The water potential is the measure of whether the solution is likely to gain or lose water and by how much. So it is concluded that pure water has the highest water potential because it has the highest number of free molecules and can diffuse from one solution to the other no matter how concentrated or dilute it may be. This again repeats the fact that the water moves from a high potential to a low potential. Once again it is important that the substance that is added to the water is soluble in water.
Osmosis is also used industrially, mainly in the process of agriculture. Sometimes in industry, quite different from agriculture, osmosis may also be used. In this case the tubing would be made of cellulose acetate in sheets or tubes used for dialysis. The size of the pores can be adjusted according to the size of the molecules or the particles of the substances and used accordingly for different purposes.
In an animal cell the water is taken by osmosis. The cell swells up and can burst if the extra water is not removed. The tissue fluid does this. The tissue fluid has the same concentration as the cell and so the substances or rather water can pass freely between the cell and the tissue fluid.
As we can see form the above representation the cell has expanded and will burst if it does not lose the extra water. That is why the tissue fluid plays an important part. The solution outside is called the hypotonic solution because it has a low concentration of the solute compared to the solution in the inside, which is called the hypertonic solution. As for the solutions between the cytoplasm and the tissue fluid have the same concentration so that they can freely exchange. These solutions are called isotonic solutions.
For the plant cells, too the same thing happens. But when the above change takes place, the cells are now turgid and this property is usually used to check the end of the osmosis in experiments and the cells that have lost water by osmosis until they are flaccid, which means that they have wilted or a starting to wilt. This is important for us to know because we will be experimenting about the plant cells when we investigate osmosis.
Plant Cells
Plant Cells have vacuoles. This makes all the difference to the turgidity. When the plant cell is surrounded by dilute solution then the water will enter the vacuole and enlarge it. The vacuole will become larger and also push against the sides of the cytoplasm. This will stretch the cell. This will happen till a limit because there is the cell wall that does not allow the cell to expand too much. This means that the cell is turgid and is exerting turgor pressure on the walls of the cell. When the water content in the vacuole decreases either due to lack of water or the loss of water due to transpiration, the cells become flaccid. The vacuole becomes smaller. The cells are said to be plasmolysed. The vacuoles have pulled the cytoplasm away from the cell walls and thus allowed the cells to become smaller.
As we have already discussed the temperature is an important factor in osmosis. As the temperature increases the rate of osmosis also increase. This is because the when the temperature increases the water molecules move faster from one solution to the other. As we proceed to the experiment, we will see that what other factors are to be kept constant and what changes occur during the procedure of the experiment.
Osmosis is important in nature. Osmosis is important in every living creature because it controls the distribution of water and nutrients. Osmosis occurs only when there is a semi permeable membrane so it keeps waste products out of the cell and provides the cell with the substances required in appropriate amounts. If there were no osmosis, red blood cells would get filled with water and burst just like a balloon. This is called hemolysis. Salty water can cause the tissues to dehydrate and osmosis may prevent this. This is because it is a salty solution. Osmosis maintains the regularity of concentration in the cells of the living organisms. Even so in plants osmosis helps to maintain the balance of water and the turgidity. Without this plants would wilt and die. The same for humans. If there were too much or too little water in cells, we may also die but osmosis prevents this. In plants this process is partially responsible for drawing water up to the stems where it can be used for respiration. Then again if the water in the soil is salty or has too much fertiliser in it, it can be harmful for the plants because the solution is then hypertonic.
Thus we can see that osmosis is a very important part of nature.
The experiment will be used to investigate the effects of changing the concentration of the salt solution in which the potatoes are placed. The experiment is given below.
Aim: To find out the molarity of the potato tubers using differing concentration of salt solutions and water.
Apparatus:
- Potato
- Cork Borer
- Cutting Tile
- Round bottom flask
- Salt solution – 0.2 M, 0.4 M, 0.6 M, 1 M
- Distilled water
- Petri Dishes
- Small Knife or Blade
- Sensitive balance
- Tissues
- Test tubes
- Labels
- Forceps
The above listed apparatus is very important. The cork borer will be used to make holes into the potatoes so as to obtain the cylinders with which the experiment will be carried out. The cutting tile will be used to cut the potatoes to peel them. This will also be used to bore holes into it otherwise we might hurt ourselves. Round bottom flasks can be used to fill the solutions and then can be taken into the test tubes as the volume is required. Salt solutions of different concentrations can be taken to test osmosis and the time taken. Distilled water is part of the experiment. Petri dishes will be used to contain the potatoes and the solutions. A blade may be used to remove the potato skin or it may also be used to remove the edges to get the cylinders of the same size. Sensitive balance is required to measure the accurate mass and also get the reading that is not affected in any way. Tissues will be used to dry the potatoes to get the final reading. Test tubes will be used to add the solution to the Petri dishes.
Hypothesis
As I have explained earlier, osmosis is the movement of water from a place of high concentration to a place of low concentration. Here we will be using varying concentrations of solutions.
From the concentrations that are being used, I think that the potato cylinders placed in solution with the highest concentration will lose mass because it has a dilute solution compared to the solution in dish and the one that is placed in distilled water will become thick and gain mass.
The other concentrations will also be affected the same way because the potato cylinders have more dilute solution than them and so this will be used to check for it. The more concentrated the solution the more the loss in mass and the more flaccid the potato tubers will become.
Fairness of the experiment/ Precautions
The experiment has to be a fair test. This means that the experiment has to be correctly done without ignoring minute details that may affect it and we cannot ignore them. They are some of the precautions that have to be taken for the experiment to be used to test osmosis.
The first thing is that all the test tubes have to be cleaned out with distilled water. Each test tube has to be cleaned with each its respective solutions. For example Tube A will hold 1 M solution and so we will have to clean it out with 1M solution and not any other one as the final result may differ. Care should also be taken to see that the flask is not broken or cracked neither are the test tubes.
The potatoes that are taken have to be from the same farm or from the same plantation or the same size, preferably. This is because different potatoes have different concentrations of solutions in them and this can vary in the experiments and so we will not get accurate reading and thus there can be no conclusion.
The potato tubers have to be cut accordingly and according to that the length should be not more than 5 cm. The length has to be the same otherwise if the length were more; osmosis would take place faster because it is exposed to more surface area and the opposite if the length was less. So 5 cm has to be taken the same for all.
Measuring the volume of the solution is also important as more solution means that osmosis takes place faster and even may also affect the reading.
The temperature also affects the rate of osmosis so the temperature should be maintained and then the readings will be more accurate than if the temperature would be varying.
The Petri dishes have to be filled up till the potato cylinders have been covered so that the osmosis takes place completely. According to the size of the Petri dishes, 5 cm3 is ideal for this.
The sides of the potato cylinders may still have the peel on them so that the peel should be removed, as the peel is not a semi permeable membrane.
The balance should be checked to see that it gives the reading 0g when the Petri dishes are added. Without the Petri dish it should be approximately –8.81 g.
It is important that the solutions are poured at the same time in all the dishes so that the process takes place at the same time.
Safety
Although safety should not be a major issue, care has to be taken when using the blades to cut the potatoes and for the cylinders, the cork borer is used. For this it is important that the cork borer is used on the cutting tile or a board to avoid any bruises or cuts of any sort.
Method
Before we start the experiment we have to take care as to the fairness of the experiment and also take care according to the safety precautions while the experiment is being done.
Firstly, we have to peel the potato. We can either do this or bore a hole in the potato. We can remove the cylinder of potato and then cut the peel on the sides of the cylinder. Then we place it near a scale and measure 5 cm and cut it. This we have to repeat for all the cylinders. We have to take 25 such cylinders.
Next will be the measurement of mass. We have to weigh each of the cylinders and then write down their mass readings. The mass readings are taken down and should be repeated to check for errors. We then take the average of all of the readings of the mass of the potato cylinders that will be put together in one Petri dish, one by one using forceps so that the water may not be absorbed.
The next step is to add the solutions to the labelled Petri dishes. The solutions should be added at the same time until the potato cylinders have been covered completely. Once this is done the Petri dishes are closed or covered with their lids and then they are put in a place where the temperature is expected to remain constant through out most of the experiment at least.
The time factor counts here. It will count because there are different solutions that have varying concentrations and so the time matters. But it is not possible to check when the experiment is over so the time is taken as on day or maybe even less than that according to convenience. It should be kept in mind that the potato cylinders should not be removed and then measured in the middle because this would upset the process.
One the defined time has elapsed, the Petri dishes are opened. We then have to measure the weight after tapping the water or the solution dry from the potatoes. Then we measure each mass and then write the average mass in comparison to the previous reading. This will be done separately for each Petri dish. We then have to note down the readings and draw a graph so that we can determine the molarity of the cell sap.
Prior test
Before doing this experiment it is essential to carry out the experiment before hand to get an idea of the results and the method. Even so it would be important to learn the difference in readings and what to expect. This also can act as a way of checking accuracy of the results obtained from the actual experiment.
The prior test is done as the same way as mentioned but in this case only two Petri dishes are used, one is distilled water and one is a salt solution. The same method is carried out and the readings are taken. The are as follows before and after the experiment, clubbed together according to the Petri dishes
Petri Dish A
Solution- Distilled water
Table 1 - Length
Table 2 - Mass
For this the potato cylinders can be compared as shown in the diagram for distilled water. They are firmer and cannot be easily bent. The amount of water has decreased in the right Petri dish compared to the left one.
Petri Dish B
Solution- Salt Solution
Table 1 – Length
Table 2 – Mass
They have become more flaccid and can easily be bent. The solution on the right has more water than the solution on the left.
The final part of the actual experiment involves the finding out of what is called the isotonic point. We can define the isotonic point as the point when the molarity of the cell sap of the potato is the same as the solution surrounding it is the same so there is no osmosis going on. This is the actual aim of the experiment so it has to be found out. Below shown is the expected graph for the actual experiment.
Change in length
and mass
X
Molarity
X marks the concentration of the potato tuber. This will be the graph obtained after the actual experiment has been concluded. The point is where the two lines intersect.
Some of my reference material includes
- Biology For You
- GCSE Biology by D.G. Mackean
- Introduction to Biology by D.G. Mackean
Obtaining Evidence
Now I will list how the actual experiment was performed and what were the readings taken.
I was taken to the laboratory where I would carry out the experiment. The experiment that I did involved the potato tubers and the effect of different concentrations of salt solutions on them. This process I have already mentioned. The list of apparatus that I was to use was as follows:
- Potatoes
- Forceps
- Cork Borers
- Salt Solutions- 1 M, 0.5 M, 0.25 M
- Distilled water
- Beakers
- Blades
- Top Pan balance
- Scale
- Petri Dishes
- Cutting Tile
- Labels
- Tissues
There is a difference in the apparatus that I would be using from the original plan. The differences are given below:
Once I got the cork borers I used them to make cylinders of the potatoes. The cork borers were selected according to size of the potato. If the potato was small, smaller cork borers were used but I used all the potatoes the same size and the cork borers should have the same hole size. Once the was done, I placed the potato cylinders on top of the cutting tile and cut off the edges that were still attached to the cylinders. By removing this I ensured that there is non permeable membrane. After doing this I weighed the potato strips. The reading I have entered later. I chose a standard size, which was 5 cm for all the strips. Then after weighing each strip I placed 5 strips in each of the 5 petri dishes. There were 25 strips altogether. I did not pat the strips dry before weighing them.
The round bottom flasks were filled with the salt solutions and marked accordingly. I first poured distilled water into the beaker and check it to be roughly 30 cm3. This would be sufficient enough to cover all the strips. This I learnt from the prior test. Then this way I filled the 4 beakers with the corresponding salt solutions. Then I stuck labels one ach of the Petri dishes as to which one would be containing which solution. After I did this I made sure that the solutions in the beakers were poured at the same time. Then I shut the lid on each of the Petri dishes and then put them separately.
The next day the following readings were taken with regards to length and mass
Petri Dish A – 0.5 M Salt solution
Petri Dish B – 1 M Salt solution
Petri Dish C – 0.25 M Salt solution
Petri Dish D – Distilled water
Length
Petri Dish A
Petri Dish B
Petri Dish C
Petri Dish D
Mass
Petri Dish A
Petri Dish B
Petri Dish C
Petri Dish D
These are the results that are obtained from the experiment. The graph for the isotonic point is shown on the next page.