If the potato tissue is placed in a solution with the same water concentration of that inside the cell, water will move out of the cell at the same rate as it enters the cell. In this case the cell will stay its original shape and there will be no change in mass or size. This is called an isotonic solution. This is when the solution inside the plant cell is of equal osmotic potential to the solution surrounding the cell.
Predictions
When I put the potato into water or dilute sucrose solutions I predict water will pass into the potato cells making them turgid, resulting in the potato cylinder expanding and becoming heavier in mass
When I put the potato into more concentrated sucrose solutions I predict water will pass out off the potato cells making them plasmolysed, resulting in the potato cylinder shrinking and becoming lighter in mass
When the concentration inside the cell is the same of that in the sucrose surrounding the potato cylinder, osmosis will occur at the same rate entering the cells as the rate leaving the cell, resulting in no change of mass or size of potato cylinder. I predict this will be a medium sucrose solution, approximately 0.2M or 0.3M solution.
Key Factors
To make sure the test is carried out fair test I will need to have three variables and stick to them:
Independent variable;
I will change the molarity of the sucrose solutions. Ranging from distilled water to a 0.7M sucrose solution as follows; distilled water, 0.1M, 0.3M 0.5M and 0.7M.
Dependent variable;
I will be measuring the percentage increase in mass of the potato cylinder in each of the differently concentrated sucrose solutions. I shall find the mass before, the mass after and the percentage change in mass of each potato cylinder.
Controlled variable;
In order to keep the test fair we will need to keep many things the same including; the same diameter of the potato cylinders, the same volume of sucrose solution added to the potato tissue, the same temperature the sucrose solutions are when added to the potato cylinder and the time the potato cylinder is left in the sucrose solution for.
Apparatus
- 15 Boiling tubes
- 3 boiling tube racks
- Cork borer
- Scales
- 75ml of distilled water (0m), 75ml of 0.1M solution, 75ml of 0.3M solution, 75ml of 0.5M solution and 75ml of 0.7M solution
- One large potato
- White tile
- Measuring cylinder
- Sieve
- Calculator
- Tissue
- Scalpel
Method
- Gather apparatus needed
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Using a cork borer cut 15 separate cylinders of potato all of the same diameter. Use a scalpel to trim these to the same length SAFETY: 1) Be careful to cut potato on white tile to protect fingers from being sliced by potato borer 2) Edges of scalpel is sharp
- Add 25 ml of each of the molar solutions and distilled water into the boiling tubes. (there will be three boiling tubes for each molarity chosen, allowing for more reliable results)
- Use scales to weigh the potato cylinder and record results
- Add one potato cylinder to each of the fifteen boiling tube (make sure you record what mass the potato cylinder in each tube is)
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Leave the cylinders in the solution for over three hours SAFETY: 1) Leave in safe position to prevent them from falling of table
- Strain each of the potato cylinders separately, in turn blot drying them, weighing them and recording the results.
- Find the change in mass of the potato cylinder, record results and finally work out the percentage change.
Strategy Of Dealing With Results
I will collect and record my results in a table as shown below.
Table Showing Change In Mass Of Potato Tissue In Different Molar Solutions
To find the percentage change in mass I will use the formula
I will then present my results on a graph:
Relationship Between Change in Mass of Potato
Tissue in different Molar Sucrose Solutions
Description Of Results
My results did not all lie on a straight line so I drew a line of best fit which passed through two of my points and close to the rest of my points, except for one slightly anomalous result at 0.4M. The line of best fit slopes downwards, we can see from the graph as the concentration of the solution increases, the percentage change in mass decreases (in a 0M solution the average % increase is 16.1 and as the Molarity of the solution increases to 5M the average percentage change in mass decreases to negative 17.4). We can read from the graph that the isotonic molarity of the potato tissue is 0.27M.
Analysis of results
In water, (0Molar) the average % change in mass is greatly positive (for 0M it was 16.1% increase). It has a large concentration gradient, therefore the water has moved from the external water surrounding the potato tissue through the selectively permeable membrane into the potato tissue to increase the mass after being emerged in the sucrose solution. The potato tissue is turgid in 0M solution and as well as increasing in mass I also noticed the potato cylinder became more stiff. In the 0.1M (a dilute sucrose solution) water again moved into the potato tissue but there was a smaller concentration gradient and so the percentage change in mass was less than in distilled water - 11.9% increase.
In strong strength sucrose solution, (0.5M) the average % change in mass was greatly negative (in 0.5M solution there was a -17.4 average % decrease in mass). This is because there was a higher concentration of sucrose outside the potato tissue, resulting in water moving from inside the cell across the selectively permeable membrane into solution to even the concentration gradients. The potato tissue is plasmolysed in a 0.5M solution, I noticed as well as there being a decrease in mass at this molarity of sucrose solution that the potato cylinder also became more floppy.
In Medium strength sucrose solutions (0.2M / 0.3M) there was no large average change in mass (in 0.2M solution the average % change in mass was +4.7. In 0.3M the % change in mass was -1.9%). This implies that the isotonic solution of the potato tissue was between 0.2M and 0.3M, as the concentration gradients were similar.
The line of best fit crosses the X axis at 0.27M showing the isotonic molarity. At this point the concentration gradient is equal and water is passing in to the potato tissue at the same rate asa it is leaving.
My results supported my predictions. I predicted the stronger the external solution the potato cylinders are placed in the lower the final mass of the potato tissue. Here are some of my results that prove my predictions correct.
Prediction for Weak Sucrose Solution:
When I put the potato into water or dilute sucrose solutions I predict water will pass into the potato cells making them turgid, resulting in the potato cylinder expanding and becoming heavier in mass
Result for Weak Sucrose Solution:
Prediction for Stronger Sucrose Solution:
When I put the potato into more concentrated sucrose solutions I predict water will pass out off the potato cells making them plasmolysed, resulting in the potato cylinder shrinking and becoming lighter in mass
Result For Stronger Sucrose Solution:
I predicted that the isotonic molarity of the solution would be between 0.2M and 0.3M my results backed up my prediction as the isotonic molarity of the solution was 2.7M. The isotonic molarity is when there is the same amount of water enters the potato cylinder as leaves it (the concentration of the solution inside the potato tissue is the same as the solution surrounding it)
During the investigation I came across one slightly anomalous result. From the graph 0.4M looks to be lower % change in mass than expected. I would plan to repeat this molarity of sucrose solution again and see if the new results lay on my line of bestfit. However if I had drawn on a different angle this point would have been close, suggesting other molarities were less reliable. If I had used class results (i.e 20 sets, as the whole class carried out the same method, my results may have been more reliable).
The method we used to show osmosis was good, as we used the percentage change in mass which allowed us to measure to two decimal places. This is more accurate than using change in length as a measure. But the following may have been sources of inaccuracy.
- When blot drying the potato tissues some potato tissues could have been dryed more than others, to try to eliminate this to the best of my ability I rolled the potato tissue gently along a piece of tissue exerting as little as possible force
- Inaccurate cutting(not same length / surface area), to eliminate this I used the average percentage change in mass of the potato tissue
- In accurate molarity of sucrose solutions
There were a few possible sources of error in my investigation. One main error that occurred would have been not drying each potato cylinder to the same extent each time, as pushing on the cylinder would have take out more water.
If I had the opportunity to repeat the experiment I would have improved it by carrying out a fourth or fifth investigation, and for each experiment have investigated a wider range of external solutions (e.g. 0.15M) having more points of the graph would have allowed me to have a more accurate line of best fit and therefore would help me to find the isotonic molarity. I also could have cut the cylinders into small pieces which would increase their surface area, giving a better opportunity for different concentrations of sucrose solutions to work on the potato strips. To make sure my results where fair a compared them with the class result.