If the liquid is exactly the same water concentration as the cells in the potato core there will be no net movement of water across the cell membrane and no change in mass.
Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The potato core will stay the same size and mass.
If the liquid has a lower concentration of water than the potato core cells (a very concentrated solution) the cells will lose water by osmosis.
Again, water crosses the cell membrane in both directions, but this time more water leaves the cells than enter it. Therefore the cell will shrink and the potato core loses mass.
The Consequences of Osmosis in a Plant Cell
Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises; eventually the internal pressure of the cell is so high that no more water can enter the cell. Turgidity is very important to plants because this is what makes the green parts of the plant "stand up" into the sunlight.
When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall.
To test my prediction I am going to carry out a small pre-test to check my method is correct and that I will achieve proper results.
Pre-test
I am going to carry out a pre-test to get information that is required for my main investigation.
Variables
What size (length) of potatoes we shall use
Volume of solution will we use
How long will we leave the core to change in mass?
What diameter of cores we will use
Method
I immersed one potato core into 20ml of 1M salt solution and one potato core into 20ml of tap water. Each core was 3cm long. I then placed the cores into either the water or the salt solution at 10:07, and the removed them at 2:47. This gave the potatoes four hours and 40 minutes in the chosen liquid.
Results
Conclusion
By looking at my results I have come to the conclusion that:
- I will use potatoes cores that are 3cm long and size 4 diameter.
- I will leave the potato cores in the chosen liquid for around hours and 40 minutes to get a good result.
- I will place the core in 20mls of liquid so the cores get completely immersed.
Variables
- My independent variable will be what strength of solution I will use for my potato cores to go in. E.G. 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 molar solution.
- My dependant variable will be the change in mass that the potato cores undergo
- My control variables will be how long the potato cores will be (3cms), the volume of the solution (20ml), and the length of time that we leave the potato cores and the diameter of the potato cores (size 4).
Apparatus
- 2 measuring cylinders (each holding at least 20mls of solution)
- Top pan balance (that must be accurate to 2 decimal places.)
- Potato cores (3cms in length, size 4 in diameter.)
- Test tubes (that must hold the potato cores and the solution)
- Knife (that must be able to cut thorough the cores.)
- White tile
- Clock
Safety
- Students must wear goggles to protect their eyes for splashes of solution
- Students must also be careful when using knife to insure no injury takes place.
Plan
This is the values of solution must use to dilute the 1.0 M salt solution down to 0.0 M (pure water):
Step 1. Take the core from the potato using a size 4 borer.
2. Cut the skin off the core then using the knife, white tile and a ruler cut the core to the correct length (3cms). A this point you must make sure the cores are perfect cylinders, you use the same borer each time and all the skin is cut off.
3. Weigh the potato cores (make sure you record the weight) then place the cores in the correct solution.
4. After the correct amount of time take the cores out of the solution, dry off any excess solution then weight them.
5. Record change in mass then calculate percentage change in mass.
6. Repeat all steps 3 times for reliability.
Results
Analysis
I found out that as the water concentration decreased (salt concentration increased) the percentage change in mass also decreased. For example at 0.8 water concentration (0.2 salt concentration) the percentage change in mass is –13.25, also at 0.2 water concentration (0.8 salt concentration) the percentage change in mass is –26.6. The reason for this trend is that during osmosis if a plant cell in this case a potato core is in a solution with a high water concentration (low salt concentration) the water will pass through the partially permeable membrane and the potato core will gain in mass.
This agrees with my prediction because in that I explained that during osmosis I thought that a plant cell would lose mass if placed in water a high salt concentration.
At looking at my graph further I have realised that there is a point where the water concentration and salt concentration is the same. On the graph this is where the line of best fit crosses the x axis (this axis shows the concentration of the salt solution) at point 0.05M. If I repeated the experiment with this concentration I will hopefully find that there would be no change in mass.
I think my results turned out like this because osmosis has taken place e.g. the potato cores have swelled and gained mass in water with a low salt concentration and vice versa (the potato cores have shrank and lost mass in water with a high salt concentration).
Evaluation
I think my investigation was good because I was able to see what happens when osmosis takes place. Although it was a good investigation my results weren’t really clear enough on my graph most of the points are in line with the curve of best fit but there are a few anomalies (these were for points 0.2M and 1.0M).
The results were clear enough to back up my predication also the graph supports my prediction by clearly showing that if a potato core is placed in a solution with a high salt concentration it will decrease in mass.
My experiment was a fair test and I did 3 repeats for each concentration so it was reliable. I carried out my experiment correctly and all results came form the same experiment so this was not the reason for my anomalies. I think that I would have improved the clarity of my results if I was able to measure the lengths of the cores more accurately and if I was able to get all the cores from the same potato. If I had used the same potato e.g. one large one rather that two smaller ones. then the results would have been more accurate because a different potato core could start off with a different salt concentration. I could have also been more accurate if I had insured that each potato core had been in the solution for the same amount of time even though I don’t think a matter of a minute or so would have really mattered.
I am confident that I did what I set out to do. E.G. prove what happens when osmosis takes place.