Plant cells are made up of mainly salts, sugars and proteins, which exist in the cell’s cytoplasm and the cell sap within its vacuole. These substances are therefore reducing the number of free water molecules that are inside the cell. A plant cell is surrounded by a cell wall that is made from cellulose and it is freely permeable, which means it allows water and dissolved substances through. However the cell membrane of the cytoplasm is selectively (semi) permeable, allowing smaller molecules, such as water molecules through into the cell.
This occurs when a plant cell is surrounded by a solution, which is more dilute than its contents. In effect osmosis will take place, the water molecules will pass into the cell through the semi-permeable cell membrane, from a high to a low concentration. When this movement takes place the plant cell swells up, as the vacuole in the plant cell expands and pushes outwards onto the cytoplasm and the cell wall.
Water diffuses into The vacuole pushes
the cell vacuole outwards, pressing
through the semi against the cytoplasm
permeable and the cell wall.
membrane. It is putting turgor
pressure on the cell wall.
The cell is described as being TURGID
The converse of this is when the solution outside of the plant cell is more concentrated than the cell’s content. Therefore water molecules will diffuse out of the cell, causing the cytoplasm to be pulled away from the cell wall, and the cell becomes plasmolysed. However this does not often happen in nature but when it does the cell is described as being limp and its pressure decreases. A plant cell in this state is known as being FLACCID.
Prediction
By using my background information on the subject, I will be making an educated prediction of the final outcome of the experiment that I will later be carrying out.
The experiment I will be undergoing is concentrating on the effects of osmosis in plants. I will be observing a plant organism, after being placed in different concentrates of solutions for a fixed amount of time and recording the weights before and after. The plant organism that I have chosen to use to show this will be a potato. I will be hoping for a change in the weight of the potato pieces, to prove that the movement of water molecules (osmosis) during the experiment has taken place.
From my knowledge on the subject of osmosis, I know that water molecules move from a region where they are at a higher concentration to a region where they are at a lower concentration. By using this simple theory I can adapt it to form a prediction. The concentrates I will be using in my experiment will range from 0%(pure water) - 3%, going up in 0.5% intervals. Therefore I predict that if the water concentration of the solution is high (dilute/weak NaCl solution) compared with the water concentration inside the potato, water molecules will diffuse into the potato, increasing it in its weight. Likewise if the water concentration is low (strong solution of NaCl) in the solution, water molecules will diffuse out of the potato decreasing it in weight. However the results of my experiment all depend on the water concentration inside the potato, and because I do not know the potato’s water concentration before I carry out the experiment this is the only assumption I can make.
FINAL PREDICTION –The stronger the NaCl solution is, in comparison to the solution inside the potato, the potato should lose weight.
Predicted Graph
Mass of
Potatoes (g)
0 Concentration of NaCl
Solution (%)
This graph is inversely proportional, which means as one of the axis increases the other decreases at a constant ratio between the two axes. As the concentration of the NaCl solution increases, the mass change of the potatoes decreases and they are in proportion to one another.
When the concentration of NaCl increases, this is the same as saying the water concentration is decreasing. So as the water concentration goes down, so does the change in mass of the potato pieces, which supports my background knowledge. The water molecules will become less frequent as the water concentration lowers, so water molecules from the solution inside the potato will diffuse out from a high to a low water concentration, causing it to lose weight.
Variables
Independent Variable
This is the variable in my investigation that I will have to change
- the concentration of the sodium chloride solution
Dependant Variable
This is the variable in my investigation that I will need to measure
- the weight of the potato pieces, before and after the experiment
Control Variables
These are the variables I will need to make sure I keep the same throughout all of my investigation to ensure it is a fair test.
- to use the same potato throughout both the experiments
- the size of the potato pieces, which will be according to the length I choose to use in cm and the size of the cork borer
- the length of time I decide to leave each potato piece in the NaCl solution for
- the amount of solution I use in each of the specimen tubes
Plan for Preliminary Experiment
Apparatus:
7 specimen tubes + lids a pair of forceps
7 different concentrates of solution a potato a top-pan balance a marker pen
weighing boat/s a cork bore
a ruler a scalpel a white tile a stop clock
a measuring cylinder or a graduated syringe a mounted needle
Method
- Collect all necessary equipment and apparatus.
- Using the marker pen, write the seven different concentrations on the sides of the specimen tubes in order to prevent confusion. The concentrations I have chosen to use will range from 0.0% (pure water) – 3.0% going up in 0.5% intervals.
- The solution I have chosen to use will be of a simple salt solution - NaCl
- Fill up the tubes with 20cc of the correct solutions either using a measuring cylinder or a syringe to be more precise.
0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0%
- After all of the specimen tubes are full, ensure all lids are screwed on to avoid any spilling or even slight evaporation.
- Using a cork borer cut out some long thin strips of potato, carefully pushing them out of the borer using the mounted needle making sure you are leaning on the white tile.
- When an adequate amount of pieces have been cut, measure them all equally using a ruler to 4cm. Cut seven pieces to the correct size using the scalpel, making sure no potato skin is left on the pieces themselves.
- When all of the pieces are close to identical, they must be weighed and the results recorded. Place a weighing boat onto the top-pan balance and then zero the scales before placing each separate piece into the weighing boat and reading its weight in grams.
- Before starting the experiment, the temperature of each solution can also be measured to check that they are all at more or less equal temperatures.
- After unscrewing the lids of each tube, using the forceps each potato piece must be put into a solution (making sure you have noted which piece is in which concentration of solution for the final results table).
- As quickly as possible the stopwatch then needs to be started, once all of the potato pieces are in a solution. Remember to re-screw on all of the lids, and then leave the tubes undisturbed for 15 minutes.
- When the time is up, you must remove the pieces as best as you can straight away, using the forceps, being as efficient as possible.
- They can then be placed on the white tile once again, after shaking of any excess solution.
- Once again, each piece needs to be individually weighed using the top-pan balance. The new weights must then be recorded of each piece alongside the original weight ensuring you have them against their previous reading for comparing them correctly.
- After all the results are recorded, you need to clear away any apparatus and equipment used and tidy up the area generally.
- This experiment should then be repeated to give you a second set of results, where an average of the two can be produced to give a good set of hopefully reliable and efficient results.
Results Table (main experiment)
Results for Preliminary Experiment
These results are for my preliminary experiment, and they proved not to be very good, as shown in the graph on the following page. The results are very different to what I would have expected to happen and the graph does not show any similarity between the predicted graph, which I first drew from referring to my background knowledge. The results seem to show no distinctive pattern emerging, so therefore I must repeat the experiment in order to obtain some reliable results that I am satisfied with.
However this preliminary experiment acts as good practice for when I will perform my final experiment, after I have made some significant changes to the plan to help improve my results.
Changes to the Plan
I was not pleased with my preliminary results so I have decided to make some changes to the format of my plan in order to make sure my main experiment flows more smoothly.
Firstly, I think it is important to make sure the amount of time I have left the potato pieces in solution for is substantial, as I want to ensure the process osmosis has taken place to its extent. Therefore in my main experiment I will leave the potato pieces in solution for a total of 20 minutes. In my preliminary experiment the pieces were left in solution for only 15 minutes, which at the time did not seem long enough, so I have added an extra 5 minutes onto the time.
Also, I have chosen to alter the length of my potato pieces. In my preliminary experiment, they each measured 4cm, which I found to be too long, amount of solution I used only just covered the tops of my pieces. As a result of this I have decided to shorten the pieces to measure only 3cm, which will ensure each individual piece is totally covered by the solutions. On the other hand I could have increased the amount of solution I used, but 20cc was a good amount to work with as it filled the specimen tubes nicely. Also the piece of equipment I chose to use to fill the tubes (graduated syringe) held exactly 20cc, so measuring the amounts was easier and would have been tricky otherwise.
Hopefully these two changes will help me achieve better results in the main experiment and give me more dependable figures that are simpler to notice any patterns surfacing from.
Analysis of Results
When observing my final set of results, I am able to make certain conclusions from using my original source of background knowledge on the subject. I can conclude from looking at my results table that, the average % change in mass of the potato pieces decreases steadily down the table, as the concentration of NaCl increases. The mass change of each piece of potato, either gained in weight, lost weight or stayed relatively the same. At 0% (pure water) there was an increase in mass, where water molecules had clearly diffused into the potato causing it to gain in weight. Therefore the water concentration inside of the potato was lower than that of the solution surrounding it. The mass change continued to increase from the original weight of the potato, but declined in relation to the last. At 1.0% NaCl concentration the mass change was almost non-existent and the reading was extremely close to zero. This shows me that at 1% NaCl solution both solutions – inside the potato and the NaCl solution around the potato – their water concentrations are almost equal. The concentrations of NaCl solutions after this, from 1.5% - 3.0% made the potato pieces lose weight, causing their average % change in mass to decrease as the NaCl solution became stronger. However, as the NaCl concentration became stronger, they were therefore becoming more dilute also (increasing in water concentration). This means, as the water concentration got higher outside the potato, water molecules began to diffuse out affecting the weight of the potato.
These conclusions I have made mostly support my prediction, especially my predicted graph. The graph I initially predicted was close to identical to the graph of my final results. From looking at the graph of my final results it shows results that seem to be a lot more consistent than my preliminary results. The plotted results form a line that crosses from the positive axis through to the negative axis, with a point marked close to the axis (0). What I have discovered from not only my graph, but also my results table does reinforce my prediction and my background knowledge on the subject of osmosis in plants, therefore it must be correct.
Preliminary Work
Here in my preliminary work I need to make decisions for my investigation, and specify any details to make sure it flows smoothly and is effective.
Firstly I need to decide what concentrates of solution to use. In my opinion and for the dependability of my final results, a wide range of concentrations would give better results because it is more widespread, making the changes easier to differentiate from each other. It is vital for me to use pure water as a basis, and then I have chosen to go up in 0.5% units until I reach 3.0%, giving me a total of seven different concentrations. The concentrates will be of a simple salt solution (Sodium Chloride NaCl). The amount of salt solution I use is also important as it could also affect my results, especially if I do not use enough. I should aim to fill the tubes, leaving only a small gap of about 1cm from the solution to the top of the tube. The potato pieces must also be fully covered by the solution. In my pilot experiment I chose to use 20cc of solution in each tube and I found this was an appropriate amount because it filled the specimen tubes well, without them over spilling, so I will not change the amount, I will leave it as it stands for the actual experiment.
After completing my pilot experiment, I have been able to choose the size of my potato pieces. They need to be a good size for the efficiency of osmosis taking place in the experiment, not too large or too small. In my pilot experiment I used 4cm long pieces of potato and came to realise they were too long, as the solution did not fully cover the pieces as well as I would have liked. Therefore I will alter their length slightly to 3cm in length when I come to do the real thing.
The last main decision I need to make is how long I am going to leave the potato pieces in solution for. In my pilot experiment I left the pieces in solution for only 10 minutes, which at the time I thought was not long enough. My results in my pilot experiment were good, but to ensure osmosis has entirely taken place I would have liked to leave them a bit longer. So in my final experiment I intend to leave them in for a total of 20 minutes.
Outline Plan
The experiment that I intend to carry out will be investigating osmosis in potatoes. I plan to use a variety of different concentrates of a particular solution, as osmosis is the diffusion of water particles from a high to a low concentration, so a solution is necessary to the investigation. I will then place equal pieces of potato into each of the different concentrations (one of them being pure water), which will have been poured into specimen tubes. They will then remain in the solutions for a decided amount of time and I will observe their changes in weight after the experiment, comparing them to the weights of the potato pieces before I started the experiment. This quite simplistic experiment should prove decent results, which I can hopefully obtain a satisfactory conclusion from, confirming that the process osmosis took place. Although there are areas in the experiment that need to be looked over and decisions must be finalised to ensure its reliability.
