Results:
There were a lot of factors that I learnt from my preliminary work that I needed to improve on for my final experiment. The first thing that I decided to change with the help of my preliminary work was the range of concentrations.
The range needs to have a variety of suitable concentrations that will cause the potato core to become longer, shorter and to stay roughly the same length in order for us to be able to determine dynamic equilibrium. The range that I decided on is the following range:
0%, 5%, 10%, 15% and 20%
I have chosen this range as I feel that it will provide me with a clear set of results.
Accuracy: It is important for the experiment to be accurate because this will make it a fair test.
- I have chosen to use a ruler with an mm scale because this will give me a more accurate measurement.
- I have decided to start with a length of 80mm because this will allow for a smaller percentage error. This is because if the potato core is short then there will be a high percentage error where as if the potato core is long there will be a low percentage error. Percentage error is the actual error in relation to the length therefore to work out the percentage error I shall use the following equation:
Percentage error = (actual error/length)x100
- To keep the surface area of the potato core the same it is important to cut the potato core at 90 degrees. Using a potato corer will also keep the surface area the same.
- We must also use accurate equipment when measuring out the solutions, for example as we are using 25 cm3 it would be inaccurate to use a 200cm3 measuring beaker and more accurate to use a 50cm3 measuring cylinder.
- We must be sure that the volume of the solution is the same for every potato core. To do this we must measure the volume from the bottom of the meniscus at eye level.
Diagram of Experiment:
Safety:
- tie long hair back
- wear lab coats
- wear safety specs
- be careful of finger when cutting the potato with the scalpel
- if any glass is broken report it to the teacher immediately
After studying the preliminary method I can see that it needs some changes and so I have provided a more detailed and accurate method.
Apparatus:
- 5 test tubes
- A tile
- Forceps
- Scalpel
- 5 syringes
- A 25cm3 measuring cylinder
- 5 different sucrose concentrations (0%, 5%, 10%, 15%, 20%)
- A test tube rack
- A millimetre ruler
- 1 potato
- A potato corer
Method:
- Firstly we have to measure out our concentrations. Our stock solution is 20% sucrose and so our maximum solution can be 20%. We shall make a maximum of 100cm3 of each solution.
- To make the first concentration (0% sucrose) we shall use distilled water as there is no sucrose present in this solution.
- For the rest of the solutions the table below will explain which amounts to use for which concentration.
- When adding the stock sucrose solution and the water make sure you mix them thoroughly
- we will core five cores from the potato. This is to ensure that the source of the potato cores is the same to ensure the accuracy and reliability of the experiment.
- we shall then measure each of the 5 potato cores individually and cut each to 80mm ensuring that the potato cores are cut at 90 degrees.
- next we will measure out 25cm3 of the first solution (0%) measuring from the cotton of the meniscus into the measuring cylinder and then pour it into the test tube.
- Measure out 25cm3 of the remaining solutions, 5%, 10%, 15%, and 20% and label them according to the solution, label the syringes used for each concentration so there is no contamination.
- Next put on potato core in each test tube.
- Leave to stand for atleast 2 hours.
- After a suitable amount of time, take each potato core out of each test tube one at a time. When you have taken the first on out measure it and record the results.
- Do this with each of the potato cores.
- Repeat the whole experiment from number 5.
- However if you are doing this experiment in a short space of time do the two repeats at the same time as the original experiment.
Prediction:
Diagram for the prediction of the graph.
Potatoes are made of cells, these plant cells have a fully permeable cell wall and the cell membrane is partially permeable. A partially permeable membrane only allows small molecules to pass through and does not allow large molecules to pass through.
The reason that the cells have a fully permeable cell wall is that when the cell becomes turgid the cell wall prevents the cell from rupturing. This means that the solution can pass through the cell wall but once it reaches the cell membrane only the water can pass through. The solute is too big to fit through the cell membrane; this is why I have chosen to use a sucrose solution as the sucrose can’t fit through the cell membrane, this means that we are actually changing the water concentration inside the cell. Therefore when the potato core is placed in the solution the water will pass down the concentration gradient by osmosis.
Osmosis is the passage of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane. I predict that when the potato cores are placed in solutions with high sucrose concentrations they will shorten in length. I think this will happen because the solution outside the cell will be more concentrated than the solution inside the cell. Therefore water will pass out of the potato cells and the net flow will travel down the concentration gradient by osmosis. This will cause the potato cells to become flaccid which is when water has passed out of cell and it has become limp, and there will be a net loss of water molecules. In my preliminary experiment the cells which were shorter in length were bendy and this supports my prediction. I predict that when the potato cores are placed in solutions with low sucrose concentrations they will get longer. I think this will happen because the solution inside the cell will be more concentrated than the solution outside the cell. Therefore water will pass into the potato cells and the net flow will travel down the concentration gradient by osmosis. This will cause the potato cells to become turgid which is when a cell has swollen to its full capacity, and there will be a net gain of water molecules. In my preliminary experiment the cells which were longer were stiff and this supports my prediction.
Diagrams:
These diagrams show what a concentrated and dilute solution look like.
Diagrams of turgid and flaccid cells
These diagrams show what the cells will look like when they are both turgid and flaccid. These can help explain the reason why, when the cells have been taking in water and are turgid the cores are firm. It also helps explain why, when the cells have been losing water and is flaccid, the cores are limp. If a massive amount of water is lost from the cells then the cells become plasmolysed which means that the cytoplasm starts to peel away from the cell wall.
Reference:
Biology For You
