Shrunk vacuole
As you see by looking at both of my results, and my prediction, the batch of chips that were placed in the solution with a concentration of 0% (i.e. total 100% H2O) the mass increased. This means that the concentration of water was higher than in the potato cell, therefore water particles have moved from the beaker into the potato cell, across a semi-permeable membrane, in this case both the cell wall and cell membrane. Mainly the cell membrane, as its job in the plant cell is to control what goes in and out of the cell.
As the vacuole took in water from the beaker, its size increases and so does the mass of the cell as it is carrying more water. As the vacuole’s size increases the cell becomes turgid but also larger, in order to cope with the increasing size of the vacuole. As water is moving from the beaker into the potato cell, we are able to recognise that the % cell sap in the cell is more than 0%. We are able to use this evidence in order to back up our prediction, as I stated that in the 0% concentration, the mass would increase as it is taking in water.
The change in mass can be worked out as follows-
Increase/Decrease in mass
Original Mass
The batch of chips that went into the solution with 12.5% starch (sugar) concentration increased in mass but on a smaller scale. This indicates that we are closer to the concentration of cell sap, however are not quite there.
In the 12.5% sucrose solution the vacuole allows water into the cell, as there is a higher concentration of water outside the cell than there is inside the cell, and due to the law of osmosis, water particles will move from a high concentration to a low concentration across a semi-permeable- membrane, until equilibrium is established. The cell once again becomes turgid, as the vacuole increases in size. If the cell did not become turgid, then the cell would simply crack, as there would be such a vast amount of pressure from the increasing vacuole acting on the cell wall, which gives the cell support. Less water particles move, as it takes fewer water particles to mover to establish.
The third batch of potato chips, which were in the sucrose solution of 25%, was odd. This is because all the other results go down, then at 25% it goes up again.
The fourth batch of potato chips, gave me an idea as to what the % concentration of the cell sap inside the potato would be. As the percentage change was only 0.7%, which was the lowest so far. This is because it takes fewer water particles to establish equilibrium; there is not a great unbalance of water particles between the two solutions.
The final batch of chips, which was put in the 100% sucrose solution, fell for the first time. There was a percentage change was –2.3%. The reason the cells mass decreased was that there must have been a higher concentration of water in the cell than in the beaker, so therefore, the vacuole released water particles travelled across a semi-permeable membrane, and then into the beaker. This would cause the cell to become flaccid. This means that the percentage concentration of cell sap, must be less than 100% and greater than 50%. I can predict that it will be closer to 50% than 100% because 50% is only a little off.
This table proves, that the percentage concentration of cell sap must be less than 100% and greater than 50%, as it is between these two points that an equal equilibrium is created without the movement of water particles due to osmosis. It can’t be higher than 100% because the cell vacuole is then releasing water to create equilibrium in the beaker and before 50%, the cell is receiving water in order to establish equilibrium.
I have plotted the results of my experiment onto a graph and the graph has negative correlation. From the negative correlation osmosis is proved, defined as when water molecules move from a higher concentration into an area of lower concentration across a semi-permeable membrane. The graph shows that as the sugar concentration increases, the percentage difference in mass decreases. Therefore proving the prediction made that, as the amount of salt increases, the level of osmosis decreases and the mass of the potato tissue will as well.
By looking at my graph it is possible to see that there is a curved line of best fit for the % change of mass and also the actual change. From that it possible to predict roughly what the % concentration of cell sap is inside the potato cell, by reading the value where my line of best fit crosses the x-axis. This is because, is the % concentration of the sucrose solution was equal, osmosis would not have occur as the balance of water particles would be equal therefore no water particles would need to move to establish equilibrium.
Therefore I am able to conclude that the % concentration of cell sap contained in the vacuole of the potato cell is approximately 65%.
From our results we can see that at the start of the experiment the potato tissue was turgid, and we can see from the different concentrations of solution which of these become flaccid through osmosis, therefore reflecting in the mass and the difference in our results.
The results obtained clearly collaborated my prediction so it was accurate.
Evaluation
I think that the experiment we did was a success. We were able to gather the results we needed to be able to draw a conclusion from them, even though we did have one anomalous result, which did not fit my line of best fit.
This anomaly could have been caused by many different factors.
- The temperature in the classroom would have changed a lot and I can’t be certain that each beaker was constantly in the same temperature surroundings as the others. The temperature should be kept the same because if it were hotter for one beaker than the other, the hotter beaker’s molecules would be given more kinetic energy from the heat and so, would speed up the rate of osmosis. Making it an unfair test.
- After cutting the potato we could not put each one into the solution at the exact same time, this may have caused osmosis to continue for a longer amount of time in some than in others.
- We could not take the chips out of the beaker and weigh them at the same time, time this may have caused osmosis to continue for a longer amount of time in some than in others.
- People could have interfered with the % concentration of the sucrose solution, as groups may have poured too much of one solution into a beaker and then pour it back into a different solution of sucrose solution, e.g. left over liquid from 100% poured into 0%. Also the solution might not have been measured to a good enough degree of accuracy to be certain it was exactly 25% sucrose for example.
- Each potato chip was a different size, whether length, width or thickness. This could mean that one set of potatoes could take in more water, or had a higher % concentration cell sap than the others as in a bigger potato chip there would be more potato cells.
- Where you cut the chips from the potato may have affected our results, as there may be a different concentration of sugar in certain parts of the potato.
- Our potato chips were from two different potatoes. As no all potatoes are the same, the % concentration in the different batches, could have affected our results. Also different potatoes could have different membranes - their permeability could be different which would affect the rate of osmosis.
- With 25% sucrose solution, which was the odd result, the original mass was much lower than all the others; I think this could have been a reason for it being such a “freak” result.
- When we dried the potato chips at the end to get rid of any excess water, we no doubt will have spent more time on one than on another batch, therefore leaving more excess water on one than other, therefore affecting the mass.
If I were to do this experiment again, I would try to eliminate most possible anomalies to make my experiment a more reliable one.
- There is not much I could do about the temperature of the classroom, but I would keep all of the beakers in a shaded area, to prevent any drastic change.
- I would use a stopwatch to accurately time the period the tissues were in the solution for.
- I would mix up my own % concentration of sucrose solution, so that we only had enough for our pair, therefore eliminating my theory of solutions being interfered with by other pairs.
- I would spend more time and effort, in trying to make sure that the size and mass of each batch of potato cells were the same.
- Even if 25 minutes was an adequate enough time space to give us clear results in which to support a firm conclusion, prove a predication and clearly show that osmosis took place in the experiment, if I were to increase the time of which the potato cells where left in the beakers, equilibrium would have had ample time to have been established.
- I would redo the same experiment another time and compare the results, using more types of sugar solution, for a longer period of time to allow osmosis to occur and I will compare the different results obtained.
- I would make sure the potatoes were new and clean, as if they had been in the air for a while, they might have absorbed oxygen and water vapour. Also by making sure we didn’t peel the potatoes until we were ready to place them in the sucrose solution would also help to overcome this problem.
- Try and make sure that each batch of potato chips is in for the same amount of time, i.e. first batch to be put in solution, first to be taken out of solution etc.
- Use the same potato throughout the experiment.
- Spend the same amount of time drying the potatoes once they have been taken out of the sucrose solution.
Incorporating these changes into our plan, would have given us more substantial results, which would have allowed us to draw a firmer conclusion. Even with these anomalies taken into account I think I can still draw a firm conclusion that as the sugar concentration increases, the percentage difference in mass decreases. But nonetheless I couldn’t draw a firm conclusion to the question,
“What is the concentration of cell sap in the potato cell?”
which was the original question we were asked. Although being able to predict that the concentration of cell sap in the potato cell was about 65%, that was only a rough estimate, and considering all of the weaknesses of my plan it is not possible to come to such a conclusion.
I personally think that there are a lot of good ways of extending the investigation, as follows.
- Try out different types of vegetables to find out what the concentration % is for them.
- Try it out on fruits, and see if there is a vast difference in the % concentration of cell sap between a piece of fruit and vegetable.
- See whether sugar needs to be in a liquid form, so instead of using a sugar solution, use plain granulated sugar.
- Try salt, instead of sugar, to see if it would work and would there be the same type of pattern.
Laura Macleod – 10PT – 10SC1
