Investigation Into the Osmotic Potential of Potato Cells.
INVESTIGATION INTO THE OSMOTIC POTENTIAL OF POTATO CELLS.
Aim:
To find the exact concentration at which there is no net osmosis in potato cells.
Relevant Information:
Osmosis - the diffusion of water molecules through a partially permeable cell membrane down a water potential gradient.
Osmosis is the main biological idea that will be looked at in this investigation, both how it travels in and out of cells, and the effect this has on the cells. Osmosis occurs mainly when there are two areas containing water molecules at different concentration, it moves from areas of high water molecule concentration to areas of low concentration. It has to occur through a partially permeable membrane, such as a cell membrane, as it allows small molecules through, like water molecules, but not larger ones such a glucose molecules.
In the diagram above the direction in which there is most water flow is from left to right. This means that the net diffusion is in this direction. The reason that it is in this direction is that there are a larger amount of free water molecules on the left-hand side than there are on the right, as the water molecules on the right are taken up by sugar molecules. This is also the theory behind the varying amounts of osmosis in different strength solute solutions. The stronger the solution the more concentrated are the numbers of sugar molecules there are and this means there are less free water molecules, this results in a large amount of osmosis from the area with high concentration of free water molecules to an area with a low concentration of free water molecules. The opposite occurs when the potato cells are placed in a very low concentrate sugar solution. This means that the water potential gradient on the left-hand side is greater than the one on the right and therefore the water molecules pass through the semi-permeable membrane until the water potential gradients are nearly equal.
There are many states that a plant cell can be in when introduced to solutions. They have very strong cell walls, which can swell a great deal without breaking. When plant cells are placed in pure water, they take up a large amount of water because of the difference in the water potential gradient. They swell and enter a 'turgid' state. This is when the cells fill up with water and become hard and increase in size. Unlike animal cells, plant cells will not burst, even when put in pure water.
The opposite a cell being turgid, ...
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There are many states that a plant cell can be in when introduced to solutions. They have very strong cell walls, which can swell a great deal without breaking. When plant cells are placed in pure water, they take up a large amount of water because of the difference in the water potential gradient. They swell and enter a 'turgid' state. This is when the cells fill up with water and become hard and increase in size. Unlike animal cells, plant cells will not burst, even when put in pure water.
The opposite a cell being turgid, is being plasmolysed. This is when the concentration of the solution that the plant cell is in is so strong that almost all the water leaves the cell and enters the surroundings.1 As the water begins to leave the cell the contents, such as the cytoplasm and the vacuole, of the potato cell shrinks and becomes 'flaccid', this is when the potato cell becomes very droopy and soft. As more water departs the cell contents pull away from the cell wall and becomes fully plasmolysed.
The cells becoming turgid or plasmolysed has a large effect on the size and weight of the potato. This is because all of the cells that make up the piece of potato are gaining or losing weight/size, and as an overall effect the potato is dramatically larger. These are the type of changes that we will be monitoring and measuring during this experiment. It is also possible to measure the droopiness of a cylinder of potato. This can be done by measuring the angle of droop before the potato is put in a solution and then after and comparing the change in angle.
Variables:
There are many factors that have to be controlled during this experiment. These are:
* Amount of Potato,
* Quantity of Concentration,
* Time in Solution,
* Type of Potato,
* Surface Area of Potato,
The independent variable, the factor that will be changed, will be the concentration of the solution that the potato will be placed in.
The dependent variable, the factor that will change in response to the independent variable, will be the change in weight of the potato discs.
Preliminary Work:
A small-scale experiment was carried out so that it was possible for us to see the methods that could be used so that it could be decided which method we wanted to use. Both length and weight were tested to see which gave most accurate answers. In this experiment only three solutions were used so that a vague idea of the results in the actual experiment could be gained. This was so that the range of solutions that is to be used in the actual experiment could also be chosen.
Preliminary Test Results:
Measurement
Solution
Starting
Final
Percentage
Method
Strength
Weight/Length
Weight/Length
Change
Weighing
m
2.12g
.7g
-20%
0.5m
2.09g
.78g
-14.09%
Dist. Water
2.00g
2.28g
4%
Length
m
4cm
3.5cm
-12.50%
0.5m
4cm
3.7cm
-7.50%
Dist. Water
4cm
4.2cm
5%
The graph shows that the sugar concentration at which there will be no net osmosis will occur towards the weaker concentrations of the solution. This is why it is decided that during the actual experiment solutions near the 1 molar level will not be used, and instead solutions with concentrations at 0.1m intervals up to 0.5molar will be used.
Looking at these results it was decided that change in mass would be the best way to measure the amount of osmosis that takes place as it gives the results to a greater degree of accuracy. This is because the scales give the weight to two decimal places. The method of using change in mass was not used as the human eye can only be accurate to roughly half a millimetre, and although half millimetre will not drastically alter the percentage change, the most accurate results possible are wanted so that it can be claimed that reliable results have been gained.
Prediction:
I think that the potato cells that are in the distilled water will gain the greatest mass. This is because the distilled water has a higher water potential gradient than the potato cell, and therefore water will diffuse into the cells and they will become turgid.
At 0.1molar solution concentration this effect will be slightly less dramatic, but still there will be a substantial gain in mass.
The gain in mass will be less dramatic at 0.2 molar as the water potential gradient will be less steep, but I still predict a small gain in mass.
By 0.3 molar I expect there to be a loss in mass by the potato cell, but only a small one.
At 0.4 molar I expect there to be a greater loss in mass, at this stage I think that the cells will be 'flaccid'.
By 0.5 the cell will have lost most of its water and there will be a large percentage loss in mass.
Finally, at 0.6 the cells will be full plasmolysed and there will be the greatest percentage loss of mass in the experiment.
I believe that the point at which there is no net osmosis will be just above 0.2 molar.
Method:
The apparatus will be set as shown in the diagram below.
Using a cork borer, six 4cm cylinders will be cut out of the potato.
Twenty 2mm discs will be cut from each of the 4cm cylinders.
The discs will be dabbed dry and weighed, these will be recorded and marked on the test tube.
The eight sets of discs will be placed into 10ml of 6 varying solutions of concentration. These will be distilled water, 0.1m, 0.2m, 0.3m, 0,4m and 0.5m.
This will be done three times. 3 x 6 = 18
The next lesson the discs will be removed using a tea strainer, dabbed dry and the re-weighed.
These weights will then be recorded on the results table.
The apparatus will be cleaned and cleared away.
The percentage change will be worked out. This is done with the following formula:
Percentage change = Change in mass x 100
Original Mass
Apparatus Diagram:
Risk Assessment:
Although this seems like a very safe experiment there are a few precautions that have to be taken. Knives will be used to cut the potato into the correct shaped pieces. Then a cork borer will be used, which has a sharp end used to cut into the potato. Finally small, but very sharp blades will be used to slice the potato into the required discs.
References:
Edited section from http://gened.emc.maricopa.edu
General Information:
Biology; Jones & Jones,
www.biology4all.com
Lucien Waran
28/04/2007 - 1 - Biology Coursework
