Diagram
Cork
Potato Cylinder
(0 Molar Solution)
Mc McCarthy Bottle
I have drawn a diagram of what the experiment will look like seen above. I have been given the best equipment to carry out this investigation thoroughly. Within each of the Mc Carthy bottles contains the different molar solutions with the potato cylinder as you are able to see above. I will be using the concentrations of 0 Molar, 0.5 Molar, 1 Molar, 1.5 Molar and 2 Molar.
TIME SCALE
The experiment will take place over the time-span of twenty-four hours. After twenty-four hours the results will be recorded and the apparatus would be washed and cleared.
FAIR TEST
To make this experiment a fair test :
- The potato cylinders should be the same size. This is because if one potato sample is 2cm long and one is 4cm long then the 4cm long sample will have a larger surface area and the rate of osmosis would be much quicker.
- Use the same potato. This is because many factors of the potato may affect the experiment. For example the age and sizes might be different, which means one potato might have more water inside than another.
- The temperature may affect the reliability of the experiment for example at extreme temperatures the cells of the potato may die and at less extreme temperatures the experiment may be speeded up. To keep this from happening, all the test tubes will be kept in the same place and at the same time of the 24 – hour experiment.
- To make the experiment as accurate as possible an average will be taken out of the 3 sets of results taken. Therefore the experiment will be repeated twice., to gain an average.
- As each test tube is filled up with the different molar solutions the syringe which would measure the amount of solution placed in the test tube may become contaminated with different molarities. To stop this from happening, the beaker and syringe must be washed every time they are used.
- Accurate amount of salt solution: More salt solution may affect the rate of solution. To make the amount of solution placed in the test tube as accurate as possible a syringe will be used to measure out the exact amount needed.
- The light intensity will be kept the same by doing all the experiments in the same room, with the same light intensity, (no extra light will be shone on the experiments, nor will there be any experiments kept in the dark).
- The potato cylinders will be left for the same amount of time – 24 hours.
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The volume of the water or the salt solution is the same – 20cm3.
Method
I plan to carry out this experiment by using all the safety issues and fair testing procedures to give me the most reliable and most accurate set of results. I will have a range of salt solutions prepared with concentrations from 0 Molar (Pure Water) to 2 Molar, in intervals of 0.5. As I have all of these in place, I shall do the following:
- Using a cork borer I shall cut out potato tissue from the potato itself, this has to be done several times as fifteen potato cylinders are required so the experiment can proceed.
- Using a scalpel and a ruler the potato pieces have to be cut, so they are 2cm long. Here I have to be careful when cutting the potato so I don’t cut myself.
- After I have cut the pieces the potato cylinders have to be weighed one by one and the results have to be recorded.
- The salt solutions have to be made and entered in the Mc Carthy bottles.
- The potato cylinders are inserted inside the Mc Carthy bottles.
- I will then record which bottles have what solutions and the weight of the potato cylinders, also with a marker pen I shall write on the test tubes which potato has which concentration.
- I will then put corks on top of the Mc Carthy bottles so the potato and solutions don’t spill out.
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Then the Mc Carthy bottles will be left for 24 hours
- After 24 hours I shall take out the potato cylinders and weigh them.
- I shall then record the results and see if the mass of the cylinders has increased or decreased.
Osmosis
Osmosis is the process of water movement from a region of low concentration to a region of high concentration across a semi permeable membrane, which is simply a thin membrane allowing the passage of small molecules. Ex: water molecules.
The solutions of high concentration and low concentration are generally referred to as hypertonic and hypotonic solutions respectively. Hypertonic solutions have less water molecules and thus are said to have lower water potential than hypotonic solutions; in hypertonic solutions, solute molecules (ex: sugar, salts) are dissolved and when this occurs, some water molecules form a cluster around them. Thus, there are less free moving water molecules. Therefore, water potential is simply the measure of whether the solution is likely to gain or lose water; it is also the pressure exerted by the freely moving water molecules. It is measured in kilopascals. (kPa) Therefore, osmosis may also be defined as the water movement from a region of high water potential to a region of low water potential across a semi permeable membrane.
Osmosis is a very important biological process as it involves the transfer of water in and out of cells. Animal and plant cells both have the presence of the jelly-like substance known as the cytoplasm, made up of about 90 percent water with dissolved sugars and salts. This forms a weak solution. Plant cells also have a large space known as the vacuole containing a weak solution of sugars, salts and water known as the cell sap. If the concentration of the solution is more concentrated in the cell than its external solution, then water moves into the cell. This is particularly important in plant cells, as they will become turgid and strong. If too much water enters animal cells, they will burst and this is damaging. However, plant cells can limit the amount of water that enters the cell due to the cell wall; when much water enters the cell, the cell membrane and vacuole push against the cell wall, applying ‘turgor pressure’ causing the cell to become firm and turgid. Since they are pushed against the cell wall, water is prevented from entering. This is known as the pressure potential and it is measured in kilopascals. (kPa) In general, the water potential in plant cells depends on the pressure potential and solute potential.
When placed in a dilute solution, water moves out of the cell and this causes wilting in plant cells as most of the cells become squashy and flaccid. They become plasmolyzed. However, if a cell is placed in an external solution of same concentration, there is no movement of water as there is no different in concentration known as the concentration gradient. These solutions are
isotonic solutions and there is no net flow of water molecules even though the water molecules continue to move in both directions.
What will happen in the experiment
If the potato cells are placed in distilled water(where there is a higher water concentration) water will move into the cell (where there is a low concentration of water) across the semi permeable cell membrane by osmosis making the cells swell and osmosise – being in a state referred to as turgid. As the stream of water enters the cell and causes it to swell and become turgid, the cellulose cell wall stretches slightly and when it can’t stretch any further it becomes firm. Now the fully stretched cell wall exerts a restraining inward force called wall pressure on the cell contents which is balanced by an equal but oppositely directed force called turgor pressure in the cell contents. This prevents the cell from bursting. The term osmosise means gaining water by osmosis.
In contrast if the potato cells are placed in a solution with a low water concentration e.g. a concentrated salt solution, water will move out of the cell (where there is a higher concentration of water) into the solution (where there is a lower concentration of water making the cells shrink or exosmosise – being in a state referred to as flaccid. The term exosmosises means losing water by osmosis.
If the concentrations of water inside and outside the potato cells are equal or almost equal, no osmosis will occur as the net movement of water molecules will be close to zero. Therefore, there will be no change in the mass, length and volume of the potato cells as the cells will be in a state between flaccidity and turgidity. This is called incipient plasmolysis.
Prediction and Justification of Prediction
First of all I predict that when the potato cylinders and put into pure water ( 0 Molar) the potato cylinder will increase in mass. When the potato cylinders are put in water, the pure water acts as the external solution. In this case, it is the hypotonic solution as it has more freely moving water molecules, higher water potential. And since osmosis is where water moves from a region of low concentration (high water potential) to a region of high concentration (low water potential), the pure water will move into the potato cells, causing an increase in mass.
I also believe that when the potato cylinders are put in higher the concentration of water in the solution, i.e. lower in concentration of salt in salt solution; the change in mass would be greater. However the change in mass would depends on the cell sap of the potato cells. The molarity of the cell sap in the potato cells is fixed for a single potato. It has a very weak concentration and so when it is placed in a higher concentration of salt solution of, for example, 1.5 M the potato cell sap acts as the hypotonic solution. This is because since the salt solution has a higher concentration, this means it has lower water potential as there are a greater number of solute molecules and so there is less number of freely moving water molecules, as they form clusters around the solute therefore it would increase in mass.
Also the lower the concentration of water in the solution the more the potato cells will shrink and decrease in mass and length volume as water molecules will leave them by osmosis – from the high concentration (inside the potato) into the low concentration.
Factors Affecting Osmosis (ADVANCED BIOLOGY – Peter Sang)
There are certain factors affecting the process of osmosis. These factors are as follows:
- Temperature
- Concentration gradient
- Size of the molecules
- Permeability of the membrane
I will now explain the effects of each of these factors on the process of osmosis in brief.
Temperature: Osmosis is merely a process where the water molecules from the dilute solution diffuse past a semi permeable membrane to a concentrated solution. By increasing the temperature, this rate of diffusion can be increased and thus, the process of osmosis is faster. This is because, by increasing the temperature, we increase the kinetic energy of the water molecules and thus they diffuse across the semi permeable membrane at a greater speed.
Concentration Gradient: The concentration gradient is actually the difference in concentration between the two solutions, separated by the semi permeable membrane. If the concentrated solution is altered so that it has a greater concentration, it will now require more water movement from the dilute solution. Thus, more water molecules must diffuse across the semi permeable membrane. As a result, the process of osmosis is increased as the water molecules diffuse across the membrane faster in order for equalization to occur. When the concentration difference is greater, we say the concentration gradient is steeper.
Size of the Molecules: When a molecule is heavier, then it will obviously take a greater deal of time to diffuse across the semi permeable membrane as the molecules speed is decreased. This is also why the solute molecules from a hypertonic solution do not move into a hypotonic solution. In cells, the cell membrane is semi permeable. However, in experiments, the membrane may longer act as a sieve and the solute molecules can diffuse across the membrane. However, since the solute molecules attract water molecules,
which form clusters around them, this causes the solute molecules to become heavier. Thus, they diffuse more slowly than the free water molecules in the hypotonic solution.
Permeability of the Membrane: In osmosis, the water molecules will move across the semi permeable membrane. This membrane in cells is generally the same size. However, plant cells have the presence of the cell wall, which contributes to the thickness of the cell membrane. Therefore, since the membrane is thicker, the rate of diffusion is greater as the water molecules must move over a greater distance. As a result, the rate of osmosis is decreased.
Detailed Info on how to prepare the solutions required
In this experiment I will increase the concentration by .5 moles, until it reaches 2 molar.
To get the correct concentrations, you need the correct amounts of salt or pre-made 2 molar salt solution. The calculations for both are shown below.
Pre-made 2 Molar Salt Solution
This substance already has the required amount of salt dissolved into the water to make it qualify as a 2 molar solution. To get lower molar values, the solution must have a certain amount of water added to it. These values are as follows:
To get: -2.0 moles in 20cm3 of water using the 2m solution, add no water
-1.5 moles in 20cm3 of water using the 2m solution, add 5cm3 of water to 15cm3 salt solution
-1.0 moles in 20cm3 of water using the 2m solution, add 103 of water to 10m3 salt solution
-0.5 moles in 20cm3 of water using the 2m solution, add 15cm3 of water to 5cm3 salt solution
-0 moles in 20cm3 of water using the 2m solution, add no salt solution to the 20cm3 of water
Salt
1 mole in 1 litre requires 58.5g of salt to be dissolved in it. Therefore as 1 litre is 1000 cm3, I was thus able to take these into account and do my measurements.
0 Moles = Pure Water
0.5 moles in 20 cm3 of water = 58.5/ 125 = 0.468g of salt
1 moles in 20 cm3 of water = 58.5/100 = 0.585g of salt
1.5 moles in 20 cm3 of water = 58.5/75 = 0.78g of salt
2 moles in 20 cm3 of water = 58.5/50 = 1.17g of salt
variables
In this experiment I shall vary the concentration of the salt concentrations, as this will me help me find out how different salt solutions affects the osmotic activity in a potato tissue. For the non-variables it will be kept the same, the experiment will be carried out in room temperature, the duration of the experiment will be the same for all of the potato cylinders involved (24 hrs)
Obtaining Evidence
Safety
Safety is an important aspect in every experiment, even if the experiment seems to be very harmless. This is why I'll be taking this into consideration.
- I should be very careful when cutting the potato with a cork a borer as it is very sharp, and also I should use the cork borer to get potato samples out and nothing else.
- I should be very careful when cutting the potato into 2cm with a scalpel, if I do cut myself I should report straight away to the teacher.
- I should also be very careful that the solution doesn’t get into my body internally, because I am not fully aware of the damage it could do to me. Therefore I should be very careful when handling with the solution.
- I shouldn’t run because I may cause spillage of the solutions that I am using, or I man run into someone without looking and cause them to spill a solution or break something.
- I should keep all my things out of the way such as my, bag, blazer, and stool should be stored underneath the desk so I don’t trip or fall.
- I shouldn’t sit down when carrying out the experiment.
- I should wear eye protection when appropriate or when instructed by the teacher.
- I should be aware of others around me.
I conducted the experiments acknowledging the necessary safety precautions I had to take. I used the required equipment appropriately. I have placed the potato on the cutting board before forming the potato cylinders preventing the chance of any injury to my hands. Furthermore, I was extremely careful when using the scalpel to cut the potato cylinders to an exact length of two centimetres each. And finally, I have performed my experiment with the utmost care following the procedure step by step.
After performing the experiment, I have noted my observations and compared them to my predictions. I have placed 3 potato cylinders in each salt solution. Therefore, for each concentration of solution, I have taken three readings as I have noted the mass readings for each of these cylinders. (Experiment repeated twice to gain average). The point of this is to obtain as much data as possible and then compile these readings together and obtain an average of these readings so that my final values are more accurate and reliable. Therefore, the results in my experiment are an average of the readings from the three potato cylinders so my readings are therefore more precise. Furthermore, the concentrations of salt solution that I have taken are 0 M, 0.5M, 1 M, 1. 5 M and 2 M as I have mentioned before. These concentrations should appropriately reveal the relation that exists between the concentrations of salt solution and the amount of water that enters or exits the cylinders that is, it should properly reveal how different variation of salt solution affects osmotic activity in a potato tissue.
I have obtained these readings after 24 hours of allowing the cylinders to be placed in their McCarthy bottles. I have then collected them, dried them using filter paper and weighed each individual cylinder on a weight scale.
The new mass of each cylinder can then be compared with its mass reading before being placed in the solution in the McCarthy bottle, thus with this information compiled together, I will be able to analyse what has really happened with the help of what I have learnt before on the topic of osmosis.
Results
In order to determine the percentage difference in mass, I will use the following formula.
Percentage Difference for mass:
(Final Mass – Initial Mass) x 100
Initial Mass
analysing evidence
From the results I have noticed a pattern that when the potato cylinders were placed in a concentrated salt solution, they decreased in mass (2 Molar). I have also realized however that with the weakest salt solution, there was actually a gain in mass and length in the potato cylinders rather than a loss (0 Molar, 1 Molar). Therefore, there was a greater percentage difference in mass. The potato cylinders that were placed in 0 Molar Concentration in the preliminary experiment also increased in mass.
From the results tables above, it is obvious that with 0 Molar Concentration, the potato cylinder actually gains length. They have a gain in mass from an average of 3.01g to 3.39g. This relies on the fact of osmosis; if a cell is placed in a less concentrated solution, then in order for equalization to occur, that is for the concentration to be equal on both sides, then water must move into the cell so that its cell sap becomes diluted and so that the concentration gradient is eliminated. Therefore the cells become turgid, as they become turgid, it is almost as if the potato cylinders have sucked in water and begin to swell up. Since they swell up with water, they will increase in mass. However there are some results that have come across as a surprise such as when the potato cylinders were put into molar concentration of 0.5, as we would have expected an increase in mass it has decreased from an average of 2.69g to 2.38g. You are able to witness this on the graph from average difference of 13% it has slumped to 11% I believe if this wasn’t here would have been a curved graph. Therefore some of the potato cylinders in 0.5 Molar Solution may be wrong.
There are reasons for this happening for example if the potato cell has a higher water potential because it is in a weaker concentration, which means there are less solute molecules and thus, more freely moving water molecules. As a result, the potato cell sap acts as the hypotonic solution, while the salt solution acts as the hypertonic solution. And again, since osmosis is the process where water moves from a region of lower concentration to a region of higher concentration, thus, water moves out of the potato cell to equalize the concentration. Therefore, there is a decrease in its mass. However this signifies an error in my experiment as generally, when the cylinders are placed in 0.5 salt solutions, the mass of the cylinders should increase. Therefore I believe this is an anomaly and this result may have happened due to human error or due to other circumstances.
Overall from the experiment I have noticed that there hasn’t been a constant decrease or increase in mass. Some have increased in mass while others have decreased. This has happened because, when a potato cylinder is placed in salt solution with a high water content at the start, it will then decrease at the end of the experiment. However, if the potato sample had low water content at the start then the sample will have increased in mass by the end of the experiment. This has happened purely due to osmosis. As water molecules travel from an area of high concentration to an area of low concentration, it will spread out randomly and spread it self out so that there is an equal distribution of water molecules, therefore both areas have the same amount of water molecules. Therefore this causes the potato cylinder to lose water and thus decrease in mass. Thus with the potato cylinder having a lower amount of water than the salt solution, the potato cylinder will gain water and thus increase in mass because, the water molecules would have travelled from the water to the salt solution and thus enter the semi-permeable membrane of the potato tissue, which is virtually inside the potato.
From this experiment patterns have emerged and my predictions have been met. I predicted that osmosis will occur and this did happen, also from the preliminary work I predicted that when the potato cylinders and put into pure water (0 Molar) the potato cylinder will increase in mass. I also predicted that when the potato cylinders are put in higher the concentration of water in the solution, i.e. lower in concentration of salt in salt solution, the cylinders will increase in mass. (Look at Justification of Prediction for why that has happened). Also from the graph a curved graph could have been drawn if the anomaly of 0.5 wasn’t present.
Evaluation
Overall I believe I carried out the experiment in an appropriate manner. I went by the safety procedures that I set myself and kept to them and worked sensibly and quietly. I used the apparatus appropriately and carried out the investigation carefully using the apparatus given. There are several improvements that could be made to the experiment. The results that I gained weren’t as consistent as I hoped. Some potato cylinders increased in mass while others decreased, therefore they weren’t very reliable as I did come across anomalies such as the 0.5 Molar concentration. However I did manage to obtain large quantity of results from which I was able to create informative graphs. I think I took easily enough results for the amount of concentrations that I was using, and the time that I used for the experiment to last was enough to allow sufficient osmosis to occur.
An anomaly that I had was the 0.5 Molar concentration. Perhaps the cylinders used were irregular, that is, perhaps they were not perfect cylinders. In this case, the amount of water that enters may differ and thus the total mass of the cylinders will differ. Thus, there are was the error of the 0.5 Molar concentration. If I was to repeat this experiment again I would have possibly found a machine to cut the potato as it would ensure that all potatoes would be the same length and dimensions. Also factors such as temperature weren’t kept constant even though the 15 potato cylinders were in the same place in the same room. a greater temperature the water molecules would move across the cell membrane of the cell more quickly and thus the rate at which osmosis occurs is increased. With a decrease in temperature, then the rate of osmosis also decreases and so perhaps the temperature was not kept at a constant during the entire experiment.
Now, in order to keep the temperature constant, I will keep the Mc Carthy bottles containing the potato cylinders inside a stored area, where the temperature is kept constant always; this will prevent the chance of any change in the rate of osmosis and thus will prevent any inaccuracies in the mass readings. I could have found a more accurate way to measure out the solutions and to determine the molar concentrations. Perhaps I could have used a burette. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a more accurate scale, e.g. not to 0.00g but to 0.0000g. Though my experiment is suitable and appropriate, it can be improved further more my using a more range of concentrations of salt solution to better reveal the change in mass of the potato cylinders when placed in these solutions but more importantly to reveal the exact concentration of the potato cell sap. Thus, in this case, it may perhaps be better to use an interval of concentrations of 0.25M. Thus, this will help to narrow down the exact concentration of the cell sap.
To investigate osmosis even further numerous experiments can be done:
1) You could do an investigation into how changing surface area and the amount of solution affects the rate/how long it for osmosis to occur.
2) You could do an investigation into how osmosis is affected when it occurs it different conditions, i.e. how osmosis occurs under high or low temperatures.
3) You could investigate osmosis further by seeing the effect of concentration of different types of potatoes or using entirely different plants, vegetables, fruits etc. This would test how different semi-permeable membrane could affect osmosis and the movement of water out and into organisms.