Controlling Variables
When conducting the experiment you must be very careful as to keep certain factors as constant as possible. In this experiment we are only intending to vary the concentrations of the solutions. To make the experiment as fair and as accurate as possible make sure you do the following:
- Check carefully the amount of distilled water and 1.0M sucrose solution you put in each test tube.
- Square ends of all chips as accurately as possible.
- Weigh all chips using same electronic balance, if there is a fault in the machine then all the chips weights will be affected and still be in proportion to each other.
- Measure all chips using same mm ruler.
- Lightly pat chips dry as to remove any extra weight on them before weighing.
- Touch chips as little as possible, use forceps as dry or wet hand may affect chips water potential.
- Try to leave all chips for the same time of one hour as they will all have same time for osmosis to occur.
- Keep all the chips in same temperature if some are kept hotter or cooler than others their results will differ.
- Keep the test tubes in the same area so they will be in the same conditions, don’t leave one in the sun and the others in shade ect.
However some variables cannot be controlled:
- The initial potato chips all vary in density which means they might be able to soak up more water than others and be able to lose more water than others. As a result all 2m and 4cm chips will vary in density.
- The initial potato chips all vary in sucrose content which means they will all have different water potentials and react differently in the concentrations. As a result all 2cm and 4cm chips will vary in sucrose content.
Safety and precautions
When conducting the experiment there are precautions that you should follow or your results or health could be affected:
- Only conduct the experiment under the supervision of an adult.
- Do not run. You could slip and hurt yourself or, if conducting the experiment with others, ruin their results or your own.
- Pay full concentration to the task at hand, messing about could cause you to make an error that could ruin your experiment.
- When using the scalpel be extra careful as it is very sharp and could easily cut you.
- Make sure you tare the balance (reset to 0) before weighing.
- When putting 1.0M sucrose solution or distilled water into test tubes via syringes, make sure you check there are no air bubbles trapped in syringe as this will affect amounts of liquid in tubes.
- Do not shout as it could disturb others concentration.
- Most importantly be sensible.
Conducting the Experiment
- Take the 12 potato chips from foil and cut single potato chip into a length of 2cm. Repeat 4 more times.
- Cut single potato chip into a length of 4cm. Repeat 4 more times.
- Organize chips into 5 pairs consisting of one 2cm chip and one 4cm chip.
- Square the ends of all chips.
- Weigh potato chips in the order you cut them in, recording them as you go. Then place the pairs in a specific order on tile so you know which recorded weight corresponds with which chip.
- Place first pair into first test tube (0.0M), second pair in second test tube (0.25M) ect.
- Note the time on the clock.
- Repeat the whole process with Chinese radish chips. Placing them into the second row of test tubes.
- Note the time on the clock.
- Note any interesting observations.
- Leave potato and radish chips for one hour.
- Pour test tube 1 on row 1 into sieve over sink. Remove chips with forceps and place them on the tile.
- Lightly pat chips with tissue paper. Weigh chips and note results. Place in order you fist put them in on tile.
- Pour test tube 2 on row 1 into sieve and repeat the process used on test tube 1 row 2. Place the chips below first pair.
- Repeat for whole of row 1.
- Pour test tube 1 on row 2 into sieve over sink. Remove chips with forceps and place them on the tile beside the potato chips.
- Lightly pat chips with tissue paper. Weigh chips and note results. Place in order you fist put them in on tile beside the potato chips.
- Pour test tube 2 on row 1 into sieve and repeat the process used on test tube 1 row 2. Place the chips below first pair from row 2.
- Repeat for whole of row 2.
- Measure lengths of every potato and radish chip and record results.
- Clear up apparatus.
- Collect results in a clear table and create graphs to illustrate them.
Predictions and hypothesis
I believe that as the concentration of the solution that the chips were placed in increases that the length and weight will decrease from the first solution, 0.0M.
I say this because distilled water has the highest water potential of all, therefore anything placed in it that contains any sucrose at all, will take on water through osmosis as it will have a lower water potential. It will gain weight and grow in length because of this. At the next molarity of 0.25M I expect that their will be very little increase or decrease in length and an even smaller increase or decrease in its weight as I have done a previous experiment similar to this one which gave me those results. At 0.50M I expect a decrease in both length and weight from the original results. A concentration of sucrose solution with 0.50M has fairly high concentration of sucrose molecules and therefore has quite a low water potential. I wouldn’t expect a potato or radish chip to have a water potential that low so I would expect a fair amount water to move through osmosis into the surrounding solution and leave the tissue lighter and shorter because of this. At 0.75M I would expect the same process to occur as at 0.50M but with the tissue losing more water and becoming lighter and shorter as a result. At 1.0M there is a very large amount of sucrose molecules thus having a very low water potential, I would expect an even further drop in mass and length than at 0.75M.
Prediction Graphs
The graphs are just rough predictions as to what the results graphs will look like. They are based upon a similar experiment’s results that were obtained prior to this experiment. The previous experiment used only potato chips at a length of 5cm; therefore they are only a rough prediction. I have no knowledge of what the radishes results may be and therefore am unable to alter my graph accordingly.
Results Tables and Graphs
These are the results obtained from the experiment. They have been sorted into two tables. The first table shows both the results for the 2cm potato and radish chips. The second table shows both the results for the 4cm potato and radish chips. The length measurements were obtained using a simple cm and mm ruler. The mass results were obtained using the electronic balance in the apparatus and mass was recorded to two decimal places as to be as accurate as possible.
There are four graphs. There is a graph to show % change in length for 2cm potato and radish chips, graph 1. There is a graph to show % change in mass for 2cm potato and radish chips, graph 2. Both these graphs get their information from results table 1. There is a graph to show % change in length for 4cm potato and radish chips, graph 3. There is a graph to show % change in mass for 4cm potato and radish chips, graph 4. Both these graphs get their information from results table 2.
Observations
When removing all chips from the solutions (before weighing or measuring) I noticed that, both types of chips at both lengths were:
0.0M sucrose solution – chips were turgid and most slightly larger than 2cm or 4cm.
0.25M sucrose solution – chips had no real change in length or turgid/flaccidness.
0.50M sucrose solution – chips appeared quite flaccid and noticeably shorter.
0.75M sucrose solution – chips were obviously flaccid and shorter.
1.0M sucrose solution – chips were very flaccid and much shorter.
When first put into the tubes and sucrose solution initial observations were on whether or not the chips floated, sunk or were submerged just below the surface. The radish chips all floated at the surface. The potato chips varied:
0.0M sucrose solution – potato chips sunk at both 2cm and 4cm lengths.
0.25M sucrose solution – potato chips sunk at both 2cm and 4cm lengths.
0.50M sucrose solution – potato chips were submerged just below the surface at both 2cm and 4cm lengths.
0.75M sucrose solution - potato chips were submerged just below the surface at both 2cm and 4cm lengths.
1.0M sucrose solution – potato chips floated at both 2cm and 4cm lengths.
Conclusion:
2cm Potato chip
The molarity at which there is no percentage change in the length of the chip is 0.50M (see graph 1).
In my predictions I suggested that the molarity at which there is no percentage change in length would be 0.62M. This prediction is fairly accurate.
The molarity at which there is no percentage change in the mass of the chip is 0.32M (see graph 2).
In my predictions I suggested that the molarity at which there is no percentage change in mass would be 0.36M. This prediction is very accurate.
2cm Radish chip
The molarity at which there is no percentage change in the length of the chip is 0.16M (see graph 1).
In my predictions I suggested that the molarity at which there would be no percentage change in length would be 0.62M. This prediction is not at all accurate.
The molarity at which there is no percentage change in the mass of the chip is 0.32M (see graph 2).
In my predictions I suggested that the molarity at which there is no percentage change in mass would be 0.36M. This prediction is very accurate.
4cm Potato chip
The molarity at which there is no percentage change in the length of the chip is all between 0.0M and 0.25M (see graph 3).
In my predictions I suggested that the molarity at which there is no percentage change in length would be 0.62M. This prediction is not at all accurate.
The molarity at which there is no percentage change in the mass of the chip is 0.27M (see graph 4).
In my predictions I suggested that the molarity at which there is no percentage change in mass would be 0.36M. This prediction is fairly accurate.
4cm Radish chip
The molarity at which there is no percentage change in the length of the chip is 0.20M (see graph 3).
In my predictions I suggested that the molarity at which there would be no percentage change in length would be 0.62M. This prediction is not at all accurate.
The molarity at which there is no percentage change in the mass of the chip is 0.27M (see graph 4).
In my predictions I suggested that the molarity at which there is no percentage change in mass would be 0.36M. This prediction is fairly accurate.
Explanation and Analysis
The molarity at which there is no percentage change is equal to the cell sap sucrose concentration of the cells in either the potato or radish chips. So according to the average change in length of both the 2cm and 4cm potato chip, the cell sap sucrose solution is 0.31M.
The average change in length of both the 2cm and 4cm radish chip, the cell sap sucrose solution is 0.18M.
The average change in mass of both the 2cm and 4cm potato chip, the cell sap sucrose solution is 0.30M.
The average change in mass of both the 2cm and 4cm radish chip, the cell sap sucrose solution is 0.30M.
Whenever the chips had a change in mass or length it was because of osmosis. If a chip lost weight it was because the chips cells had a higher water potential than the solution it was put into. So, water molecules then moved out of the cells and into the solution it was put into in an attempt to achieve equilibrium. The solute sucrose molecules could not diffuse into the cell, which is why no sucrose moved only water. This loss of water causes the cells to lose weight or shorten as a result. If all of cells in the tissue all lose a tiny amount of water then the tissue will lose the amount of all of them combined. The amount of water lost depends on the difference between the water potential of the individual cell and the surrounding solution. Water will continue to be lost until there is a balance between the two; therefore as the difference gets greater then more and more water will be lost. I f a chip gains weight or lengthens it is because the cells have a lower water potential than the solution it was put into. So, water molecules move into the cells from the surrounding solution in an attempt to achieve equilibrium. The gain of water causes a gain in weight and length as the cells swell, as a result the whole tissue gains weight and swells making it longer.
Evaluating:
Suitability of Procedures
I believe that the method used was well suited for the experiment. It was a standard investigation into osmosis and to find the point of equilibrium for two different types of plant tissue. The method let me do this effectively and let me illustrate my results well. However the method was not perfect if one wanted an almost 100% accurate investigation as there are uncontrollable factors in the method.
Accuracy of Observations
The accuracy of some of the measurements were very good, especially for the mass results. The length measurements were not very accurate because we only measured one dimension of the 2cm and 4cm chips allowing a wide range of surface areas on chips that were supposed to be equal lengths. This would explain some of the results being a lot different to the predicted results or the results of a different tissue type of the same length.
Reliability of the Evidence
The evidence that suggest that the cell sap sucrose solution is 0.30M is very reliable. It was the average result for 2cm and 4cm chips of both potato and radish tissue types. It was obtained from the mass results I would believe that the actual cell sap sucrose concentration is very close to this. The mass results are far more accurate than the length results which are not very reliable. The mass results were recorded to 2 decimal places, using an electronic balance rather than a simple mm ruler. The mass results are very reliable, the length results are not though.
Improvements for further work
If repeating this experiment then I would do the following to increase accuracy overall and specifically to find the point of equilibrium:
- Allow more time to exaggerate results.
- Measure length, width, and height of each chip.
- Repeat the whole experiment then average both sets of results.
- Use more specific molarities closer to the possible range of the point of equilibrium.
- Conduct the experiment in a room in which the temperature could be constant at all times and the same in all parts of the room.
- Use more chips at different lengths for comparison.
