The 0.28 molar solution of sucrose, I think, should make a slight difference now, noting that it should be a small difference between the two concentration gradients. And therefore the weight should decrease, not very noticeable for us but none the less there.
The difference between the water concentration in the potato and the 0.32 molar solution of sucrose will be relatively big, and the water in the potato should be transferred from the potato, through the permeable membrane, to the solution surrounding the potato. As a result, I am sure that the weight of the potato will increase by quite a measurable amount. This is because the potato tissues are surrounded by a stronger solution and it will probably become smaller, and shrink. But because of such high molars of sucrose, the water can diffuse all the way, throughout the two substances, equalling the concentration gradient of the two substances.
My theory in this experiment is that, due to the difference in the water concentrations of the two substances, I believe that the weight of the potato will start decreasing when it is tested on 0.2 molar solutions of sucrose and greater.
Scientific explanation of what would happen:
Osmosis is defined as the movement of water molecules from an area of high water concentration to an area of low water concentration, across a semi-permeable membrane.
In a high concentration of water the amount of solute (e.g. sugar) is low. This could be called a weak or dilute solution.
In a low concentration of water the amount of solute (e.g. sugar) is high. This could be called a strong or concentrated solution.
When two such solutions are divided by a semi-permeable membrane the water will move from the area of high concentration to the area of low concentration, until both sides are equal (have reached equilibrium).
This can be seen in living cells. The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration) water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid. If done with potato cells the cells would increase in length volume and mass because of the extra water.
If these potato cells were placed in a solution with a low water concentration, then the opposite would happen. Water would move out of the cell into the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmolysed. The potato cells will have decreased in length, volume and mass.
The greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater the amount of water that leaves the cell.
However, there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred.
Using this information a graph and prediction can be made, which can be seen below:
I predict that as the concentration of the salt solution outside the potato increases the potato mass will decrease because more water is lost through osmosis.
As well I predict that there will be a certain level of concentration where the potato will neither gain or loss mass and below this concentration the potato will gain mass.
I also predict that at a certain concentration the potatoes will plasmolysise and at levels of concentration higher than this the change in mass will be exactly the same as the level where the potato first became plasmolysed.
Apparatus I will need:
· Cork Borer (diameter of 0.5cm),
· Water,
· Measuring cylinder,
· Tile,
· Potato,
· 0.2M, 0.24M, 0.28M, 0.32M, 0.36M and 0.4 molar of sugar solutions,
· Weighing balance,
· Test tubes,
· Beakers,
· Paper towels,
· Sticky labels,
· Ruler,
Diagram:
I have drawn a diagram of what the experiment will look like seen below. I have chosen the best equipment given to me to carry out this investigation thoroughly.
Fair Testing:
In this experiment on osmosis there should only be two changing variables:
1: Different molarities of sugar solution,
2: Change in mass of the potato samples.
To keep these two variables the only variables in the experiment I must:
1: Keep the potato samples the same lengths (5o, 55 & 60mm). This is because if one potato sample is 1cm long and one is 3cm long then the 3cm long sample will have a larger surface area and will osmosis much more quickly.
2: Use the same potato. This is because many factors due to the potato may affect the experiment. For example the age and sizes might be different, which means one potato might have more water in them then another.
3: Stop the evaporation of any of the molar solution. This is because if the sugar solution evaporates past the level of the potato, then the potato sample will have less surface area in the solution so this would make osmosis happen much slower. To stop any solution evaporating a cork lid can be placed on top of the test tube.
4: Accurate amount of sugar solution: More Bathing solution may affect the rate of solution. To make the amount of solution placed in the test tube as accurate as possible a syringe will be used to measure out the exact amount needed.
5: Contamination: As each test tube is filled up with the different molar solutions the syringe which would measure the amount of solution placed in the test tube may become contaminated with different molarities. To stop his from happening, the beaker and syringe must be washed every time they are used.
6: Average: To make the experiment as accurate as possible an average will be taken out of the 6 sets of results taken. Also any clearly anomalous results will be ignored.
7:Temperature: The temperature may affect the reliability of the experiment for example at extreme temperatures the cells of the potato may die and at less extreme temperatures the experiment may be speeded up. To keep this from happening, all the test tubes will be kept in the same place and at the same time of the one-hour experiment.
Safety procedures:
Safety is an important aspect in every experiment, even if the experiment seems to be very harmless. This is why I'll be taking this into consideration.
I will be using a very sharp knife, which could injure someone if it's not handled properly. I will also be very careful that the solutions don't get into our bodies internally, just in case, because we are not fully aware of the damage it could do to us. I will also be wearing an apron due to any solutions getting to my clothes.
But other than that, there weren't any bigger matters to be cautious of.
Method:
I plan to carry out this experiment by using all the safety issues and fair testing procedures to give me the most reliable and most accurate set of results.
I plan to have a range of sugar solutions prepared with concentrations from 0.2M to 0.4M, in intervals of 0.04. Then sections of potato will be cut using a cork borer and knife to lengths (between 5omm and 60mm) keeping the surface area constant. I will then measure each tissue of potato using a measuring scale, measured in grams, when I do this I will place the Chip on the scale. I will then add the potato into each concentration of sugar in two test tubes, giving me a total of 5 test tubes, and labelling each molar reading. Then to each test tube 3 chips will be added. These will be left Overnight. Then the potato pieces will be removed from the test tubes, and surface solution on the potato will be removed using paper towels. I will then measure the potatoes again, recording its change in mass by weighing them. I will then be taking the two readings of each molar concentration. This is because I will be doing a repetition of the experiment, as it will save me valuable time. These results will be taken for each concentration in order to get an average and reduce the effect of anomalous resulting.
As it is difficult to get the cut potato pieces to the same mass it was decided that I would use a percentage change in mass, which will be used to compare the data in the results, as, this would be far more accurate. I will do this by taking the difference in mass; divide it by the mass before of the potato and multiplying it by 100.
After looking at my preliminary work it suggested that there wasn't enough concentrations of sugar to compare each result to each other, so their will be 6 different sugar concentrations to choose from for the actual experiment. The timing of the potatoes weren't left for enough time in the preliminary experiment, as the results didn't have much change, therefore I will leave them overnight instead of one lesson to give a greater mass change. I will also be and measuring the mass change as a percentage as it will be more appropriate so that reasonable results could be obtained.
I will also be using the same raw data table I used in my preliminary results seen on Last page.
Analysis
The graph quite clearly shows the relationship of how increasing the concentration of the sucrose solution forces the potato to lose more mass. This trend can be summarized as: If you increase the concentration of the sucrose solution, the mass of the potato will decrease as well. The line is not a straight line indicating to me that the graph shows no directly proportional or indirectly proportional link. Or it could be that my points are slightly wrong and I have made an error. However, the graph indicates to me that I have kept within a good accuracy and consistency level. So from the graph I am able to find that there is no directly or indirectly proportional link to the loss of mass and sucrose solution, but there is a relationship that as you increase the concentration the amounts of mass change increases. I did however notice that in my preliminary work the middle of the graph the line was straight, but it started to tail off towards the extremities of the concentrations used, so I made the modifications. I feel this was due to the fact that the cell walls were unable to expand or contract anymore, so therefore put up more resistance and prevented the shape from being altered more.
Evaluation
Comments on the Procedure and Method: This experiment was done two times and during the repeat method an altered and improved method was used, therefore when discussing the efficiency of the procedure I must look at both the original method and the improved method.
My original method was inaccurate and suffered from some problems; the results from the first set of results indicate that this is true. During my first experiment the method was accurate, however, it contained certain problems. Firstly, the potato pieces were too small. They only measured 5mm long. The pieces were cut in an accurate manner with a 15cm ruler that measured to the nearest mm: more than sufficiently accurate in terms of this experiment. The fact that the potato pieces were to small meant that osmosis did not have enough time to occur over the twenty minute period. By increasing the size of the potato the surface area that osmosis can occur over is increased, therefore the rate of osmosis is increased and osmosis can occur sufficiently during a twenty-minute period to give us results that contain no anomalies. I also increased the amount of solution from 5ml to 10ml. This had the same effect as increasing the size of the potato. These two problems were corrected after the first run and allowed the procedure to be improved during the second results.
In terms of accuracy, the experiment was a good one. The accuracy was satisfactory in numerous ways, firstly the potatoes were all accurately the same size, and the cork borer ensured that each cylinder was precisely the same size. Once the cylinder had been bored a 15cm ruler that measured in mm ensured that the potato were all either 5omm, 55mm or 60mm long. The ruler measured in mm a measurement, which is more than satisfactory for this experiment; this ensured that all the potato pieces were the same size. The solution was only put in by hand using our eyes to check that they were level. This meant that the amount of solution in each of our test tubes was different, so I could have improved this by measuring it next time. The electronic balance was also accurate to the nearest 0.01g. This may seem accurate overall, however, when put in the context of this experiment this accuracy is needed because this experiment deals with very small masses, e’g the change in mass was very small.
Comments on the Data Gathered: The reliability and amount of anomalies from the first set and second set of results varied by a large amount.
The first set of results showed a pattern that could be drawn into a best-fit line. However, this best-fit line couldn’t be trusted because it contained anomalies. I.e. the results showed a relatively low negative correlation. Because a majority of the results fell away from the best-fit line the conclusion drawn from this best-fit line couldn’t be trusted on the basis that it was concluded from anomalous results. These poor results for the first set of results can be explained due to the problems of the procedure of the original method. The main problem was the fact that the potato was not of a large enough size to allow enough surface area to be exposed for osmosis to occur on. Therefore osmosis couldn’t occur properly over the twenty-minute period. Since osmosis couldn’t occur properly the results received in the first run of the method contained anomalies.
My second run provided high quality results. They produced a best-fit line of a Straight line crossing the x-axis at about 3.7M. All the results fell close to the best-fit line and as a result it provided a good solid conclusion. These good results were due to discovering the poor results in the original method and correcting the faults in the method for the repeat results. Using a small a potato size e.g. 5mm was a large problem in causing anomalous results.
Although my repeat results contained a few anomalies, how good was the reliability of the experiment. Reliability is essentially the amount of repeated results there are, an experiment with four repeat runs is said to be very reliable but an experiment with no repeated runs is unreliable. This experiment has only one repeat runs, more would have been done but time was a limiting factor. However, only one repeat run is required to provide sufficiently reliable results for this experiment. But, reliability is not solely the number of times an experiment is done, if you do an experiment twice and both times the results come out exactly the same then the experiment is reliable, but, if you do a experiment six times and every time you come out with a completely different set of results then the experiment is unreliable. Therefore this experiment is only fairly reliable because its repeated once but there is a discrepancy between the first and second set of results.
How Firm is my Conclusion?
The firmness of my conclusion depends on the number of anomalies and the reliability of the results. Although the second set of results greatly support the conclusion and tie in with it, the first set of results detracts from this because they contain a number of anomalous results. Fundamentally, my best-fit line supports the conclusion: that at low concentration, there is a gain in mass and at high concentrations; there is a loss in mass. The original results detract from the firmness of the conclusion because of the anomalous results and the fact that they reduce the reliability of the experiment.
However, the conclusion can be said to be firm because both the experiments supported the statement and the repeat run firmly indicated that the conclusion was entirely true.
Improvements and Further Work: There are number of alterations and improvements that could be added to the method which would allow us to investigate further into the area of osmosis in biology.
-You could investigate osmosis further by seeing the effect of concentration of different types of potatoes or using entirely different plants (e.g. plants, other vegetables, fruits or a different type of potato). This would test how different semi-permeable membrane could affect osmosis and the movement of water out and into organisms.
-You could investigate the time it would take for potato cells to plasmolysise. I.e. you could investigate using extremely low or high concentrations with potatoes. This would allow you to see how long it would take the potato to plasmolysise and die.
-You could do the experiment using a longer period of time for osmosis to occur. By doing this you could investigate how long it takes for osmosis to fully complete. It could also be connected to the above improvement in finding how long it takes for the potato cells to plasmolysise. E.g. use a 3-hour or longer 24-hour period.
-You could do an investigation into how changing surface area and the amount of solution affects the rate/how long it for osmosis to occur.
-You could do an investigation into how osmosis is affected when it occurs it different conditions, i.e. how osmosis occurs under high or low temperatures. You could also investigate the amount of heat that the semi-permeable can withstand before the membrane is disrupted. After this you could expose the potato to a certain amount of heat, a large amount (but not enough to completely disrupt the membrane) and then you could see how exposing the potato to heat beforehand affects osmosis.