Effect of Osmosis on Potato Cylinders.
The write-up of my investigation.
Aim:
This experiment is to investigate osmosis on potato cylinders in different moles of sucrose solution ranging from distilled water to 1.0 mole in even steps.
Prediction:
Osmosis occurs when water is transported from a high concentration to a low concentration, through a partially permeable membrane. In this experiment we should find that the cylinders in the distilled water should become turgid because of water diffusing into the vacuole, making the cells increase in weight and length.
The cylinders in the higher concentration of sucrose solution should loose weight and length and so become plasmolysed because of water diffusing out of the vacuole. This causes the cytoplasm to pull away from the cell wall and so making it flaccid.
After collecting the data I would expect a graph to look like this. The distilled water would become turgid and the strongest molar solution would become plasmolysed. I would expect to find that the weight and the length would increase or decrease by the same amount for each cylinder and so they would either be the same curve or slightly off as shown in the graph opposite.
Below is a diagram showing what happens when diffusion occurs across a partially permeable membrane. The membrane has small gaps, which a low small cells through, such as water and other molecules. Larger cells are not aloud through, these are molecules such as proteins, sucrose, and salt.
Though I do not know what to use as a control at the moment, after the experiment I can find the place at which the line on my graph crosses the x-axis and then this could be used as my control if I chose to re-carry it out. This would also be the point to find out how many substances are in the potato; this is also called the sap. This too would be shown on the x-axis where the line crosses it.
I will carry out the method I have chosen because it doesn't seem to be too complicated, but easy to follow through. Also during year 9 I carried out an experiment very similar to this one, using potato cylinders and watching how osmosis would occur. Though the experiment had the same idea, we only used one type of solution, tap water. I can use the notes I took from this investigation to help me plan my method and I can also compare my results and see if they are similar or not. Hopefully they will be, but if not, I can then see where I went wrong, (if the experiment does actually go wrong in any way!). From this old experiment I can also decide how I will measure my results. I will measure the potato cylinders in the experiment to the nearest millimeter and weigh the cylinders to the nearest milligram. These results should then be reliable enough for me to evaluate the investigation I have just carried out.
Plan:
To carry out this experiment I must follow through this list of instructions:
. Set up six different petri dishes each containing 30cm3 of different molar solutions of sucrose; distilled water, 0.2, moles 0.4, moles 0.6, moles 0.8 moles and1.0 mole, and label them (also indicating the place where the A, B, and C cylinders should go).
2. Cut off the skin of the potato and then using a cork borer to make several potato cylinders each weighing 1 gram and with a length of 3 centimeters.
3. Put the three cylinders ...
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Plan:
To carry out this experiment I must follow through this list of instructions:
. Set up six different petri dishes each containing 30cm3 of different molar solutions of sucrose; distilled water, 0.2, moles 0.4, moles 0.6, moles 0.8 moles and1.0 mole, and label them (also indicating the place where the A, B, and C cylinders should go).
2. Cut off the skin of the potato and then using a cork borer to make several potato cylinders each weighing 1 gram and with a length of 3 centimeters.
3. Put the three cylinders (A, B and C) in the different concentrations of sucrose solution and leave for 15 minutes.
4. Take out the cylinders, dry them on a piece of filter paper, then weigh and measure them.
Apparatus:
For this particular experiment I would need the following list of requirements. The six different moles solutions, six petri dishes, a measuring cylinder, a cork borer, a pair of tweezers, a knife, a cutting board, a piece of filter paper, a weighing scale, a ruler, a stop watch, and a marker pen.
For this experiment the measuring cylinder will be a tall and thin one to ensure that the amount of solution we have out in is as accurate as we can get it. Also we shall measure our results with a weighing scale which can measure in decimals, and a ruler which can measure in millimeters, again to ensure the readings which we will obtain are the most accurate.
Fair Test:
So the experiment was a fair test several variables were kept the same to ensure that our results were accurate and correct. These were:
. The volumes of the sucrose solutions were always kept the same throughout each experiment.
2. The same size of cork borer was used in each experiment.
3. The measuring cylinder used to measure the volume of the solutions was washed out after each measure was taken.
4. The temperature was not altered during the experiment, room temperature was used throughout.
5. The weighing scale was always put on zero at the beginning of each reading
6. Each experiment was conducted three times, just incase any readings were read incorrectly or a fault happened during the experiment.
Safety:
My group and I carried out several safety checks to make sure no harm could be inflicted on our selves or other members of our class:
. Sharp objects such as the cork borer and knife were kept in the tray when not in use.
2. When cutting off the potatoes skin and boring holes, a board was used, it was not done in our hands
3. For obvious reasons, the potato and the sucrose solutions were not eaten or drunk.
Method:
For our first experiment, and for our second experiment, we started off as we had planned, leaving the cylinders in the solutions for 15 minutes. Unfortunately the results we obtained were not what we had expected and had hoped for. Instead of a dramatic change in the weights and length of our cylinders we had very little change and even the odd gain. So for our third experiment we changed our variables a little, making the waiting time 2 and 3/4 hours. We also increased the length and weight of our cylinders as we thought this too would improve our results, as there was more of a surface area for osmosis to occur. As we did not have enough time to re-repeat the experiment we stayed with the results we had and found the averages from the cylinders A, B and C.
Analysis:
After completing this experiment I have found out that the results I have obtained support my hypothesis. The potato cylinders expanded in the high external water concentration, and the cylinders in the low external water concentration contracted. From the graph I have drawn of the average percentages of increase or decrease, it clearly shows that the stronger the molar solution the more the cylinders decrease in mass and length, whereas if the external solution is weak the cylinders increase in mass and length.
The experiment using potato cylinders in distilled water, increased in mass and length. This is because the cells saps osmolarity has a lower water concentration than the concentration of sucrose that the cylinders were in. Therefore water travels across the concentration gradient from the concentration of sucrose in to the cells.
The experiment using potato cylinders in a sucrose solution of 0.2M decreases in mass at 3% but in length it increased by 1.1%. There is obviously an error in one of these results but this could have been due to poor weighing or measuring techniques, which could be improved by using more sensitive apparatus. Th decrease in mass suggests that the concentration of water in the potato cell was lower than the water concentration in the solution outside the potatoes. The increase in length though, suggests that the concentration of water in the potato cell was higher than the water concentration in the external water solution.
The experiment using potato cylinders in a sucrose solution of 0.4M is very similar to that of the results in the experiment using a sucrose solution of 0.2M. There was a decrease in mass of 13.6% but an increase in length of 3.3%. In this experiment there must also be an error, as the length could not increase if the mass decreased. Th decrease in mass suggests that the concentration of water in the potato cell was lower than the water concentration in the solution outside the potatoes. The increase in length though, suggests that the concentration of water in the potato cell was higher than the water concentration in the external water solution.
For the experiment using potato cylinders in a sucrose solutions of 0.6M there was a decrease in mass of 33.2% and a decrease in length of 20%. This shows that the concentration of sucrose inside the potato cells was weaker than the concentration of sucrose in the solution. The water traveled across the potato's semi permeable membrane and into the solution outside the potato.
For the experiment using potato cylinders in sucrose solution of 0.8M there was a decrease in mass of 42.3% and a decrease in length of 7.8%. This shows that the concentration of sucrose inside the potato cells was weaker than the concentration of sucrose in the solution. The water traveled across the potato's semi permeable membrane and into the solution outside the potato.
For the experiment using potato cylinders in sucrose solution of 1.0M there was a decrease in mass of 18.2% and a decrease in length of 15.6%. This shows that the concentration of sucrose inside the potato cells was a lot weaker than the concentration of sucrose in the solution. The water traveled across the potato's semi permeable membrane and into the solution outside the potato to make the concentration on both sides of the potato's semi permeable membrane equal.
Both the mass and the length show high negative correlation. From this graph I am also able to estimates how much sap is within the cells of the potato, this is about 0.1grams and is shown where the mass line crosses the x-axis. This is where the water concentration is equal to the external sucrose solution.
Evaluation:
Though I am fairly happy with my end results you can see from my graph that there are a few anomalous'. Using 0.6M of sucrose solution both the mass and the length dropped dramatically from the line of best fit. This could be because the petri dish was too close to heat or something may have gotten in to the solution besides the 0.6M of solution. This happens again at 1.0 M for the mass, but this time it increased dramatically. I would have expected it to have a decrease of about 45% but instead it had a decrease of 18%. This again could have been because of poor readings or because some thing else went wrong wen setting up the experiment.
I f I could repeat this experiment I would still put the cylinders in their solutions longer and maybe increase their surface area. Also I would have a more of a vary on the sucrose solutions like having 0.1M, 0.3M, 0.5M, 0.7M, 0.9M and even others to find the exact osmolar concentration of the cells sap. This would make the results I obtained even more reliable. Other wise I think the method my group and I used was good, and successful. The results were also reliable, even though I could get yet more reliable results, as I have explained above, for the experiment I carried out.
Osmosis on Potato Cylinders.
Kathryn Hinchcliffe.
