Temperature: This is important as it affects the amount of energy that the water molecules will have, and if they have different levels of excitation, as when this occurs the plant cells will move faster then the Osmosis may occur at different rates, the temperature may also affect the rate at which the water evaporates, lowering the level of water in which the chips are immersed, and increasing the concentration of the water which the chips are immersed in. So I must try and maintain the same temperature throughout all my experiments.
Pressure: Pressure is quite an important variable; as if there is a change in the external pressure my results will be drastically changed. If it were changed to a low external pressure, then there would be a high level of water loss from the cell and vice versa. So I must ensure that the pressure is kept constant otherwise it may alter the water-flow.
Time of Exposure: The time of exposure is also very important, as I must make it long enough for the cells to reach an Osmotic equilibrium, I also must ensure that all of the chips get the same time of exposure to the solutions. So I will try and remove all chips and place them all in at the same time.
Prediction
I predict that in a concentrated solution where there is little water present, that there will be a nett loss of water from the cells. In a more dilute solution, where there is more water present I predict that there will be a nett gain of water.
Preliminary Experiment
The reason that I conducted a preliminary investigation was so that I could find out the best method in which I should conduct my main experiment and to see whether or not the concentration of the solutions really does affect the rate of Osmosis in potato cells. This trial run also helped me to explore the variables that may affect my experiment in order to change these when I redo my experiment and this time use the knowledge I have acquired to set up a fair test in order to obtain a valid set of results. This is how I conducted my experiment.
Apparatus
- 7 Petri dishes
- 1 scalpel
- 1 white ceramic tile
- 7 concentrations of salt solutions
- 1 ruler
Preliminary Method
First of all we sliced the potato chips into quite fine 5cm long strips, with the scalpel on the ceramic tile in an attempt to try and increase the accuracy. We then poured a small amount of the seven concentrations into each of the uncovered Petri dishes. We then placed the chips in one at a time for 15 minutes; we hoped that this would be enough time for the potato cells to reach osmotic equilibrium with the salt solution. Afterward we removed the chips from the solutions and measured the potato chips length and below are our results.
I predicted that there will be an increase in length of the potato chips in the salt solutions of lower concentration, as the concentration of the solution rises the change in length of the potato chips will decrease.
Preliminary Results
Conclusion
In my results I can see that from the concentrations of 0.5M to 0.125M the length of the chip decreases, and from 0.06M to 0M the length of the chip increases, so the more dilute the solution the longer the length of the chip.
From my results I can see a general trend showing that as the concentration increases the change in the length of the chip will decrease, however my results are not significant enough to identify a clear trend, so I would not be able to draw out a conclusion. However I now know that I can use potato cells for my main experiment.
Evaluation of my preliminary method
Good Points
In my experiment there were a few positive aspects which I will try and keep the same for my main experiment for example the length of my chips was kept constant throughout and I measured them very accurately with the ruler. Also the timing of the experiment was fairly good as all the chips were removed and placed in at roughly the same time. However I was dissatisfied with the length of time my experiment was conducted for, as I felt it was an unsuitable time limit for the cells to reach Osmotic equilibrium.
Bad Points
I have quite a few criticisms of my method, as now that I have looked back I can see that my method is quite flawed, as throughout my experiment I did not measure the volume of the salt solutions that I was putting into the Petri dishes, as that way I could've made them all the same. Also because I only put a shallow pool of water at the bottom of the Petri dish I did not manage to fully submerge any of my chips, so the possible surface area for absorption was decreased. The surface area and width of my chips were also drastically different from one another, so some would have had a larger surface area than others again giving some chips an unfair advantage. Another bad point of my experiment is that my Petri dishes were left uncovered, this means that some evaporation may have occurred. The timing of the experiment was not very good as all the chips were removed and placed in one-by-one so some had a longer time of exposure than the others. The final criticism I have is how I measured the chips, as the length is an inappropriate measurement of the rate of Osmosis as the chips not only elongate but expanded as well and we did not take this into consideration that the width will increase as well, also the chip may take water in without any outwardly physical signs like an increase in length, however if I were to measure the length again then I would use a calliper.
In my main experiment I will change the following aspects:
- I will increase the period of the chips submergence, in order to ensure that osmotic equilibrium is achieved in all the solutions.
- I will measure using weight instead of length, as the chips will expand as well as elongate, whereas the weight will give a measurement of all the water lost/gained.
- I will measure and maintain the volume of all 7 salt solutions in all seven Petri dishes.
- I will use three chips in each solution in order to increase the number of results and reduce the risk of anomalies. As data is always more reliable when a sample is used.
- Increase the volume of the salt solutions enough to fully submerge all the chips, so I will now use 40mls of water in each Petri dish.
- To prevent excess evaporation I will cover my Petri dishes in my main experiment.
- Before weighing the chips both before and after I will pat off any excess water, so that I may get a result showing the average change in weight.
Revised Apparatus
7 Petri dishes-The Petri dishes have shown to be quite useful, as when filled they allow total immersion, and they have lids to help reduce evaporation.
1 scalpel-to cut the chip into roughly the same size/shape/width.
1 50ml measuring cylinder-To measure the volume of water that I am filling my Petri dish with.
1 weighing scale-so I can measure the change in weight of the chips, as a way of measuring the osmosis.
7 salt solutions-to get these I took pure salt and mixed it into one liter of water, this gave me the 0.5M solution, and then I just kept adding another liter of water to the solution to dilute it by half
1 white ceramic tile-to cut the chips on
7 pieces of filter paper-to remove excess water from the chips
Revised Method
First of all I cut the chips, so that they would all have generally the same weight and shape and then patted the excess water off and weighed each one out so that I could measure the change in the weight, I then measured out 40ml of each solution in the measuring cylinder and poured them out into separate Petri dishes, which I then labeled accordingly. We then placed all of the potato chips in at exactly the same time, making sure that all of them were fully submerged. We then placed the lids on all the Petri dishes and waited for 40 minutes, as we believed this would be enough time for the chips to reach an osmotic equilibrium with the water. We then removed the chips, and patted the excess water off them and weighed them very quickly.
We carried out the experiment and got a good set of results, they are conveyed in the following tables.
My prediction is going to remain the same. I think that, just like in the preliminary experiment, the increase of the concentration of the salt solutions will cause the increase in weight of the potato chips to decrease ending in a resultant decrease in the weight. This is based on the fact that the direction of the nett water intake will change as the concentration increases/decreases accordingly.
Tables of Results
Result Set 1
Result Set 2
Table of Averages
Analysis of Graph
In my Graph of Averages there is a line of best fit with a negative correlation, this means that the water loss/ gain and the concentration of the salt solution are inversely proportional.
Gradient:
d y
d x This is the formula for working out the gradient
-0.95
0.44 Here I replaced all known values
-2.15909 This is my gradient.
This means that for every 0.1M increase in concentration the weight decreases by -2.15909 and for every decrease of 0.1M in the concentration the weight increases by 2.15909.
From my graph I can see that when the chips are hypotonic from the concentrations of 0M to 0.012M the potato cell cytoplasm was above the concentration of the salt solution and therefore the chips mass decreased. However when the chips were hypertonic from the concentrations of 0.12M to 0.5M the potato cell cytoplasm was below the concentration of the salt solution and therefore the chips mass increased.
From my graph I have discovered that the concentration of the potato cell cytoplasm is 0.12M, this means that this is the point were the potato is isotonic to the solution. As this is the point at which the mass of the chip stays the same which means that the chip and the solution are already at Osmotic equilibrium and there was no need for Osmosis to occur. This is only an average though and will not be exactly the same as there will be some variation.
Evaluation
Although I believe that my experiments were quite successful, I think that there are still some elements that if I were to repeat my experiment I would change. One such thing is the surface area of the potato chips, as when there was variation between the chips, some had an unfair advantage over the others, as there was a larger surface area for absorption.
Another possible improvement for my experiment is that after my experiment was completed I left some of the chips out to dry for too long. So this may take a small section away from the mass that should have been. The room temperature may also be a factor that affects my results that I did not take into consideration as there was no control over the two and I did not check whether or not it was the same over the two days. Also when I removed the chips from the solutions I used my hands which was a very ineffective method, as this may cause water to rub off onto my hands, or I may apply too much pressure and squeeze some water out accidentally.
The clock we used was also unreliable, as I believe that if we were to have used a computerized clock the timing would have been much more reliable and accurate, as with a computer the power supply is constant all the way throughout, whereas with a battery powered clock slowly the power source decreases and the clock will gradually slow down, so the timing may not have been precise and the chips may not have been able to reach osmotic equilibrium.
I think that if the experiment was to be repeated then I would perhaps use Visking Tubing however I realized that this would defeat the whole purpose of the experiment as it is to find the average concentration of the cytoplasm of the potato cells. However if there were some way in which I could incorporate this in to my experiment then it would be usefuls there would be little to no variation between the Visking tubing unlike the potato chips.
So if I were to repeat this experiment I would:
- Use an electronic clock for more accurate readings
- Use tweezers to remove and place the chips into the solutions
- Make sure that I quickly weigh all of the potato chips immediately after the time limit is up
- Ensure that the room temperature is the same on both of the two days or perform the experiment in a room where the temperature is monitored
- I would ensure that all of the potato chips surface are were identical in area.
Conclusion
Overall my experiment was successful, as in the end I was able to draw a decent graph that showed a clear trend in the data and allowed me to draw a line of best fit. Although my experiment was not perfect and there were a few anomalous results, I believe my experiment was successful. My prediction has also been proved by my experiment and I have discovered the average concentration of the cytoplasm of potato cells and it is 0.12M.