The mixture of solute potential and pressure potential determines water potential .
Apparatus
In order to carry out this practical element of the investigation, I will choose the most appropriate equipment from the following lists.
- To collect potato samples and cut to size accurately: -
- Cork borer (50mm in length and 6mm in width)
- Kitchen knife
- Razor blade
- Scalpel
- Forceps
To extract the sample of potato tissue from the potato tuber, I will use the cork borer instead of other apparatus available, as it will provide the exact surface length for each sample and is a much safer method.
The scalpel will be used to provide similar surface area by cutting the samples to similar lengths to maintain this seeing that it may affect the accuracy of the results. To prevent an external factor such as too much pressure being put on the samples when putting in or taking out of solutions manually, forceps will be used that do not damage the samples or pressure is exerted.
- Beaker (100/250ml)
- Boiling tube *6
- Boiling tube rack
To hold the solutions, I will apply the use of the combination of six boiling tubes and a boiling tube rack. They will be used as the solutions within the boiling tubes will contain the surface area of the samples, but in beakers the solution will not, as it is in a small quantity and a beaker has a larger area.
- To measure the volume of water and sucrose solution
- Measuring cylinder (10/20ml)
- Beaker (100ml/250ml)
- Pipette (5/10ml) *2
Using a pipette of 10ml will be the most reliable method to measure the exact quantity of the distilled water and the sucrose solution. The measured solution can be released at the lowest point of the boiling tube, instead of the solution droplets exerted on the sides. This could waste liquid and could also result in inaccurate quantities that would affect the reliability of my results. I will use two separate pipettes for the distilled water and sucrose solution to avoid any flaw of concentrations.
- Manual scales
- Electronic balance
I will use an electronic balance to achieve accurate readings of the mass of potato samples to two decimal places with no possibility of manual error.
- Potato tuber
- Stickers (to label boiling tubes)
- Filter paper
- Bottle of distilled pure distilled solution
-
1 mol dm³ of sucrose solution
Safety
Whilst carrying out the experiment safety precautions will have to be considered when using the apparatus, especially the scalpel that poses the main safety risk because of the sharp blade. Therefore, when using the scalpel to cut the potato samples to mass, the forceps will be used to hold the sample to evade it slipping and the blade striking another surface that could lead to any injuries. Another precaution I will follow includes after using the scalpel and cork borer for their purpose, they will be immediately cleared away to avoid any accidents.
Variables
To achieve reliable results and plan this investigation to be carried out fairly, I will have to control certain key factors that could affect the accuracy of my findings. These are outlined below: -
All the potato samples will spend only twenty-four hours in their assigned solutions. If the samples spent varied periods of time in the solutions inconsistent results could be produced, i.e. the samples that spent more time than the others would have had more time to carry out osmosis.
Only boiling tubes will be used to contain all the samples in their solutions, as if I were to use different types of containers i.e. beakers and boiling tubes, this could result in accurate results. This is due to the potato samples in boiling tubes may be surrounded closely by the boiling tubes sides unlike in a beaker it would have a large amount of space. Therefore, the volume of the containers differs and the rate at which diffusion occurs could vary owing to this.
The solutions will be kept at room temperature, which is between 15°C and 20°C. They will not be placed in direct sunlight due to light intensity or near sources of heat e.g. radiators. Temperature must be maintained as the rate of diffusion, including during osmosis, increases as the temperature increases caused by molecules gaining kinetic energy than at lower temperatures.
- Mass and size of potato samples
All the samples of potato tissue will initially weigh between 1.11grams and 1.16 grams, measuring 20 to 25mm in length and a width of 6mm. The surface area of all samples must be taken account of as ‘greater the surface area, then the more molecules can cross it at any one moment, and therefore faster diffusion can occur.
- Amount of total solution/concentration.
This must be maintained seeing that if there were differences in the total solution present, i.e. use of too much sucrose solution or distilled water, the percentage change in mass may occur to an extent it shouldn’t.
These factors were noted during my preliminary experiment and could affect the results of my final experiment. By carrying out the preliminary experiment, was made aware of these variables and therefore, I will be able to control when the experiment is repeated.
Method
To understand how osmosis occurs across a range of concentrations, I will use the following in this investigation: 0 mol dm³ (distilled water), 0.2 mol dm³, 0.4 mol dm³, 0.6 mol dm³, 0.8 mol dm³ and a 1 mol dm³ sucrose solution. The 1 mol dm³ solution is one in which 1 dm³ (a litre) of solution contains the molecular mass (one mole) of the substance in grams. In this case, the sucrose 1 mol dm³solution, 342g of sucrose will be dissolved in 1000 cm³ of distilled water.
For this investigation, such a large quantity of concentration is not required and will instead be made up of one quarter of the stated amounts, which will still make up a 1 concentration; 85.5g of sucrose in 250cm³ of distilled water.
Below is the procedure I will follow: -
- Collect apparatus and other requirements needed to carry out the experiment stated.
- Label boiling tubes with stickers and the concentration in mol dmˉ ³ to be contained within them.
- Make up solutions by following ratio figures outlined in the following table:
Using a pipette and placing it halfway in the sucrose solution can
carry this out. Draw up required ratio amount of sucrose solution (none is
required for the 0 mol dm³) and release in the boiling tube labelled with that
concentration. Repeat the same process for the amount of distilled water in
the bottle (if required), using separate pipettes for the distilled water and
sucrose solution. Total amount of solution in each tube should total 10ml.
- Bore out six cylinders of potato samples from the potato tuber, using the borer.
- Transfer each sample in turn using the forceps onto the electronic balance and recording initial mass. This should be done to two decimal places and the mass of each potato sample should be between 1.11g and 1.16g. Thus, the length should be 20mm to 25mm and a width of 6mm. If the sample exceeds the mass required, use scalpel to cut very small amounts of tissue from the sample until an appropriate mass is achieved.
- Transfer one sample to each boiling tube and solution using forceps as each one has been weighed, noting solution in which it has been transferred.
- Put boiling tube rack in a location not facing direct sunlight or any potential sources of heat. Note time and leave undisturbed for twenty-four hours.
- After twenty-four hours, use forceps to remove potato samples from tubes in the order they had been placed in their solutions. Remove surplus liquid immediately by gently placing on filter paper or paper towels using forceps. Reweigh each potato sample and record its final mass. Do this for each sample.
- Throw waste materials away and spill all solutions, taking care to avoid any spillages.
Prediction
My prediction will be mainly based upon the results of my preliminary experiment, which are presented below: -
The results of my preliminary experiment support the concept that osmosis affects the mass of potato samples in the different concentrations. For that reason, I predict that as the concentration increases, so does the mass of the samples increasingly decrease. Thus, the potato tissue in the 1 mol dm³ solution will decrease in mass the most. This will eventually increase the solute potential (nearly 0) of the solution until an equilibrium is reached, as the protoplast within the potato samples’ cells shrink until the pressure potential is zero. This means the protoplast is no longer exerting any pressure on the cell wall of the cells. Consequently, plasmolysis may occur owing to the diffusion of water molecules from the potato samples cells; the mass of the sample will decrease.
As the solutions become increasingly diluted, the potato sample within the solutions will increase in mass. Thus, the 0 mol dm³ (distilled water) will gain the most mass. I predict that this will be the result, since as the cells of the sample will have a lower water potential compared to that of the concentration surrounding it, water molecules in the concentration with a higher water potential, will diffuse through firstly the freely permeable membrane of the cell and then all the way through the partially permeable membrane. This will increase the cells solute potential (nearly 0) and during the process, the cells may become turgid.
The molarity of the solution at which the cells of the sample neither lose or gain mass will fundamentally decide at which solution or concentrations osmosis causes the cells to lose or gain in mass. The values that are more diluted than this particular solution, will gain mass due to osmosis and these molarites in my preliminary experiment were 0 mol dm³ and 0.2 mol dm³. The solutions that were increasingly concentrated than this molarity will lose mass, in this case 1 mol dm³, 0.8 mol dm³, 0.6 mol dm³ and 0.4 mol dm³. Consequently, the molarity at which the sample neither loses nor gains mass is between 0.4 mol dm³ and 0.2 mol dm³. I predict my final experiment will follow this trend, as the process of osmosis will repeat the trend of these results.
Results
The following results table includes the results recorded during my investigation.
The table beneath shows the relationship between molarity and solute potential of sucrose solutions. I have only shown the molarities relevant to this investigation: -
Comparing these results with those of my preliminary, I noticed some slightly contrasting results and therefore repeated the experiment for the 1 mol dm³, 0.8 mol dm³ and 0 mol dm³ solutions. Here are those results: -
Analysis
The results of my experiment were consistent with my prediction, as the process of osmosis did affect the mass of the potato samples in the various concentrations of sucrose solution. My prediction was very accurate due to the subsequent findings during my preliminary experiment. This is clearly supported by graph 1, which shows the results of this experiment. The shape of the graph is curve-like from the negative axis (showing the result for the 1-mol dm³ solution) to a positive axis (for the 0 mol dm³). This can be summarised as, from left to right.
However, comparing these results to those of my preliminary, I had noticed some slightly contrasting results for the 1 mol dm³, 0.8 mol dm³ and 0 mol dm³ concentrations. Therefore, I repeated the experiment for specifically these concentrations, in order to get an overall consistent and accurate set of results. Using these figures, the table below shows the average ‘mean’ percentage change of the two but sometimes three sets of results.
Graph 2 shows these results simply support what I had previously predicted would occur. Initially, the potato samples gain mass in the 0 mol dm³ concentration and then as the solute potential decreases (more negative) so does the mass they gain decrease. Nonetheless, eventually the samples will begin to gain in mass in solutions of higher solute potentials and the highest being the 1 mol dm³ solution. This relationship between molartiy and solute potential is shown in graph 3.
The process of osmosis causes this to occur since as we add solute to water, the water molecules from a shell around each solute molecule. So this decreases the number of free water molecules that are able to exert a pressure on the membrane. Therefore, the water potential of the cells becomes lower.
The process of plasmolysis may have occurred in such conditions because the cells of the potato sample lose water due to osmosis, the protoplast shrinks away from the cell wall.
The solute potential in the potato samples is higher than that of the concentrations, which causes them to gain mass as water molecules diffuse through the cells of the sample due to the difference of the concentration gradient. The cells of the sample may have become turgid due to the build up of internal pressure (pressure potential).
Using these findings, regarding how specific concentrations of solutions affected the mass of the potato samples, I can determine the concentration at which the potato samples are neither lose or gain in mass due to osmosis. This means, I can use my results to work out the solute potential of the potato samples. This is determined by the point at which the average percentage change line equals zero (osmosis does not occur in this concentration due to equal concentration gradient) and crosses the x-axis. For that reason, the solution at which the sample would not gain or decrease in mass is approximately 0.280 mol dm³, which according to graph 3 indicates the solute potential of the potato samples is around –850 kPa.
