Hypotheses
I predict that the potato cylinders in the higher concentration sucrose solutions are more likely to lose mass through osmosis, and in the lower concentration solutions the potato cylinders would gain mass – especially in the distilled water. I predict the rates of osmosis will look like this:
As we can see from my graph prediction, I think that the two lowest molarities – distilled water and 0.2 mol will increase the mass of the potato cylinder, whereas I think that from 0.4 to 0.8 mol of sucrose solution will decrease the mass of it.
The reason I think this is because of the principals of osmosis. Osmosis is the movement of particles of a substance from a high concentration to a low concentration through a partially permeable membrane. In the case of our potato cells, this means that water would move out of the potato cells if there was a higher concentration of impure water outside the cell than inside. However larger molecules either would not be able to pass through or would pass thorough very slowly, so the two solutions could not ever be the same concentration.
The following graph is what I predict will look like if I plot the end results of the experiment for rate, onto it. It shows the % mass change of the potato cells changing according to the molarity of the sucrose solution it is put in.
This graph shows that there would be no mass change in the potato cells if they were placed in a solution of 0.27 Moles. I am able to predict this because in one of my secondary sources of information, (Roberts A – Level Biology) it states that the molarity of a potato is about 0.27 Moles. This means that potato in a solution of this molarity would neither gain nor lose mass as the water molecules would not be moving as there is no concentration gradient. The straight line suggests a directly proportional change in mass to concentration and the curve shows the rate of osmosis being much faster at extremes of concentration, and the rate of osmosis decreasing as the concentration is closer to 0.27 Moles.
As the time goes on as we perform the experiment, the rate of osmosis decreases. If I were to plot a graph of rate of osmosis against time, it would decrease quickly at first, but level out after:
The graph on the left shows the rate of osmosis decreasing as time passes, and the right graph shows the initial rate as high on either side of 0.27 Moles, but decreasing towards the stated concentration of potato cells – 0.27 Moles.
Method
In this experiment, we will need to make it a fair test by making sure that all other factors that are not being tested are constant. This investigation seeks to investigate the factors affecting the rate of osmosis, so each experiment will need to be carefully timed.
We will need potatoes that will be cut up for the experiment, several beakers with various concentrations of salt or sugar solution, an accurate balance of grams to 2 decimal places, and we will need to time the experiment. In order to make the test as fair as possible, we will use a cylinder of metal which can be pushed into the potato and drawn out, bringing a cylinder of potato with it. These cylinders will be uniform in length and width and height, so the results obtained will be as accurate possible:
Potato cylinder cylinder cutter
Potato
Several of these cylinders will be cut from the potato and put into Petri dishes containing different molarities of sucrose solution – distilled water, 0.2 mol, 0.4 mol, 0.6 mol and 0.8 mol of sucrose solution. The ends of skin must be cut off so the solution is allowed to enter all the cells and so they are the same length. Each cylinder shall be taken out and weighed every 5 minutes on electronic scales. Through weighing them as often as this, it will be possible to create an idea of the rate of osmosis.
Because there will be solution on them when they are taken out, they will be rolled once forward on a piece of kitchen towel by the same person so that the solution on it does not affect the mass of it. The same person must roll the cylinder so it will be more likely to be a fairer test as the person would dry them the same amount each time, so would be more likely to be as dry as each other.
Results
The above table shows percentage mass change as it was too hard to make each cylinder the same mass to obtain fair results.
Analysis
Our results seemed quite accurate, apart from a few irregular ones, as they showed what I expected. The results we have obtained seem to show us that as you decrease the molarity from between 0.4 and 0.2 moles, the rate of osmosis into the potato cells increase, but as you increase the molarity from between 0.4 and 0.2 moles, the rate of osmosis out of the potato cylinder increases. The fact that it is only somewhere after 0.2 moles that the potato cylinder starts to lose mass, shows that Roberts A – Level Biology book is probably right in saying that the molarity of potato cells is 0.27 moles.
The reason behind why the potato cylinders gain or lose mass is stated in my hypothesis – the water molecules will move in the direction of the concentration gradient; if the solution the cylinder is in is 0.6 moles, that is higher concentration than the potato cells, so an act of balancing out takes place. Water molecules leave the cells through the cell membrane, so it is becoming more concentrated in potato, and losing mass. At the same time, the water molecules are moving into the solution, therefore diluting it and making the concentration closer to that of the potato cells. This process is shown in the following diagram:
Plant cell at start of experiment:
Large water-filled vacuole
0.6mol solution
Cell wall cytoplasm
Plant cell at end of experiment:
The sucrose molecules are too large to pass through the cell membrane, however the water molecules can pass freely through the membrane. Because of this, in the experiment with distilled water, the potato cells can not be the same molarity as the distilled water because they will always have some impurities in them, but the distilled water will not.
Evaluation
I think the method we used to conduct this experiment was good, and could have produced better results if we had been more careful. The experiment went quite well considering the amount of accuracy needed to obtain valid results, although due to a few mishaps there were some anomalous results. The potato cells were not dried exactly the same amount each time, they were not each the same in mass, the scales had residue on them from previous measurements of weight. From the results, it seems as though the 0.8 and 0.6 may have been mixed up, which is why the 0.6 generally had less mass. The scale we used was near to the open window sometimes affecting the results when there was a breeze. It is my opinion that this is the reason that our results for the mass of the potato cylinders in the 0.6 mol solution was not as expected.
If I were to perform the same experiment again, I would cut each potato cylinder the same length (making them the same mass) and from the same potato. I would also dry each cylinder the same amount by rolling it the same length each time, using a new sheet of kitchen towel each time.
If we were to further investigate rates of osmosis, we could see the effects of the other factors: pressure, heat and surface area (pressure would be too hard for us to carry out with the limited apparatus we have).
To investigate heat, we could use the same concentration of solution each time, the same mass and surface area of potato cells, but each solution could be kept at a constant heat. We could place one experiment in a fridge, one at room temperature, and one could be kept on a low heat on a stove or other safe heating apparatus. Each experiment would have a thermometer with it to check the temperature. It would, however, be quite hard to measure rate of osmosis, especially in the case of the experiment in the fridge.
To investigate surface area, we could cut potatoes so that they each had the same mass, but varying surface areas. Then the same method of finding rate would be used as done in the concentration experiment but the concentration would be uniform in all cases.
Bibliography
- Advanced Biology 1
- Roberts A – Level Biology