In plant cells water potential has two main components which are solute potential and pressure potential.
Solute potential is the effect of dissolved substances on the potential energy of a solution and a hypotonic solution has a high solute water potential while a hypertonic solution has a lower water potential. Solute potential is a always 0 or below with pure water having a solute potential of 0 and a hypertonic solution having a smaller solute potential. Solute potential is the key factor in water potential because as we know the major factor in osmosis is the movement of water from a higher water concentration to a lower water concentration which is in effect from a solution with a higher solute potential to one with a lower solute potential.
Pressure potential is hydrostatic pressure and so a turgid plant cell with a large hydrostatic pressure has got a higher pressure potential than that of a plasmolysed cell. In a turgid cell the pressure potential is positive while when a plant cell is not turgid the pressure potential is negative and so pressure potential is usually negative.
The equation linking water potential, solute potential and pressure potential is:
Ψw = Ψs + Ψp
Water potential = Solute potential + Pressure potential
In a plant water will move by osmosis into and out of cells due to differences in water potential between the cell and its surroundings. The speed at which water moves in and out of a cell is dependant on the difference between the water potential of the cell and its surroundings. If there is a very large difference between the cell’s and the surrounding water potential than the water will diffuse in or out of the cell very quickly but if there
Investigation Outline
Potatoes of the same size will be placed into situations where the solute potential is lower in the solution surrounding the potato then that of the water potential in the potato cells and then my partner and I will see what happens to the potato. We will do the same thing but also in situations where the solute potential is higher in the solution surrounding the potato then that of the water potential in the potato cells. From the results of the changes in the potato we will also then be able to work out the water potential of the potato cells as that will be where there is no change in the potatoes’ mass and length. At this point the cell’s pressure potential will be 0 and from the concentration of the sugar solution we will be able to work out the solution’s solute potential at room temperature. To make the experiment fair we will try to keep all the variables the same e.g. temperature, length of time kept in solution. We will also place three potatoes into each solution and use an average to get more accurate results. The solute potential of the sugar solution that the potatoes will be placed in will be varied by the molarity of the sugar solution. The potatoes will all be placed into the same volume of sugar solution. The potatoes will all be of the same type and they will have been all peeled but not washed as then osmosis could occur.
Predictions
I predict that the potato cylinders will increase in size as the original water potential of the sugar solution increases but not proportionally because, as explained above, the rate of water diffusion is dependant on the difference between the water potential of the cell and its surroundings therefore as the original water potential of the solution decreases there is a smaller difference between the water potential of the solution and that of the cell’s and if the original water potential of the sugar solution is lower than that of the cell’s the potato cylinders will decrease in size.
Experiment planning
In order to be able to check whether my main experiment would work well we conducted a preliminary experiment which we then evaluated to see how we could improve for my main experiment.
Preliminary Experiment
Method
We placed three lengths of 3.5 cm of potato, which had been cut using a scalpel and measure using a ruler, into 5 Petri dishes. In one Petri dish we poured in 10cm3 of tap water and 10cm3 of 1 Mole/Litre sugar solution making a concentration of 0.5 moles/litre. We then used different ratios of water and 1M sugar solution to produce 20cm3 solutions with concentrations of 0.2M, 0.1M, 0.05M and 0M in the other four Petri dishes. We measured the volumes of the sugar solution and water using measuring cylinders. As this was the preliminary experiment we only waited two and a half hours before measuring the new lengths of the potatoes.
Preliminary Results
Preliminary Conclusion
These showed the expected trend though they can be made more accurate as will be explained next.
Evaluation
To make this experiment more accurate for the main experiment a number of differences to the method can be made. This includes leaving the potatoes longer in the Petri dishes in order to be able to see as big a gain or loss in size as possible, using mass as well as length to make sure the potatoes are all similar, using more accurate equipment for measuring the lengths of the potatoes possibly callipers, making sure sensitive electronic scales are used to measure the mass of the potatoes, using a borer to bore cylinders all of the same diameter, possibly using more accurate equipment to measure the volumes of water and sugar solution than a measuring cylinder and we should use distilled water rather than tap water.
We decided to use 4cm rather than 3.5cm in the main experiment for the lengths of potato cylinders just to make it easier to measure and 0.87g was the average mass of the cylinders in the main experiment. We used the same concentrations again and we decided using three cylinders and then averaging out the lengths was accurate enough.
Main Experiment
Method
We placed three cylinders of diameter 0.5cm, length 4cm and mass 0.87g of potato, which had been bored through a potato using the same borer, then the lengths were measured using a ruler to the nearest millimetre and the masses were measured using sensitive electronic scales to the nearest hundredth of a gram, into 5 Petri dishes. We used different ratios of water and 1M sugar solution to produce 20cm3 solutions with concentrations of 0.5M, 0.2M, 0.1M, 0.05M and 0M in five Petri dishes.
The ratios are as follows:
We measured the volumes of the sugar solution and water using measuring cylinders. We waited six hours before measuring the new masses and lengths of the potato cylinders. We dried the potatoes before weighing them as to prevent the weighing of excess sugar solution but we made sure that we did not squeeze the cylinder as that could have changed the length.
Results
Table of Results of Change in Length of Potato Cylinders
Table of Results of Change in Length of Potato Cylinders
Graphs
Conclusion
As can be seen from the graphs as the original water potential of sugar solution surrounding the potatoes increased the potatoes’ length and mass increased though not proportionally as predicted. Under a certain original water potential of the surrounding sugar solution the mass and length of the potatoes decreased from their original size.
The line showing % change in length intersects the x-axis at approximately 0.38M (2.d.p.) which is when there is no change in length and so the water potential of the contents inside the potato and the sugar solution are equal. The line of % change in mass intersects the x-axis at approximately 0.43M (2.d.p.). These two values give an average original concentration of sugar solution when there is no change in size of 0.405M.
Therefore using the values given in this table:
Table of solute potentials of sucrose solutions (at 20°C)
It can be worked out that when the original concentration of sugar solution is 0.405M the water potential (as pressure potential is 0 in solution) is
approximately -1134 kilopascals and as we know when there is no net loss or gain of water by the cell the water potential of the cell and that of the solution’s must be equal and so the original water potential of the potato cylinders we used is also approximately -1134KPa. The results then go on to show that when the original concentration of the sugar solution is 0.5M the original water potential of the sugar solution is -1450KPa and this is lower than that of the potatoes’ and the potato cylinders decreased in size at this original concentration. This shows that water does travel from a higher water potential (the potato cell) to a lower water potential (the surrounding solution) as predicted. The opposite then happens when the original concentration of the sugar solution is above 0.405M because than the original water potential of the solution is higher than that of the potato and also pressure potential begins to play a role as pressure potential is positive when the potato is increasing in size and so this increases the potato’s water potential meaning that if the original water potential of a solution is 5% more than the potato cell original water potential (about -1134KPa) the water will move in at a slower rate than if the original water potential of the solution is 5% less than that of the potato cell because of pressure potential as it will increase the water potential of the potato cell increasing in size.
Evaluation
We unfortunately were not able to use all of the suggestions I made in the evaluation after the preliminary experiment so the experiment could have been made more accurate by using distilled water, more sensitive length measuring equipment than a ruler, more accurate equipment to measure the volumes of the sugar solution and water and we could have also measure the volume of solution outside the potato when measuring the results to see whether there was a net gain or loss of water outside the potato.
The results were all quite accurate showing no real inconsistencies.