INVESTIGATING OSMOSIS AND WATER POTENTIAL IN POTATO CORES

WRITTEN BY SARA KHAN

AIM: The aim of the experiment is to investigate osmosis and water potential of potato cores using different concentrations of sucrose solutions.

INTRODUCTION:

Plant cells contain a cell wall and a cell membrane. The cell wall is a permeable layer whilst the membrane is a selectively permeable membrane, this means that it will only allow certain molecules to pass through it and keep out others. An example of this is water and sucrose; water is a small molecule that can pass through the selectively permeable membrane, whilst larger molecules such as sucrose are unable to. Root hairs of plants contain within them many different substances. The way in which plant root cells take up water is by using this selectively permeable membrane to allow ‘osmosis’ to take place. Osmosis is a specific type of diffusion concerning only water molecules; it is the net movement of water molecules from a region of high concentration to a region of low concentration. When plant root cells take up water, water from the soil is passed across the selectively permeable they move from a high concentration in the soil to a region of low concentration inside the cell.

The effect of dissolving solute molecules in pure water is to reduce the concentration of water molecules and hence to lower the water potential. All solutions therefore have lower water potentials than pure water. How much lower the solution is than pure water is known as solute potential. Solute potential is a measure of the change in water potential of a system due to the presence of solute molecules.

Water potential is the tendency of water to move from one place to another. Water moves from a place of higher water potential to a region of lower water potential. A solution with higher water potential (less negative) has a greater tendency to leave. If two systems are in contact, water will move from the system with higher water potential to one with lower potential.

PREDICTION

When the potato core is left in a pure water solution, the mass of the potato tissue will increase and therefore so will the volume. This is due to the fact that as the water potential is greater outside than inside the vegetable then water will move inside the cell across the selectively permeable membrane (osmosis). As the solute potential of the solution containing the potato tube is increased then the increase in mass will also decrease until a point is reached when the water potential inside and outside is the same and no change in mass will be recorded. Once the water potential is larger inside the root vegetable than the solution the mass of the root vegetable will decrease because water will be leaving it. From this a prediction can be derived that if the concentration of the sucrose solution is increased then the mass of the potato core will decrease.

VARIABLES

To make sure that the results obtained are reliable it is important that a fair test is planned and carried out. Below are listed the variables and also the variables that will be kept constant as to make sure that the experiment is a fair test.

The mass and shape of the potato core will be kept constant. It is essential that this remains the same because then a comparison can be made on the change of the mass of each potato core. To maintain the size and mass a potato borer will be used to cut the potato this will give the same mass and dimensions of the potato core. Mass is bound to vary slightly this is why a percentage will be used to compare the change in mass using the original mass and the end mass. This will assure that ...

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The mass and shape of the potato core will be kept constant. It is essential that this remains the same because then a comparison can be made on the change of the mass of each potato core. To maintain the size and mass a potato borer will be used to cut the potato this will give the same mass and dimensions of the potato core. Mass is bound to vary slightly this is why a percentage will be used to compare the change in mass using the original mass and the end mass. This will assure that the experiment is reliable and also a fair test. If the length and diameter are kept the same then the volume will also remain constant.
The period of time the potato is left in solution will also be kept constant. This is important because if there is a variation in the time that the potato core is left in the solution then one piece will have a longer period of time over which osmosis can take place, to prevent this taking place the potato core will be placed in the sucrose solution for the same period of time, 3 and a half hours, this will assure a fair test.
The volume of sucrose will be kept constant at all times, if it was not so then a fair comparison could not be made this allows a fair test and reliable results.
The temperature can make a change to the experiment. This is because a change in temperature may increase or decrease the kinetic energy of the molecules, which could alter the rate of osmosis to counter this; the experiment will be kept at a constant temperature by undertaking the experiment in room temperature. This once again will aid in giving reliable and accurate results.
The concentration is an important variable, which will be varied throughout the experiment.
The change in mass is the variable that changes which is dependent on the concentration of the solution.

PRELIMINARY WORK:

Preliminary work that was done was to measure the change in length, mass and position of a potato tissue in sucrose solution. This is because as the potato cells gained water they would increase in length but as the cells are made of cellulose and are rigid this is not very accurate. Below the results of the preliminary experiment are shown:

As can be seen the results obtained are inconclusive and so this means that the results are unreliable. To avoid this when doing the further tries of th4e experiment, the following modifications will be made:

The different concentrations used would not give an appropriate amount of results so 5 different types of concentrations will be used instead.
Only one recording was taken of the length and the mass of the potato tissue so to find more accurate results three readings will be taken from the same concentration and then the average will be found.
Also, to get more accurate results, conclusions will be based on the change in volume and the %change in mass because this will help provide more reliable conclusions.

APPARATUS:

Top-pan balance- is required to measure the mass of the potato tube before and after it has been left in the sucrose solution. The balance is a precision instrument and is accurate to 0.01g.
15 cm ruler- the ruler will be use to make sure that the same size potato tube is used each time and also to find out the change in size. The ruler is an accurate instrument and can measure to the required measurement, which is 0.1 cm.
Distilled water- the pure water is needed because pure water is required to make up each concentration of sucrose and also for the control experiment.
Water bath- water bath is required to regulate the temperature.
Sucrose solution- the solution will be 30 cm3 for every set of potato tubes.
Syringe - this will allow equal volumes of each solution to be put in each test tube.
15-test tubes- these will provide the container in which the solution and potato tissues are to be placed in.

METHOD:

Label the test tubes with the concentration of the sucrose solution that will be placed in them. (Each concentration will be done three times including the control).
Set bath water to 25 degrees Celsius.
Take the syringe and extract 30cm3 of distilled water and place it in the test tube. (Make sure the syringe is clean).
Take a potato borer and a potato and use the potato to obtain a cylindrical piece of potato.
Weigh the potato cores using a top pan balance and record this weight. All cores should have the same mass.
Measure the diameter and length using the 15cm ruler.
Now place the potato tissues in the test tubes and leave in test tube rack for 3 hours. After this take out the potato tissue and measure its mass, length and diameter.
Record these results.
Repeat the experiment twice more and then record an average to have reliable results.
Repeat the experiment for the other sucrose solutions.

The five different sucrose solution concentrations to be used are 0% (distilled water), 5%, 10%, 15% and 20%.

RESULTS:

On the following pages are tables displaying all the results collected for each sucrose solution. There is also a table of all the averages calculated for the volume and % change in mass.

The first graph displays the %change in mass and the second graph shows the volumes before and after the experiment.

When working out the percentage change in mass the following formula was used:

(Final mass – original mass) / original mass *100 = % change in mass

ANALYSIS AND CONLUSION:

From the graph that has been obtained a simple trend can clearly be identified. From the graph it can be seen that as the concentration of the sucrose solution is increased then the mass decreases. This means that at 0% concentration, the percentage increase in mass is 18.4% but as the concentration increases this number decreases and then at higher concentrations it turns into a negative value. This is clearly indicated on the graph by the line of best fit.

Graph 1 of the volumes of the potato cores displays two lines of best fit. It can be seen that the volumes before are all constant and the same and a straight, horizontal line of best fit represents this. The line of best fit for the volume after is a line going straight down in a diagonal direction. This shows that as the concentration is increasing the volume is decreasing steadily. This is shown clearly by the results because at 5% the volume after is 0.74cm3 and at 15% the volume after is 0.33cm3.

Graph 2 shows the percentage change in mass. It can be seen clearly that as the concentration of sucrose solution increases, the % change in mass decreases and becomes negative as the potato core shrinks. At 5% the % change in mass is 7.8% and at 15% the % change in mass is –11.6%.

The conclusion that can be reached from the results that have been gained is that as the concentration of sucrose is increased then water potential is decreased, if the water potential is decreased then more water will move out of the potato and this will therefore decrease the mass. This can be seen from the graph obtained at the concentration of 20% the mass decreases by 0.28g. When the concentration of sucrose is low, the mass of the potato core increases; this can be seen on the graph as well. When the concentration is 0% the mass increases by 0.23g, the reason for this increase in mass being that water has to move into the potato core. The water either moves out or into the potato core to bring both solutions into equilibrium. If the sucrose solution is high and the water potential is higher inside the potato core, then the water will move out the potato to balance the solutions and bring them into equilibrium. If the sucrose solution is low and the water potential is higher in the solution, then the water molecules will travel into the potato.

EVALUATION:

The techniques that have been used can be said to have been suitable for the desired task, the reason for this is that the results that have been obtained show a clear trend and also directly correlate with the prediction that was made. Also a procedure was obtained that allowed the accurate measurement of the change in mass of the potato with the concentration as the variable. The results that were obtained were reliable and consistent. Also they had a high level of reliability, as the difference between repeated readings was small. This is a clear indication that a suitable method and technique was used. The experiment was also reliable because variables were controlled such as the temperature were kept the same. Also, the apparatus was accurate and to high precision meaning that the results were reliable. The evidence for this is that the difference between repeated readings is small and also all results lie close to the line of best fit indicating that the results were reliable.

ERRORS:

From the graph obtained it can be seen that the results are spaced away from the line of best fit and so this means that some errors must have occurred during the experiment. Below is a list of some possible errors that could have incurred:

When removing the potato from the solution and when placing it into the solution, the potato core may have been squeezed too hard, therefore squeezing water out of the potato. This would definitely affect the mass of the potato and so give inaccurate results.
Other errors could have come form the top-pan balance not properly working. This means that the measuring of the mass was incorrect and error could come from this. Also, the top-pan balance is a very sensitive device, and by just leaning on the surface the balance is on, this affects the mass the top pan balance shows.
It is presumed that by using the same potato all the cores will have the same cells, however, even by using the same potato, the cores could have different cells. This would cause great errors as different cells have different water potentials. However, this error was minimized as much as possible by using the same potato.
Concentrations could have been made incorrectly and this would therefore cause large errors. The errors can significantly affect the results. The fact the results obtained are close to each other means that many errors did not occur. Errors such as those mentioned would have made the conclusion reached invalid and therefore they could not be used as preliminary work for another experiment or as a basis for another investigation.

The effect of these errors can be said to be minimal and thus resulting in a reliable experiment.

The experiment that was carried out was reliable. This is mainly because of the steps that were taken whilst carrying out the investigation. These include the use of precision instruments, which meant the results were accurate and also reliable. The repeated readings are very close together and this shows that no large errors took place because this would have been noticed like the one anomalous result was seen on the graph.

The apparatus used were to a high degree of accuracy but there are modifications, which could have been made to improve the experiment. These are all listed below:

MODIFICATIONS:

Further test can be done for higher concentrations such as 50% sucrose solution. By having at least 10 different sucrose solutions this would give a more accurate graph from which more deductions can be made.
The use of a digital balance accurate to 0.001g could be used to gain increasingly more accurate results.
The readings can be repeated more than three times to increase the accuracy.
More precise instruments could be used to get all the potato cores to be of exactly the same length before placing them in the experiment to make it a fairer test.

The modifications listed above would help in eliminating sources of error and also increase the reliability of the results. The modifications also suggest further work that can be done.