrane has tiny holes in it which allows small molecules to pass through but not large, sucrose ones, therefore the cell membrane is semi-permeable.
I believe that any potato samples placed in beakers containing bathing solution weaker than the isotonic point will increase in mass because it is gaining water due to the fact that the concentration of water within the potato sample is less than that outside. Any potato samples placed in beakers containing bathing solution stronger than the isotonic point will decrease in mass, because it is losing water due to the fact that the concentration of water within the potato sample is higher than that outside. The change in mass will be proportional to the change in the concentration of the solution. At the isotonic point the overall mass will not change because the concentration in the cell will be equal to that in the surrounding solution, as there is equal water movement in and out of the potato sample.
I think there will be a greatest increase in mass at 0M and a greatest decrease in mass at 1M. I predict that whatever the loss of mass is at 0M this will equal to or roughly equal to the increase in mass at 1M and I believe this relationship will occur throughout my results in a direct proportion.
Results:
Table –
Conclusion:
My results are sufficient proof that osmosis does have an affect on plant cells because there is evidence that tells us when osmosis occurs there is a change in mass within the plant cell (if there is an increase osmosis has occurred, whereas if there was a decrease there has been water loss within the plant cell)
I recorded my results in a table to begin with, as this is the simplest and quickest way to record them as I gain them. I then plotted my results into a graph, this way I was able to identify any trends within my results.
The graph I drew was the ‘Change in Mass (%) against the Concentration of the Sugar Solution.’ This graph was a straight-line graph, but my results do not run exactly along the 45º line of best fit, which I think is due to slight error and/or bad inaccurate measurement upon my part. From the graph I can safely say, with sufficient proof that the concentration is proportional to the % of mass lost. I can also say that my prediction was correct, the stronger the concentration of the solution the greater decrease in mass there is to the potato cutting, and the more dilute the solution the greater the cutting increases in mass. The higher the concentration of sugar, the more water is lost, and vice versa, the lower the concentration of sugar, the more osmosis occurs and the greater the intake of water is therefore the greater the mass of the cutting is.
My results show that the greatest increase in mass occurred in a solution of 0M. With no sucrose in the beaker there was an increase of 0.63 grams (25%). I predicted this and stated it in my prediction. I also said in my prediction that I thought the greatest loss in mass would occur at 1M and my results prove this also to be correct. In a solution of 1M, were no water is present (a sucrose solution) there was a decrease in mass of 29% (0.755 grams) which is the greatest loss in mass throughout the experiment.
Also in my prediction I stated that I thought there would be a direct proportion. My graph shows proof that this happened. Although the results are not all exactly on the line (probably due to minor mistakes on my half) there are definite signs of the increase at 0M being roughly equal to the decrease at 1M, and a similar relationship throughout my results. This relationship stemmed from the isotonic point where there was 0% change in mass. Anything that had a positive percentage change in mass (an increase in mass) was more dilute than the isotonic point and anything with a negative percentage change in mass (a decrease in mass) was stronger than the isotonic point.
The isotonic point is where the concentration of the solution meets the % change in mass at 0%. On my graph the isotonic point is 0.467M. At the isotonic point (0.467M) the cuttings are neither flaccid nor turgid. The amount of water entering the cutting and the amount leaving is equal and the concentration gradient in the potato and in the solution are equal. Therefore the potato cells are at a normal size and are at the same mass they began at, as there is no overall change in mass. At anything more dilute than the isotonic point there is a positive % change in mass, meaning the mass of the cutting increases at any solution weaker than the isotonic point. This is due to the fact that osmosis occurs because the concentration gradient of the solution in the beaker is less than that in the plant cell sap. Therefore, the water in the beaker will move from a dilute solution through a semi-permeable membrane along the concentration gradient into the potato cutting, causing an increase in mass.
If the cuttings are lighter there must be a water loss, which occurs through osmosis. For this to occur the concentration of the solution in the beaker must be greater than that in the plant cell sap, so that the water in the plant cell will move from a dilute solution along a semi-permeable membrane, and out of the cutting. This is what happens from the isotonic point and onward (0.467M – 1.0M).
Evaluation:
Overall I think my experiment was fairly successful, due to the fact I had sufficient evidence to prove that osmosis had an affect on plant cells. Using my results I was able to show a direct relationship between the change in mass and the concentration of the solution. I found the hardest part of the experiment to be the cutting of the potato cylinders and to prevent this, if I was to do any further investigation, I would have to design a mechanism to make it easier to cut accurate potato cylinders. I would still use the core borer to gain the cylinder-like shape of potato then using a flat, vertical upright, such as the spine of a book, I would line up the cylinders in a row and cut them all in one movement using a cat wire. My first mistake was not making all the cuttings the same mass from the beginning, but it was virtually impossible to get them all exact. Because of this it could have affected the surface area of the cuttings and therefore the overall rate of osmosis would be altered.
My results are reliable because in each of the separate pots the results are within 0.05grams of one another. There is evidence of this in my results table. The mass of the potato cylinders at the start and end of the experiment, in each case (solution), in both pot 1 and 2 are always identical or within a few hundredths of a gram. This minor difference could be something as little as not removing a few water molecules from the outer layer of the potato cylinder when rolling it in a paper towel.
If I was to repeat the experiment I would make quite a few improvements:
- I would design a mechanism to make it easier to cut accurate potato cylinders, so that all the cuttings would be of the same mass from the beginning. I would still use the core borer to gain the cylinder-like shape of potato then using a flat, vertical upright, such as the spine of a book, I would line up the cylinders in a row and cut them all in one movement using a cat wire.
- Instead of using tap water I would use distilled water because the chemicals in the tap water may very have varied the overall outcome.
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I would do at least one more trial at each concentration to create more accurate averages. I think two was not enough because if one result was inaccurate it was difficult to even it out for the averages. With only two trials if you were to get an odd results that were completely different to one another you would not be able to tell which one was the incorrect result and which was the correct result, whereas if a 3rd trial was added using an additional potato cutting you would be able to establish which was the odd result more easily and reliably.
- Instead of increasing the sugar concentration in 0.2M intervals I would go up in intervals of 0.1M to make my results more accurate. Instead of using 6 beakers I would therefore have to use 10 and I would end up with 10 average changes in mass which would help me plot a more accurate and reliable graph with a more accurate line of best fit, as greater proof that osmosis has an affect on plant cells.
- I could either find a more accurate way to measure the solutions or find a more accurate top pan balance that goes into 3 or 4 decimal places.
- When measuring out the solutions I could use a measuring cylinder of a far smaller scale so that my solutions would be more exact. I could also weigh the mass of the solution as this is equal to the volume (1cm³ of solution = 1g of solution) and using top hand balancing that are to 2 decimal places would increase the accuracy of my weighing drastically, and would avoid any visual error when reading off a measuring cylinder. Obviously when weighing the solutions I would have to find the mass of each beaker and subtract this from the reading of the overall mass (solution and beaker).
One factor I feel could not really be measured or improved too greatly was drying the excess water off the cuttings once they are removed. Rolling them in a paper towel can not exactly be measured, we could say roll them each 3 times for example but I still think that each cutting will not lose all the excess water therefore the results will always be inaccurate, unless a form of measuring this method is discovered or another, more reliable way to remove any excess water is found.
In my experiment I only assessed the change in mass in 0.2M intervals, if I was to do further work I would analysis the changes in mass of solutions in 0.1M intervals to make my results more accurate. Instead of using 6 beakers I would therefore have to use 10 and I would end up with 10 average changes in mass which would help me plot a more accurate and reliable graph, as greater proof that osmosis has an affect on plant cells.
Other factors which I could investigate are the:
Temperature – if the temperature of the conditions were the test is taken place are changed, the rate at which osmosis takes place would also change. I would expect the greater the temperature the faster the rate of osmosis. The hotter the solution the more energy/vibration there is within the sucrose molecules, therefore the quicker the diffusion of the water molecules will occur.
Different plant cells within the potato (e.g. the peel) – I would carry out a similar experiment to the one above, but instead of keeping the type of cutting identical throughout, I would investigate any trends when varying the type of potato plant cell. The rate of osmosis would vary from cell to cell and it would be interesting to compare the results and find any links or connections as t o why there are comparisons.
Different types of potato (e.g. grown from different areas of the world, under different environments) – I would do a similar investigation as this one only using potato from different environments and climates. Once I find the isotonic solution of the different potatoes I could compare the rate of osmosis within the different cuttings (e.g. I could compare the potato cuttings from a hot climate and from a cold climate and investigate whether there are any connections or patterns with the rate of which osmosis occurs and the environment in which the potato is grown in).
Different plants/vegetables (e.g. apple, carrot, etc.) – I could investigate if there is a difference in the rate of osmosis within the different plant/vegetable cells. There would definitely be varying results with the different plants/vegetables, as some are root vegetables (e.g. carrot, potato), some are ground vegetables (e.g. peas) and some are open vegetables (e.g. apples, pears) and each have a different function, for example the function of a root vegetables is to hold all of the nutrients needed to promote the potato seedling to grow without sunlight.
Different dimensions and surface areas of a cutting – In the experiment I have just completed I used cuttings of equal dimensions and surface areas, but instead I could very this slightly and vary the dimensions of the cutting which would therefore very the surface area. It would be interesting to see if there was a difference in the rate of osmosis when several cuttings of the same mass, but different dimensions were investigated.
My results were fairly accurate and even when they were plotted into a graph I cannot see any obvious anomalies.
GCSE Biology Coursework – How Does Osmosis Affect Plant Cells?
(Preliminary Work)