An Example of Osmosis in a Root Hair Cell
When a cell is placed in distilled water (high water concentration) water will move across the semi- permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. The cell is now referred to as turgid. If a potato cell was placed in the same circumstances the cells would increase in length, volume and mass because of the effects of osmosis. If the same potato cells were placed in a solution with a low water concentration, then the effects would be the opposite – water would move out of the cell into the area of lower concentration, the water. Thus, the potato chip will decrease in length, volume and mass. It is then known as flaccid. In more extreme cases, the cell membrane would break away from the cell wall and the cell is then referred to as plasmolysed.
Osmosis in Plant Cells
The greater the concentration of water in the external solution, the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution, the greater the amount of water that leaves the cell. In spite of this, there will be a point where the concentrations of water inside and outside the potato cells are equal. This is called the isotonic point, and at this point there will be no change in the length, volume or mass of the potato, as the net movement of water will be zero (no osmosis has occurred).
Prediction Graph
Method Plan (for Pilot Experiment)
- Start the experiment by preparing a range of sucrose solutions with concentrations of 0 molar, 0.1 molar, 0.2 molar, 0.3 molar, 0.4 molar. This will be achieved by adding the varying amounts of sucrose solution to the distilled water, to make a total solution of 10ml.
- Then, cut sections of potato using the potato borer. Using a ruler and scalpel, measure out equal sections of potato of 3cm in length. This preparation must be done very accurately as a change in surface area will allow less or more osmosis to occur.
- Weigh each potato chip and record its mass using the scales.
- Then, place one chip in each different test tube of solution.
- The potato pieces will be then placed in the different test tubes and left for 24 hours.
- After this amount of time the potato pieces will be removed from the test tubes and surface solution will be removed using the blotting paper. The potato chips will now be re-weighed and their weights recorded.
Pilot Results
Graph of Pilot Results
From my pilot results, I can see that the results are inconclusive. The first three experiments (concentrations of sucrose solution between 0 M and 0.2 M) seem to follow the trend that I identified in my prediction, but the rest of the results are inconsistent, and it is obvious that some mistakes were made in the experiment. The figures do not follow my predicted graph. From this, I can see that my method must be changed and revised in order to obtain correct and accurate results.
Equipment List
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1 M sucrose solution – to add in varying amounts to the distilled water to achieve correct strength of solution.
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Distilled water – to dilute the sucrose solution to the correct molar.
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3 potatoes of similar size and weight – to make the potato tubes from.
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3 Test tube racks – to hold the test tubes.
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15 test tubes – to hold the different solutions.
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15 Labels – to help remember which test tube contains which solution.
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1 Measuring Tube – to measure the correct amount of solution.
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2 Pipettes – for more accurate measuring of the solution.
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1 Scalpel – to cut the potato tubes with.
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1 Potato Borer – for making the potato tubes.
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1 Ruler – to measure the length of the potato tubes.
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1 Stopwatch – to measure the length of time the potato tubes were left in their solutions for.
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1 Set of Scales – to measure mass of potato tubes before and after being put in the solutions.
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Sheets of blotting paper – to blot off any excess solution after removing the potato tubes from their solutions.
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1 roll of cling film – to put on top of the test tubes to prevent the solutions from evaporating.
Changes in Method
Due to the questionable results in the pilot experiment, the following method was adapted and corrected. The subsequent changes were made:
- The total amount of solution was increased from 10 ml to 25 ml, in order to have enough solution to properly cover each potato chip.
- More tests were added, so that three test tubes of each concentration of sucrose solution were tested instead of just one. This way, an average result can be obtained for each concentration.
- Cling film was placed over the top of each test tube before being left, in order to prevent any solution evaporating.
- The total amount of time that the potato tubes were left in the solutions was decreased from 24 hours to 2 hours. This would give us a better and more accurate set of results.
- More accurate scales were used.
Final Method
- The experiment started with a range of sucrose solutions with concentrations of 0 molar, 0.1 molar, 0.2 molar, 0.3 molar and 0.4 molar being prepared. This was achieved by using the measuring tube and pipettes, and by adding the varying amounts of sucrose solution to the distilled water. To achieve the concentrations for a 25 ml solution:
Each solution was divided between 3 test tubes (a total of 15 test tubes) and labelled carefully,
- Using the potato borer, tubes of potato were made. Using a ruler and scalpel, equal sections of potato of 3cm in length were measured out. This preparation was done with great care, as a change in surface area would allow less or more osmosis to occur.
- Each potato chip was weighed its mass recorded using the scales, so that a change in mass could be calculated.
- Then, one chip was placed in each different test tube of solution so that an average could be obtained for each solution.
- The potato pieces were then placed in the different test tubes and cling film was placed over the top of each one in order to keep the solution from evaporating. They were left for 2 hours, in a place kept out of sunlight and at room temperature.
- After this amount of time the potato pieces were removed from the test tubes and surface solution was removed using the blotting paper. The potato chips were then re-weighed and their weights were recorded, so that a change in mass could be calculated.
Results
Initial Results Table
Average Results Table
Rough Graph of Average Results
Computerised Graph of Average Results
Analysis
In order to make a line graph of results, I calculated the averages of each concentration and recorded the average change in mass. I used these results to make the graph above. My best-fit line (shown in green) shows that there was one particularly anomalous result, the concentration of 0.4 (circled in red).
My graph shows that between the concentrations 0M and 0.1M the average change in mass decreases. From the concentrations 0.1M to 0.3M it continues to decrease slowly. Between the concentrations 0.3M to 0.4 the average change in mass decreases more dramatically, but this could simply be due to the fact that this last result might be anomalous.
Osmosis is the process in which water moves in and out of cells. In osmosis, water moves from high to low concentrations. Osmosis moves from higher water potential to lower water potential. The higher the concentration of sucrose, the lower the water potential. For osmosis to occur, you need a semi-permeable membrane, and 2 solutions, one either side of the membrane. An example of a semi-permeable membrane is visking tubing (used in kidney dialysis machines). Its tiny holes allow water to move across it, but nothing larger can move through it.
In my experiment, the water molecules are attracted to the sucrose molecules. This slows down the overall movement of the water molecules in the sucrose solution; therefore there is more kinetic energy in the water than in the sucrose solution. The net movement is the water molecules travelling to the sucrose solution.
As in my prediction, I know that inside the potato cell is a number of dissolved solutes and ions in the cell sap and cytoplasm, because potato cells are a type of plant cell. If you place a potato cell into a solution that is more dilute than it’s contents, water will move into a cell. The cell swells, the cytoplasm pushes on the cell wall, and a pressure is created. The cell is now known as turgid, and its mass increases. If you place a plant cell into a solution that is less dilute than it’s contents, water will move out of the cell. The cell shrivels, and is known as flaccid. Its mass decreases.
In osmosis, there is a point where the concentration is the same both in the solution and inside the potato cell (the net movement of water is 0) – this is known as the isotomic point. From my graphs and my line of best fit. I can predict that this point could be reached at about 0.42 M.
From this information and the my results collected, I can see that my prediction was correct - the greater the concentration of water in the external solution, the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution, the greater the amount of water that leaves the cell.
Evaluation
Due to the fact that there were only slight anomalies in my experiment, I can draw the conclusion that my results were relatively accurate. There are not any large differences in the range of results, and therefore the results must be relatively precise.
The slight anomaly in the concentration of 0.4 M may have been caused by a number of factors:
- The range in the size of potato chips in the three test tubes may have been very large therefore making the average seem higher or lower than it was.
- The 0.4 M sucrose solution may have been made up incorrectly, and the concentration may have been higher or lower than it was labelled.
- The reading on the scales may have been read incorrectly.
To prevent such anomalies occurring in the future, I can improve the accuracy of my experiment. This can be done in the following ways:
- The range in the size of the potato chips should be decreased. This should be done by more accurately measuring each potato tube. Using a more accurate way of cutting the potato pieces, other than just a ruler and scalpel, could also do it, as this method was relatively imprecise.
- The accuracy of each concentration should be improved. Being more careful when mixing up the sucrose solutions, and using the measuring equipment such as pipettes and measuring tubes more accurately should do this.
- The reading on the scales should be read more accurately. This could be achieved by making sure that no other interferences (such as standing to close to the scales) impair the results.
- The scalpel could be sharper.
Other experiments that could be carried out involving potatoes could be:
- Using higher concentrations of sucrose to put the potato tubes in. This could expand the range of results, and see how much higher sucrose concentrations affect the potato cells.
- Using different types of potatoes. This would allow me to determine if different types of potato have a higher or lower water potential than others.
- Using different vegetables. This would allow me to determine if different types of plant tissue have a higher or lower water potential than others.
- Varying the temperatures of each solution. This would allow me to observe how varying temperatures affect the rate of osmosis in plant cells.
Despite the slightly anomalous results in my experiment, I think that it was a fairly reliable experiment and my results were relatively precise and accurate.