Discover whether the concentration gradient affects the rate of osmosis in a potato cell.
OSMOSIS IN A PLANT CELL
DAVID WILLEY
PLANNING
Osmosis in a plant cell
Aim-
To discover whether the concentration gradient affects the rate of osmosis in a potato cell.
Basic outline plan-
9 potato cores will be cut to the same size, and weighed. Three will be submerged in water, three in 0.2 molar sugar, three in 0.4 molar, three in 0.6 molar, three in 0.8 molar, and three in 1.0 molar, all of which will have the same volume. One will be placed in cling film and not submerged in any thing this will be the control. They will then all be removed at the same time and weighed. To make sure my experiment is reliable I will do the experiment three times.
Apparatus-
6 Petri dishes
Potato
4+5 mm potato borer
0.2, 0.4, 0.6, 08, 1.0 molar sugar and water
Knife
Ruler
Cutting tile
Thermometer
Cling film
Stopwatch
25cm3 Measuring cylinder
Prediction-
I predict that the weight of the potato cores will increase when they are submerged in water, and will slowly decline in the rate of increase until they start to decrease when a higher gradient of molar sugar is added. There are a number of reasons why I believe this. The first is due to a prior experiment. In this experiment potato cores were placed in water and 1 molar sugar, and their weights were recorded before and after. The potato core in water gained weight, and the potato core in sugar lost weight. I am led to believe that the results are two from a series which is a steady rate. I also believe there will be equilibrium in the results, where no weight change will be present, because the concentration gradient will be the same on either side of the semi- permeable membrane. I also believe my results to be correct due the theory of diffusion. Diffusion dictates that the higher the concentration gradient, the faster osmosis will occur. It also dictates that Osmosis will occur from highly concentrated areas to less concentrated ones. Hence the water will diffuse from the solution to the potato if, the concentration of water in the solution is greater than that in the potato, and vice versa. The rate of osmosis will affect the mass far greater than the diffusion of sugar, because the sugar will diffuse very slowly and the results will be barely noticeable. I got this information also from a GCSE revision book which explains that, 'Osmosis moves with the concentration gradient.'
There are certain variables, which we should monitor, in order for the experiment to be a fair test.
The following I obtained from the GCSE revision guide, where it states that the variables which affect the rate of reaction are temperature, the use of a catalyst, the surface area, and the percentage of the solution. We will not have to keep the percentage of the solution the same as this is the variable we are changing in the experiment.
I also found much of this information by looking at notes from Miss Spilsbury's class, which state that the kinetic theory says the hotter the temperature the faster the rate of reaction. That the bigger the surface area over mass the faster the rate of reaction, and the use of a catalyst can be used to speed up a reaction.
I predict that if the temperature is changed this will affect the rate of osmosis. Kinetic theory dictates that the hotter the temperature, the faster the rate of osmosis. This is because that particles and atoms move faster when they are hotter, and therefore will diffuse faster. They move faster, because they get 'excited', we also know heat is a catalyst in most reactions.
Surface area affects the percentage rate of diffusion. The bigger the surface area in comparison to the volume of the cell, the faster the rate of diffusion. This is shown in life with Amoeba, single celled organisms that survive on diffusion. If they were bigger then they were their surface area would not be adequate to diffuse in the nutrients that they need to survive. This is why Amoeba is so small, and giant Amoeba can not exist because the rate of diffusion would not be adequate for them to survive. We also know this to be true, due to a previous experiment, in which blocks of different size Agar, were submersed, in Potassium Permanganate. All the blocks were left in for an allotted, time and when they were removed, they were cut open, and the amount of diffusion, was easy to calculate because, the purple Potassium Permanganate, shows up easily, on the white agar. It was proven that no matter what the surface area the solution will still diffuse at the same rate. However, if something diffuses, 2mm into a 4mm wide block, 50% of the agar will be covered. However, if 2mm diffuses into a 10mm block, it has only diffused, to 20% of the block. So surface area only affects the percentage rate of diffusion. Also according to the GCSE revision guide, the collision theory is affected by the surface area of an object, the greater the surface area the faster the rate of reaction.
The cores do have to be submerged in the solution for the same amount of time. This is because as the experiment continues more osmosis will occur, but the concentration gradient will be decreasing, and so will the rate of Osmosis.
If the semi-permeable membrane is changed, it will also affect the rate of osmosis and also could allow different substances through. That is why the skin of the potato has to be removed on each one. If the potato skin was left on certain cores, it might not allow water to pass through into the ...
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The cores do have to be submerged in the solution for the same amount of time. This is because as the experiment continues more osmosis will occur, but the concentration gradient will be decreasing, and so will the rate of Osmosis.
If the semi-permeable membrane is changed, it will also affect the rate of osmosis and also could allow different substances through. That is why the skin of the potato has to be removed on each one. If the potato skin was left on certain cores, it might not allow water to pass through into the core and the experiment would be ruined.
Explaining my diagram.
The following diagram represents the molecules of sugar and water in the potato cell and the solution. The arrows are colour coordinated to represent the direction and amount of substance is diffusing. The sugar molecules are shown as pink circles and their direction as pink arrows. And the water is shown with pencilled circles and pencilled arrows also. The sugar molecules are bigger than the water ones to represent the difference in size of the molecules.
A diagram to support my prediction
Detailed Plan
- I will compare Osmosis in potato cells when they, are completely submerged in different gradients of molar sugar. I will accurately cut 18 potato cores, from the same potato, to the same size, and submerge them in the following substance after weighing them.
0.0 (water)
0.2M Sugar
0.4M Sugar
0.6M Sugar
0.8M Sugar
.0M Sugar
My theories are supported by the Biology GCSE revision guide. Under the heading of the collision theory, it tells you that; temperature, the use of a catalyst and surface area are the things which speed up and effect the rate of reaction. Also included in the explanation are the differences in concentration of solution. This is the variable that I am changing.
To make sure the only variable which is affecting the rate of osmosis is the gradient of sugar, all the other possible variables, will be carefully monitored and kept the same, by the use of a thermometer.
For example, we know that temperature affects the rate of osmosis, the higher the temperature, the faster the rate of osmosis, and vice versa. There for temperature will have to be carefully monitored and kept the same. The temperature affects the rate of osmosis because, particles move faster and have more energy as they get warmer, and move slower and have less energy as they get cooler. More movement and collisions occur as temperature increases. We also know heat is a catalyst in most reactions.
The surface area of the potato also affects the percentage rate of diffusion, we know this due to a prior experiment, where agar blocks of different sizes were submerged in Potassium Per-manganate, and left for an allotted time. When the agar blocks were removed and cut open the colour of the Potassium per-manganate, enabled us to see how far the substance had diffused in. On small blocks, because the ratio of the surface area to volume was smaller, a higher percentage of diffusion occurred, and vice versa.
An other thing which affects the concentration gradient is the substance you are diffusing into, i.e. potato, because potato may have a different concentration gradient of water in it than say apple. Also the semi-permeable membrane will be different, and a different membrane could allow different substances in, or allow these substances, but in a slower or faster time.
Time also affects the rate of osmosis. The longer the time the more osmosis will occur, but the rate of osmosis will also slow down as the time goes on, because the concentration gradient will be getting less.
I will weigh and record the weight of each potato core before they are submerged in the solutions. I will keep all of the cores completely submerged in the solution for the same time. I will then weigh them again and work out the change in weight whether it is positive or negative. I will then work out the percentage change in mass. And record my results.
The amount of the solution will be kept the same because he more solution you have the more osmosis will occur over a longer period of time. The amount of solution does not affect the rate of osmosis, because the percentage of sugar in the water does not change when you add more solution, and this is not affecting the concentration gradient at all.
To make sure the results I obtain are accurate I will do the experiment three times, because this will let me know if there are any results which are inaccurate, and I can therefore ignore them.
Safety: Eye protection must be worn
Method:
8 potato cores were cut to an equal size and weighed. We recorded what each core was, and placed them so that we could easily remember which one they were when they were put in the solution. Then 25ml3 of water 0.2, 0.4, 0.6, 0.8, and 1.0 molar sugar was measured and each placed in a separate Petri dish, their temperature was measured to make sure it was a fair experiment. Three cores were then completely submerged in each solution and left for twenty minutes. Each core was placed so that we could remember what each was. After the twenty minuets which were timed on a stop watch, they were removed dried and weighed again, and there results recorded.
Diagram
OBTAINING EVIDENCE
Results Table
A table to show the change and average change in weight for each core, when placed in each solution and left for twenty minutes.
All units shown are in grams.
Core 1
Water
0.2 molar
0.4 molar
0.6 molar
0.8 molar
.0 molar
Weight Before
2.33
2.33
2.35
2.32
2.25
2.29
Weight After
2.39
2.36
2.34
2.15
2.05
2.02
Change in weight
+0.06
+0.03
-0.11
-0.17
-0.2
-.27
Core 2
Water
0.2 molar
0.4 molar
0.6 molar
0.8 molar
.0 molar
Weight Before
2.34
2.40
2.39
2.40
2.37
2.25
Weight After
2.38
2.37
2.25
2.23
2.13
.98
Change in weight
+0.04
+0.03
-0.14
-0.17
-0.24
-0.27
Core 3
Water
0.2 molar
0.4 molar
0.6 molar
0.8 molar
.0 molar
Weight Before
2.24
2.36
2.28
2.34
2.39
2.29
Weight After
2.3
2.39
2.23
2.17
2.17
2.04
Change in weight
+0.06
+0.03
-0.05
-0.17
-0.22
-0.25
Average Change
+0.05
+0.03
-0.10
-0.17
-0.23
-0.26
A table to show percentage change in weight.
Percentage weight change = Average Change in weight x 100
Average Initial weight 1
Percentage change in weight
Water
+2.17%
0.2 molar
+1.27%
0.4 molar
-4.27%
0.6 molar
-7.23
0.8 molar
-9.83%
.0 molar
-11.40%
ANALYSING EVIDENCE AND DRAWING CONCLUSIONS
Conclusion
The results show that when potato cores are submerged in different gradients of sugar there is a change in weight. It also tells you that this change in weight will alter, when the gradient of sugar is changed in the solution. From the results we can tell that the change in weight is not random and follows a pattern as the concentration gradient is changed. The weight of the potato cores increased when they were submerged in water, the rate of increase then declined until the weight decreases as a higher gradient of sugar is added to the solution. The change in weight from negative to positive happened between 0.2 molar and 0.4 molar, I can assume that the state of equilibrium where no weight change would be recorded, because the concentration gradients were both the same, in the potato and the solution would happen between these two points.
I think I can explain my results through the theory of diffusion. Diffusion dictates that substances diffuse from higher concentrated areas to less concentrated ones, through a semi-permeable membrane. And it also says that as the concentration gradient increases the faster the rate on osmosis will be. When there is no sugar in the solution and its pure water (0.0). There will be a higher concentration of water in the solution then in the potato. The water would therefore diffuse from the solution into the potato. The sugar from the potato would also diffuse from the potato to the solution, but because the molecules of sugar are so big in comparison to the water molecules, not a lot of sugar would diffuse and it would not greatly affect the change in weight. When the level of sugar is increases to a level where the concentration gradient of water in the solution is still bigger than the gradient of water in the potato but not by as much a gain in weight can still be noted, but it is not as drastic in a higher concentration of water. This is supported by my results, because there is not as drastic a rise in weight in 0.2% molar sugar as in pure water. However there is still a gain in weight. When the gradient of water in the solution is less than that in the potato, a change in weight was still noted, but it was now a negative change. As the concentration of water is getting less, when the gradient of sugar is increasing. The decrease in weight becomes more apparent because the concentration gradient of water is decreasing in the solution, so now, the higher concentration will be present in the potato core. And it will be diffusing from the core to the solution.
I found that the results supported my prediction to a certain extent. In my prediction I said that I thought the weight of the potato cores will increase when they are submerged in water, and will slowly decline in the rate of increase until they start to decrease when a higher gradient of molar sugar is added. This did happen, so in this way my prediction was correct. However I also stated that I thought that the rate of osmosis would be a steady incline or decline, however you can see through my graph and results table that it was not. Between water and 0.2 molar, there is a difference in the change in weight of 0.02g. When we look at the difference between 0.2 and 0.4 molar, it is a difference of 0.13g. The difference in the change in weight now does not continue to increase, for the next difference, between 0.4 and 0.6 is 0.7g. I can not explain why this is; there must have been some inaccuracies, in my experiment, for these results to be present.
Explaining my Diagram.
The following diagram represents the molecules of sugar and water in the potato cell and the solution. The arrows are colour coordinated to represent the direction and amount of substance is diffusing. The sugar molecules are shown as orange circles and their direction as orange arrows. And the water is shown with black circles and black arrows also. The sugar molecules are bigger than the water ones to represent the difference in size of the molecules.
A diagram to support my conclusion
EVALUATING EVIDENCE
Evaluating evidence
On the whole I thought my experiment went very well. The graph show no outstanding anomalies or deviations, suggesting that my results were accurate. However I feel there were some improvements that could have been made. For instance, to obtain accurate results, I put three cores in each solution; these were all in the same Petri dish. I noted some inaccuracies in my results. Between water and 0.2 molar, there is a difference in the change in weight was 0.02g. When we look at the difference between 0.2 and 0.4 molar, it is a difference of 0.13g. The difference in the change in weight now does not continue to increase, for the next difference, between 0.4 and 0.6 is 0.7g. I can not explain why this is; there must have been some inaccuracies, in my experiment, for these results to be present. I believe that the difference in the change in weight between each one should have been a steady increase of decrease, not a random change like that that is present. If I had put each core in a different Petri dish perhaps my experiment would have been more accurate and these inaccuracies would have not been recorded.
I thought my experiment was reasonable suitable for my results. As stated above if I had separated each core into different Petri dishes, the results may have been more suitable. As we were weighing the weight of one core before and after not the combined weight of three. Maybe the three cores in the same Petri dish had some affect on the results I don't know, I can only say that if this altered my results it is likely to have happened to all of them, and therefore not really ruined my experiment.
My results were fairly accurate and allowed me to draw a fairly safe conclusion. We did each core three times, and so the results must have been fairly accurate. My results also supported my prior knowledge to the experiment, and followed the same trends as the previous experiments I have done. It also supported the theory of diffusion, and so I could say that my results were accurate enough to draw a firm conclusion.
If you look at my graph and my results table, you can see that in the Petri dish filled with water that the average change in weight, looks out of place. When I draw a line of best fit on my graph, it suggests that the change in weight should have been somewhere around 0.1g. However it is only 0.05g. I think that there must have been some inaccuracies in my results. For instance the water that I used may not have been pure, so there may have been other substances in the water that could alter the results.
If I did this experiment again to make sure the results I obtained were accurate, I would do the following things. I would measure all my solutions in an accurate pipette so that one volume could have not been larger than others. I could accurately measure the temperature of the substances with, and electrical thermometer. I also could have used a more accurate method to cut my cores to the same size.
For a future experiment, as well as the previously mentioned changes which are just to achieve higher accuracy, I could also change another one of the variables. We know that the concentration gradient affects the rate of reaction, and that other variables do to. If I kept the same percentages of solution as previously mentioned, but raise the temperature to around 40oC, I would predict that my results would be more wide ranging than before. This would make less reliable results easier to spot, and it would also make differences in weight easier to see as well. Perhaps if I did this I could also tell if the rise in temperature affects some solutions in their rate of reaction more than others. The risky part is that if I raise the temperature too high, the potato cells could become denatured and the Osmosis would stop, because the cells are no longer able to transfer fluids through the semi-permeable membrane.
