Osmosis is defined as the movement of water molecules through a semi- permeable membrane from an area of high concentration to an area of low concentration until a state of equilibrium is reached. Equilibrium is reached once enough water has moved to balance the solute concentration on both sides of the membrane. Semi-permeable membranes let small molecules such as water and oxygen to travel through it but large molecules such as proteins are prevented from going through it as it is only partially permeable. Osmosis occurs across a partially permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane. When this happens to the cells they will either become turgid if water flows into them, or flaccid if water flows out of them.
In this diagram the membrane separates pure water from a solution of sucrose and water. The membrane is semi permeable, water molecules can cross it but the sucrose molecules are too large. The water solution will move from a lower concentration of solutes to one with a higher concentration of solutes. A solution with pure solutes has more molecules that are free to move about and follow their concentration gradient across the membrane; as a result there will be net movement of water across this membrane from left to right (shown by the yellow arrows on diagram above).
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Sugar solution has a low water potential (not many water molecules).The water potential in the solution would be lower than that in the potato cell so the net movement of water will move out of the potato through its membrane into the sugar solution and so the potato's mass will decrease. The opposite happens in water where the water potential is much higher than in the potato (almost 0). Here the net movement of water will move into the potato in order to reaching equilibrium and so the weight of the potato increases.
To investigate the effect of varying concentration of a certain sugar solution on the amount of osmotic activity between the solution and two vegetables (potato and sweet potato) of the same mass. To investigate if the additional glucose in sweet potato has an effect on its mass compared to the normal potato.
Before I start to carry out an experiment to investigate my aim, I am going to conduct a simple experiment to verify my knowledge of osmosis. Doing this experiment will also help me to improve my main experiment.
Plan: For my preliminary experiment I will be using three different solutions: sugar solution, pure water, a solution consisting of 50% water and 50% sugar solution. I will also be using potato chips which are all equal in mass. A cork borer will be used to cut out 3 potato cylinders, the mass of the potato cylinders will be Measured to make sure that they are equal in weight. Then the mass of each of the potato chip will be recorded. Next 3 measuring tubes will be placed on a test tube rack. Using a measuring cylinder tube 20ml of water, sugar solution and the solution consisting of 50% water and 50% sugar solution will be measured out and put into three different test tubes. In order to avoid any mix up's each test tube will be clearly labelled. One potato chip will be placed in each solution and left for half an hour. Afterwards the potatoes will be taken out of the solutions and their masses will be measured and recorded.
Starting mass (g)
Mass after (g)
50% Water and 50% sugar
100 % Sugar
From the results that I have obtained, I can see a negative correlation between the concentration of the sugar in the solution and the mass of the potato chip at the end of the experiment. While the solution containing 100% sugar decreased in mass by 0.11g and the solution containing 50% sugar decreased in mass by 0.03g, the solution that contained 0% sugar increased in mass by 0.07g. This shows that in the two solutions containing sugar the mass decreased because water molecules moved out of the potato chips into the solution. The opposite happened with the water solution; water moved from the solution into the potato chip.
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Overall I was please with the results as they agreed with what I have already learned about osmosis. The results that I got also showed trends that could later be expanded in my main e experiment.
Over all my preliminary experiment was a success and the results I obtained were precise and useful. However for my main experiment I am going to make some improvements so that I can get the best possible results. Firstly I am going to experiment with sweet potato chips as well as potato chips because I would like to investigate if the additional glucose in the sweet potato has an effect on its mass. In addition I am going to be using a wider selection of sugar concentrations in order to collect a larger quantity of accurate results. I am going to use water and 4 other concentrations. This will enable me to write a conclusion that is more accurate. In my preliminary experiment I only did the experiment once. I am going to repeat my main experiment three times, by doing so I will gain more confidence in my results as they will be more reliable. By repeating the experiment I will be able to identify any anomalies. Further more in my preliminary experiment I could not take the potato chips out of the solutions at the same time. I also encountered this problem when putting the potato chips into the solutions. For my main experiment I am going to overcome this problem by asking a few of my associates to help me put the potato chips into the solution and take them out at the same time. This will make my results more accurate and fair.
I predict that for the potato chip in water the mass will increase because water has a higher water potential than the potato chip so water molecules will move from the water into the potato chip, causing it to increase in weight. The opposite happens with sugar solution because sugar solution has low water potential so water molecules will move out of the potato chip into the sugar solution. Thus I predict that as the concentration of sugar in the solution increases the mass of the potato cylinders will decrease. I would expect sweet potatoes to have a lower water potential (not many water molecule) than normal potatoes, because they are 'sweet' and therefore must have contain sugar. Hence for the sweet potato chips I predict that at first the mass of the sweet potato will increase but, as the concentration of sugar in the solution increases past the isotonic point the mass of the potato cylinders will decrease.
I predict that the graphs for the both the vegetables will be similar.My prediction can be demonstrated in the following graphs:
While I carry out this experiment, it is important to consider safety. Lack of safety in science labs can not only endanger me and my fellow class mates but can also produce poor and useless results. Following safety procedures protects the lab, its people and the environment. Safe labs also help produce effective results. These are the safety measures I took:
All bags were kept well away from the area of work so that no pathways were being obstructed and the risk of someone tripping over was reduced.
All stools were tucked in under the tables in order to avoid a passer-by tripping over. Both this point and the one above can prove to be fatal if the victim is holding any dangerous equipment e.g. Scalpel, acid etc. It can also cause spills.
Loose hair was tied up; sleeves rolled up and head scarf's tucked away. Hair and clothing can get in the way while working. It is also dangerous
and can cause spills, ruin experiments and even potentially harm me or others around me.
No earphones, scarves or jewellery was worn as these can catch on equipment and cause spill or damages.
Safely glasses were worn at all times to ensure our eyes were protected from and potential danger.
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It was made sure that the work area was clear of any unnecessary items such as reading books and folders as these can get in the way and cause accidents.
Working quietly and carefully. Talking unnecessarily can be a distraction to me and others around me, when distracted it is easier to make mistakes that can not only be dangerous but can also affect my results.
Chipped and cracked glass wear was not used as this could break more easily if handled a lot and create a mess. It can also cause injuries. Extreme care was taken when using equipment made of glass e.g. test tubes.
Eating and drinking in the labs is forbidden as the food may come into contact with some harmful substance. This could be unsafe for the consumer.
When using knifes a non-slip surface was used to cut on so that no one was injured.
After finishing the experiments, I washed my hands because my hands could be contaminated with chemicals, even if they were not used by me.
All equipment was safely stored away after use as leaving unwanted equipment around can cause accidents and get in the way.
Accuracy and Reliability
There are the tasks, which I undertook to ensure reliable and precise results were produced.
To eliminate all possibilities of faulty errors; I repeated my experiment three times. In addition, I had no less than 5 different concentrations, so I obtained adequate results.
In order to increase the reliability of the results, I found more precise values of mass by using an Electronic balance.
When measuring the potato and sweet potato cylinders, I waited for at least 10 seconds to ensure that the figure displayed on the digital scale was not flicking.
If any readings seemed anomalous, I took an additional reading just to be sure.
I collaboratively, with the help of 3 other members, measured the mass of the potato and sweet potato cylinders individually. This gave more reliable results as the masses of all the potato cylinders were measured about the same time.
I used pieces from the same potato and sweet potato for each of the 5 pieces in each test.
I used exactly the same method for each test to make the comparison much more reliable.
Goggles: - These were worn for safety reasons to avoid anything from coming into contact with the eyes.
Electronic balance: - I used this apparatus to measure the mass of the potato and sweet potato cylinders before and after they were put in their molar solutions. Electronic balances are also more accurate that normal balances.
Test tubes: - These were used to hold the molar solution and the potato/ sweet potato cylinder for each molar solution for every experiment and test.
Healthy potatoes: - This was one of two main vegetables used for the experiment.
Healthy sweet potatoes: - This is the second vegetable that was used for the experiment. This vegetable was mainly used so that I could investigate if the additional glucose in sweet potato has an effect on its mass compared to the normal potato.
Chopping board: - This was used to cut the potato cylinders on. Using this not only kept the lad neat and tidy but also prevented injuries as it is a non-slip surface.
Knife: This was used to cut the potato cylinders if the mass was too much.
Timer/stop watch: - This was used to time the experiment. It is also more accurate than using a clock.
Paper towels: - These were used to pat the potatoes dry before being measured at the end of the experiment. If the potatoes were measured without being dried first then the additional liquid on the potato can cause it to increase the mass.
Test tube rack: - This was used to hold the test tubes.
Sugar solutions (1 molar and 2 molar): - These solutions were very significant for the experiment, as they, with the potato cylinders, caused osmosis to occur.
Water: - Used to make certain concentrations of sugar solution.
Cork borer: - To cut the same shape potato pieces.
Measuring cylinder: - To measure out certain amounts of water and sugar solution.
Labels: -To labels the test tubes so it is easier to identify the contents inside each test tube.
In this investigation I had three different variables:
The independent variable - This is a factor that can be changed.
The dependant variable - What you measure and what is affected in the experiment.
Fixed variable- The factors you keep the same.
The independent variables:
I changed the concentration of the sugar solution. I used; water, 0.5 molar sugar solution ,1 molar sugar solution, 1.5 molar sugar solution and finally 2 molar sugar solution. I also changed the types of potato used (sweet potato and normal potato)
The dependant variables:
I measured the mass of the sweet potatoes and potatoes before placing in the solutions and after. This is something that is affected during the course of the experiment.
All the potatoes and sweet potatoes I used were that same in mass (0.64g). The volume of each solution in each test tube was also the same (20ml). In addition I kept the potato and sweet potatoes in the solutions for the same length of time (40 minutes)
It is only by carrying out a fair test that I can be sure that it is what I have changed (independent variable) that is affecting what I measured (dependent variable).
In order to get accurate results I needed to ensure that my test was fair.
Therefore in this experiment I controlled:
The volume of solution in each beaker
The mass of the potatoes and sweet potatoes at the start of the experiment
The length of time the potatoes and sweet potatoes were left in the solution.
I ensured that the potato and sweet potato cylinders were put in the solutions and taken out of the solutions at the same time.
Firstly get 2 test tube racks and place 5 test tubes on each rack label one test tube rack "sweet potato" and label the other rack "potato"
Get some sticky labels and label each test tube with the solution it is going to contain (water, 0.5 molar, 1 molar, 1.5 molar, 2 molar,). Do this for the test tubes on both racks.
Using the measuring cylinder measure out 20ml of 1 molar sugar and pour into the test tube labelled "1 molar".
Then measure out 20 ml of 2 molar sugar solution and pour in to the test tube marked "2 molar".
After measure out 20 ml of water and pour in to the test tube marked "water"
Next make the different concentrations of sugar solution. (1 molar and 2 molar are already provided).
-To make 0.5 molar: use the measuring cylinder to measure out 5 ml of 1 molar sugar solution and 15ml of water. Mix these together and pour into the test tube labelled 0.5 molar.
To make 1.5 molar: use the measuring cylinder to measure out 15 ml of 1 molar sugar solution and 5ml of water. Mix these together and pour into the test tube labelled 1.5 molar.
(Do the above 5 points for all the test tubes on both racks. By the end of point 6 you should have two test tube racks one labelled "potato" and the other labelled "sweet potato", each rack must hold 5 test tubes, each test tube must be labelled with the solution that it holds e.g. water, 1 molarâ€¦etc.)
Afterwards, get a healthy potato; make sure the skin is peeled. Using the cork borer cut out 5 potato cylinders.
Using the electronic balance, weigh all the potato cylinders. They should all weigh around 0.64g. If they don't then alter them accordingly using a chopping bored and knife and place to a side. Remember to record the masses.
Subsequently, get a healthy sweet potato. Once again make sure the skin has been peeled. Using the cork borer cut out 5 potato cylinders.
Using the electronic balance, weigh all the sweet potato cylinders. They should all weigh around 0.64g. If they don't then alter them accordingly using a knife and a chopping bored. Remember to record the masses.
Then both the sweet potato and normal potato must be put in to the solutions, each test tube must contain either 1 potato cylinder or 1 sweet potato cylinder. Make sure all the cylinders of potatoes and sweet potatoes are dropped into the solutions at the same time (ask help from classmates if needed). The timer should also be started now.
(Leave the solutions in this state for 4o min)
After 40 min, the potatoes and sweet potatoes must be removed from their solutions. Again, they should all be taken out at the same time (again ask for help if needed). Make sure the potato cylinders do not get mixed up; ensure that you know which cylinder came from which solution.
Dry each cylinder and measure it's mass one by one. Ensure that the masses are recorded once again.
(All the above was repeated 2 more times to increase the reliability in my results)
During the experiment I noticed that some of the potato and sweet potato cylinders in higher concentrations of sugar started to float. In the 1.5 and 2 molar sugar solutions the potato has higher water potential that the sugar solution so water molecules from the potato cells would have moved into the sugar solution, this made the potato cylinders float to the top of the solution rather than stay at the bottom of the beaker because it became lighter. This happened after approximately 10 -15 minutes. After approximately 20 minutes the potato cell in 1 molar sugar solution started to float as well. This is because as the potato cells are losing water molecules, they gradually become lighter. The potato in water did not float, nor were there any visible changes. All the above applies to both the sweet potato and normal potato.
A difference I noticed between the two types of potato was that while the potato in the 0.5 molar solution floated a slightly, the sweet potato in the 0.5 molar solution did not float at all, it remained still, similar to the potato cylinder's in water. I also noticed that the potato cylinders floated higher than the sweet potato cylinders.
Below are the calculations I did to work our various aspect of the results:
Percentage change in mass = Change in mass x 100
Averages = All three masses in one category added
The number of different masses
Change in mass = New Mass - Original Mass
Both the graphs show negative correlation between the concentration of the sugar in the solution and the mass of the potato chip at the end of the experiment. It is clear from the graphs that osmosis has taken place.
Potato: For the potato cylinder put in water (0% sugar) the potatoes mass has increased 21.87% form 0.64 grams to 0.73 grams, this shows that because water has a higher water potential than the potato cylinder, water molecules have moved from the water into the potato causing its weight to increase. When this happens the cell becomes turgid (hard and swollen).For the potatoes in all the other solutions (1 molar, 0.5 molar, 1.5 molar and 2 molar) the mass of the potatoes decreased. This shows that water molecules have moved out of the potato cell into the sugar solutions. When a cell loses water in such a way the cell become flaccid, which is the opposite of turgid.
Sweet potato: For the sweet potato cylinder that was placed in water the mass increased as expected. This happened because water moved from the solution into the sweet potato.
For all the other molarities, except the sweet potato placed in 0.5 molar solution, the changes were the same as for the potato. For the sweet potato placed in 0.5 molar solution the mass increased from 0.64 grams to 0.65 grams. There was a 0.01 gram increase in the mass of the sweet potato by the end of the experiment. This suggests that my prediction was correct: - I expected sweet potatoes to have a lower water potential than normal potatoes because they are 'sweet' and therefore must have contain sugar. Hence for the sweet potato chips I predicted that at first the mass of the sweet potato will increase but, as the concentration of sugar in the solution increases past the isotonic point the mass of the potato cylinders will decrease. There was a 0.01 gram increase in the mass of the sweet potato by the end of the experiment. 0.01g is extremely close to 0g. On the graph, if you look at this point, it is clear that the range bar is in between 1.56% and 0%. If there had been a 0% change in mass, it would mean that a state of equilibrium had been reached. This is a point where the concentration of water inside the sweet potato cell is the same as the concentration of water in the solution which the sweet potato is in. my results imply that the reason the mass of the sweet potato in the 0.5 molar sugar solution increased in because the water potential of the sweet potato was slightly lower than that of the water. As a result water molecules moved in to the sweet potato cell. The sweet potato in 1.5 molar sugar solution decreased weight because the concentration of sugar in the solution had increased passed the isotonic point (equilibrium).
My graphs show that osmosis has taken place. There was a negative correlation between the concentration of sugar in the solution and the mass of the potato or sweet potato cell at the end of the experiment. Over all the results that I obtained have answered my aim; varying the concentration of the sugar does have an affect on the mass of the vegetable and the additional sugar in sweet potato does have an effect on its mass when compared with normal potatoes.
With the results I have obtained I can conclude that the higher the concentration of sugar in the solution, the more mass the potato looses. The graph also shows that the opposite happened when the concentration of water in the solution is higher. So I can conclude this by saying: the higher that concentration of water in the solution the more mass the potato gains. This applies for both the potato and sweet potato. However because of the additional sugar in the sweet potato, the mass of the sweet potato will only decrease after the concentration of sugar in the solution increases past the isotonic point (point of equilibrium).
To make full sense of the conclusion, a recap on osmosis and the affect on cells are required. When there is a high concentration of water in the solution and a plant cell with low concentration of water into water, the water would move into the plant cell by osmosis. Osmosis is defined as the movement of water molecules through a semi- permeable membrane from an area of high concentration to an area of low concentration until a state of equilibrium is reached. Equilibrium is reached once enough water has moved to balance the solute concentration on both sides of the membrane. Osmosis occurs across a partially permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane. When this happens to the cells they will either become turgid if water flows into them, or flaccid if water flows out of them.
The results that I have obtained in this experiment support the prediction that I made using scientific theory and my preliminary experiment.
Overall I was please with the results that I gained as they agreed with what I have already learned about osmosis. I believe I have gained reliable results, which supports the scientific theory. After making changes to my main experiment from the preliminary experiment I faced no difficulties. The equipment that I used was suitable, I managed to collect adequate data and the investigation that I organised was effective, as can be seen from my results. I was able to get enough results to come to a strong and explanative conclusion. I believe that my results were reasonably accurate as I did the experiment 3 times and found that the repeated results were mostly very close together. With these results I was able to construct two, very accurate and informative graphs.
My results were free from any major anomalies that did not fit the regular pattern. However there were one or two very minor anomalies that could have been caused by the following:
Some limitations or experimental errors could have occurred during the investigation, which I did not realise and could have led to some miscalculations in my results. The main limitation could have been the change in temperature of the room during the experiment. High temperatures could have altered the rate at which osmosis takes place. Another potential problem in the investigation may be tampering, as the investigation was carried out in a lab which is in constant use. This could have resulted in students playing with the apparatus, which could have affected the out come of my results. Also when drying the potato chips after the allotted time it was inevitable that they would have excess water on them, and so we dabbed them with paper towels to remove it. However, in doing this we could have removed water that was not excess, and altered our results.
To make my investigation more reliable and accurate I could make some improvements. Firstly I could use a wider and more precise rage of sugar concentrations. If I did this I would have a wider range of results to work with so it would be more accurate and I would be able to come to a more secure conclusion. Another improvement I would make if repeating the experiment is to dry all the potato cylinders the same. I used paper towels to dry the potato cylinders, which caused variation between the potatoes as sum where dried more thorough and for longer than others which affects their mass, and those potato cylinders that have not been dried as much as others will have more excess water and therefore a greater mass. In the future I would develop a method to dry the potatoes in a way that is fair.
Further investigations may be to investigate using lengths or shapes of potatoes, to see affect of osmosis upon them. I could do a comparative study, where I see the effects of osmosis on potatoes which have been altered to do specific breeding, e.g. grow in a hot climate, etc. I could do a study into surface area or the effects of temperature upon osmosis, to identify the best temperature in which osmosis occurs. I could also investigate the affects of osmosis on different vegetables such as aubergine, turnip, carrots.