Weak Strong
Solution solution
Water in a dilute solution can move more freely than in a concentrated solution, because solute molecules attract water. (Water in any type of solution moves less freely than in pure water). Water molecules are more concentrated in weaker solutions than they are in stronger solutions.
Turgor pressure
Turgor pressure is when the cells contain as much pressure as it can hold and are blown up like a balloon. The cell walls stop the cell bursting and we say that the cell is turgid. This is useful as it gives the stem of plants support and it gives support to green plant tissues and stomatal guard cells. An example of turgidity if we put chips in water they will soon go firm. Flaccid is when a plant is lacking water and it wilts and the cells become Flaccid as water has moved pout of the cell. If a lot of water leaves the cell, the cytoplasm starts to peel away from the cell wall we say the cell has undergone plasmolysis. The water evaporates and for example the potato feels flabby and soggy and goes smaller. Equilibrium is when there is equal water concentration inside and outside the cell.
All my research I have collected from:
Biology for you- Gareth Williams. Revised national curriculum edition for G.C.S.E
As biology, and Edexel modular science- revision guide: book 2 (CGP)
Preliminary run
Before carrying out my experiment I did a trial run to help me understand more about the experiment and more about Osmosis.
- Firstly I cut out 4 pieces of potato 1cm x 1cm on a cutting tile.
- Next I placed each potato in a Petri dish. And covered each in one of the following: water, 2% glucose solution, 4% glucose solution and 8% glucose solution.
- I left the potatoes for 45 minutes.
Results
The results were as expected; the potato in water expanded the most because water moved into the potato by osmosis. As there were more water molecules outside the potato than inside, water molecules moved through the semi-permeable membranes of the potato cells into the potato so it swells.
The 8% glucose solution shrank most because water moved out of the potato through osmosis. As there were more water molecules inside the potato than outside, water molecules moved through the semi permeable membranes out of the potato so the potato shrank.
Improvements
After carrying out my trail run I realised that I could have carried out my experiment more accurately to get more reliable results. I will carry out my investigation the same except I am going to change or add the following:
- I am going to use 4-8% glucose solution, as it will enable me to obtain more refined results and find the exact point where the net movement of water molecules is at zero and the system equilibrium.
- I will measure the volume of solution I cover each potato with and make it equal; so one potato does not get more have more volume covering it than the other.
- As an alternative of cutting the potatoes to 1cm sizes I will use a cork borer, this way all the potatoes will have equal surface area for osmosis to work on and it will be easier to have the same mass.
- Instead of a Petri dish I will use a boiling tube, as less evaporation will occur.
Variables
There will be two changing variable in this experiment:
- The change in mass of the potatoes.
The concentration of glucose solution.
Fair testing
After carrying out my Preliminary run I realised that I could have made the test more accurate to get more reliable results. In my experiment I will take in to consideration to keep anything that could change the same following:
- I will use the same potato because some potato types may have slightly different semi permeable membranes in their cells, which could affect osmosis, because one potato might have more water than the other
- I will keep the temperature same, because it affects the rate at which the water molecules are moving around so this would change the rate of osmosis too.
- I will keep the potato samples same length, because if 1 potato sample is 1cm long and the other 3cm long then 3cm will have a larger surface area and Osmosis will happen much quicker.
- I will try to make sure any evaporation of glucose solution is stopped, because if the solution evaporates past the level of the potato, then the potato sample will have less surface area in solution so this would make osmosis happen slower. To stop any evaporation instead of Petri dish I will use a boiling tube.
- I will keep the length and mass of the potato same and cover each potato in the same amount of volume as one potato could have more volume which could affect the osmosis rate.
- I need to make sure I have clean and dry hands each time I handle the potato. This will stop any extra water entering the potato and affecting the osmosis rate.
- I will work on a flat surface, so I make sure there is the right amount of volume and when transferring the water from the measuring cylinder to the test tubes, I will try not to spill any.
Safety
Whilst doing this experiment there will be some precautions I need to take to ensure safety. Safety is very important in a scientific experiment even if the experiment may seem harmless. To make the experiment safe I will handle sharp equipment such as Cork borers with a lot of care. Moreover any spillages I will wipe/ clean straight away to avoid further accidents. I will make sure I do not run as there could be an accident and one could get hurt. Also I will keep the desktop that I am working as clean as possible, so that there is nothing on the desktop, except for the apparatus needed thus; an accident does not happen. In addition I will ensure the test tubes are placed correctly in test tube racks so they do not roll of the desk and break. Lastly I will be very careful that the solutions don’t get into the bodies internally as it could be harmful.
Prediction
When a potato is placed into a concentration of glucose solution, which is greater than a certain level the potato will contract. However if there is less glucose solution then the potato will expand. Therefore when a potato is placed in distilled water (high water concentration) water will move across the semi-permeable membrane in to the cell (lower water concentration) by the process osmosis, making the cell swell up. The cell will then be turgid. The potato cells increases in length, volume and mass because of extra water. If a potato were placed in glucose solution with low water concentration, then the opposite would happen. Water would move out of the cell in to the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmosysed, the potato cells will have decreased in length, volume and mass.
I predict that the potatoes with 4%, 5% and 6% glucose solution will go turgid. This is because the more hypotonic the concentration i.e. the lower the concentration of sugar the more water molecules will diffuse in to the cell through the semi permeable membranes making the potatoes swell up and will consequently, gain mass.
I think water molecules will continue to move until the water concentration is the same on both sides of the membrane. Therefore after carrying out my preliminary run I think that the isotonic point will be at 7% glucose solution.
I believe the potato in 8% glucose solution will go flaccid. This is because this solution will be hypertonic; it will have the higher concentration of solutes. I know that the higher the concentration of the sugar the greater the loss of water, so more water molecules will move out of the semi permeable membrane of the potato, by the process of osmosis. As there are more water molecules inside the potato than outside, water molecules move through the semi permeable membranes out of the potato, so the potato shrinks.
Scale
I have chosen to use 4-8% glucose solution for my experiment, and I will place each potato in one of these concentrations. After looking at my preliminary work I feel these are suitable concentrations, for me to investigate at which concentration the system will be equilibrium.
Expected results
Change in Mass
(%)
Isotonic point
Concentration of Glucose (%)
Apparatus
- Boiling tube
- Cork borer
- Mass scale
- Ruler
Method
1. Bore out 5 pieces of potato using the cork borer. (Make sure they are all the same size).
2. Measure the mass of each potato on a mass scale.
4. Place each potato in a boiling tube
5. Measure the equal concentration of each glucose solution and cover each potato in a separate one.
6. Leave experiment for at least 24 hours.
7. Record the change in mass of the potato by weighing it again and then using the following formula:
(Mass after-mass before) x 100
Original mass
8. Repeat the experiment however many times required to obtain accurate and reliable results.
OBTAINING EVIDENCE
Results
Conclusion
I found out that as the concentration of glucose solution increases the mass of the potato decreases, from these set of results I know that my prediction was correct.
From my results indicated in the graph I can see that it agreed with my prediction. I predicted that the graphs would look as follows:
Change in Mass
(%)
Isotonic point
Concentration of Glucose (%)
A curved line shows that as glucose concentration increases the percent change in mass decreases. This shows a clear pattern, because it confirms that as the concentration of glucose increases the size of the potato decreases as water molecules move out of the potato through the semi-permeable membrane.
I can see that my graph starts from top left and finishes at bottom right (negative correlation). The gradient of the graph is decreasing. The reason for that is after a certain concentration the potato cannot lose any more water i.e. maximum water loss. After that if I increase the concentration of the solution surrounding the potato, it will not lose any more water. Therefore, the more concentrated the solution is, the smaller the mass gain should be.
The curved line produced on my graph is valid, as there is an obvious link between the concentration of glucose and the percent change in mass. Also my graph agrees with the osmosis theory as it shows that at a higher concentration of glucose solution there is a bigger percent change in mass. Although I have one anomaly it is not very anomalous and overall my graph follows the curve I expected.
Some of my results agreed with my prediction as I stated that 4%, 5% and 6% would go turgid, as more water molecules enter the potato through the semi permeable membrane making it swell up. After carrying out my preliminary work I predicted that the isotonic point would be at 7% glucose solution however in my experiment it was at 6.5%, nevertheless the result is close to my prediction. As a result the 7% glucose solution was anomalous and on my graph it was shown as flaccid. Lastly I said that 8% would go flaccid, as there are more water molecules inside the potato than outside so water molecules move out of the potato’s semi permeable membrane as a result the potato shrinks, my prediction was correct and this is shown in my graph.
EVALUATION
I had one anomalous result (labelled on the graph). This could be due to due to my measurement of the volume of glucose concentration being incorrect. However it is quite close to my curve so it shows that my graph pattern is valid.
After studying my graph I know that they are good enough to support a valid conclusion because overall the percent change in mass decreased. Because my anomaly is very close to my curve and follows the overall trend it shows that my results are reliable. Also it backs up my theory that as glucose concentration increases there is a decrease in the size of the potato.
If I repeated this experiment I would get similar results, as my results are overall; close to what was expected
Improvements
In general the procedure used was suitable for the experiment, however I think that to obtain more reliable and valid results, improvements could be made.
- Instead of using measuring cylinders to measure the volume a (Burette) pipette could be used to obtain more accurate measurements.
- In addition to calculating the percent change in mass, I could calculate the percent change in volume of glucose solution and the percent change in length of the potatoes.
- I could use two potatoes for each concentration of water, so if I obtained a rogue result from one piece of potato I would have another one
Overall I think my experiment went really well. I feel that I carried out my experiment reasonably, and I encountered no problems. As a result I ended up with a valid set of results.
Further work
Another experiment I could carry out to investigate osmosis would be to use a different fruit or vegetable. A potato contains lots carbohydrates in the form of starch, which is not soluble. A starch produces less osmotic pressure. If I used a fruit or vegetable that had sugar it would affect the osmotic balance, as lots of water would rush in to the cell making it very turgid. An apple could be an idea as it stores carbohydrates in the form of sugar (fructose). As a result I would expect the isotonic point would be much higher. I could use glucose solution from 6% up to 20%. I could then compare the results. If I used an apple I would expect the graph to look as follows:
% Change in mass Apple
Potato
Concentration of glucose solution
The same variables would apply for this experiment as those for my actual experiment. Also to ensure that the test was fair so reliable results were obtained I would have to use the same points for fair testing which I mentioned for this experiment. In addition safety would be just as important.
I could use the same apparatus and idea of the method, which I used in my experiment. However one thing I could change would be to add the improvements I mentioned for this experiment.
