An investigation into the effect of Sucrose concentration on osmosis

Aditi Gupta LVM

AIM:

The aim of my experiment is to investigate the effect of sucrose solution on osmosis in potato cells, and what the point of equilibrium would be. I shall be observing how the mass of the potato chip changes in five different concentrations of glucose solutions.

HYPOTHESIS:

Osmosis is the diffusion of water molecules from a high water potential to a lower water potential through a partially permeable membrane. A partially permeable membrane, such as a cell wall, allows small molecules, such as water molecules, pass through it, but does not let bigger molecules such as sucrose through it.

Cells placed in distilled water take up water by osmosis. This happens because the cell contains dissolved chemicals and therefore has a lower water potential than the distilled water surrounding it. As there are more water molecules outside the cell, more go in to the cell than out of it. The net movement of the water molecules is into the cell, and the cell will increase in mass. Eventually the cell stops taking up water, even though the concentrations inside and outside are not even. This is because the cell wall becomes stretched and prevents any more water entering. The cell is said to be turgid.

Cells placed in a concentrated sugar solution lose water by osmosis, as the water potential is higher inside the cell. As there are more water molecules inside the cell, more water molecules leave the cell than enter it. The net movement of the water molecules is out of the cell. This causes the cell to become flaccid, and for it to decrease in mass.

When a cell is placed in a concentration of sugar solution that has the same water potential as the cell sap, an equal amount of water molecules go in and out of the cell. There is no net movement of water molecules and the mass of the cell will stay the same. This is called the state of equilibrium.

For this investigation, I predict that the lower the concentration of the sugar solution in the test tube, the larger the increase in mass of the potato will be. This is because the water molecules pass from an area of high water potential, the water itself, to an area of low water potential, in the potato cylinder. Therefore, the potato cylinders in the concentrations of sugar with lower water potential will decrease in mass, as the water potential is higher inside the potato cylinder, than in the sugar concentration.

By looking at my graph for the results from my pilot study, I also predict that the point of equilibrium will be at 0.2 Mole sugar solution.

APPARATUS:

1 test tube rack – to hold the test tubes in during the experiment.
1 large potato – for the potato cylinders.
10 test tubes – 5 to hold each concentration, and 5 for the repeat experiment.
1 ruler – to measure the length of the potato cylinders to ensure they are all 3cm.
1 chinagraph pencil – to label the test tubes and the different solutions.
1 digital balance – to measure the mass of the potato cylinders.
Distilled water – to make up the different concentrations.
1.0 Mole sugar solution – to make up the different concentrations.
2 syringes – 1 for the distilled water and 1 for the 1.0 M solution to measure the amounts needed to make up the different concentrations accurately.
1 beaker – to hold the 1.0 M sugar solution.
1 plastic cup – to hold the distilled water.
1 cutting tile – to cut the potato cylinders on.
1 knife – to cut the potato cylinders to the right length.
1 apple corer – to cut out the potato cylinders with.
1 sieve – to empty out the solutions and catch the potato cylinders.
Tissue paper – to dry the potato cylinders before weighing them.
Clock – to measure the time the potato cylinders are in the solutions.

METHOD:

Make up 20cm³ of each of these concentrations of sugar solution:

0.0 M
0.2M
0.5M
0.7M
1.0M

To make up each concentration follow this table:

Pour 20cm³ of each solution into separate test tubes.

Cut out 5 cylinders of potato using an apple corer, and cut each of the to a length of 3cm.

Dry each cylinder of potato thoroughly with tissue paper and weigh each one on a balance, rounding to 2 decimal place.

Put each cylinder of potato into a ...

This is a preview of the whole essay

METHOD:

Make up 20cm³ of each of these concentrations of sugar solution:

0.0 M
0.2M
0.5M
0.7M
1.0M

To make up each concentration follow this table:

Pour 20cm³ of each solution into separate test tubes.

Cut out 5 cylinders of potato using an apple corer, and cut each of the to a length of 3cm.

Dry each cylinder of potato thoroughly with tissue paper and weigh each one on a balance, rounding to 2 decimal place.

Put each cylinder of potato into a test tube of sugar solution, one in each concentration.

Leave them in their respective solutions for 25minutes.

Empty out each test tube of solution, one by one, using a sieve to catch the potato cylinders.

Dry the potato cylinders thoroughly with tissue paper. Weigh each one again and note the results in a table.

For reliable results, repeat this experiment again.

To ensure that my results are as fair and accurate as possible, certain aspects of the experiment will have to be kept the same, whilst one key factor is changed. I have chosen to vary the concentrations of the sugar solutions, whilst keeping everything else the same. This will be my independent variable, and will give me a varied set of results from which I hope to make a good conclusion. If any of the non-variables below were not kept constant, it would not be a fair test.

Firstly, I shall choose the same variety of potato for each cylinder because the permeable membrane will be similar. This will stop this possible difference from affecting my results.

Secondly, I shall attempt to prepare the cylinders so that they are cut uniformly. This will keep the surface area of the cylinders as constant as possible, so that they all have an equal amount of space for osmosis to occur. I learned from my pilot study, that the best shape to use for the potato is a cylinder. This is because it is quick and easy to cut out, using an apple corer, saving you time, as you do not have to measure all the sides. This also ensures that the potato cylinders will be more equal in shape, and it also has sufficient surface area for osmosis to take place effectively.

Thirdly, I shall leave each potato chip in the solution for exactly 25 minutes. In my pilot study, in preparation for this experiment, I found that this length of time obtained a sufficient amount of results. If I had left them for 15 minutes the results may not have been noticeable and so more difficult to measure.

Fourthly, I shall take care when removing the chip from the solution, particularly when removing any excess fluid, by blotting. Any excess fluid will affect the recorded mass when weighing.

Lastly, I shall use the same volume of solution for each test tube. The potato cylinders must be totally covered in the solution, and the amount of solution will be kept the same, as all the potato cylinders are the same size. In my pilot study, I found that 20cm³ of solution is a suitable amount for this experiment. This is because 10cm³ of solution only just covered the potato cylinder, whereas 20cm³ covers it fully and has space left over. This number is also easy to make solutions of different concentrations from.

I chose to do these concentrations of sugar solution:

0.0M 0.7M

0.2M 1.0M

0.5M

This is because in my pilot study, I found that my point of equilibrium was close to when the concentration was 0.2M. I tried to make my concentration fairly evenly spaced, so that I would get a good line on my graph.

The mass of the potato cylinders is a dependent variable, and this means that it will be measured throughout the experiment. I will measure the mass in grams, measuring to 2 decimal place, using a digital balance so my results will be precise. The potato cylinder will be weighed before it is put into the solution, and after. This will allow us to whether osmosis has taken place, and to what extent.

To make my experiment as reliable as possible with the amount of time given to carry it out, I shall repeat this experiment once and use the average of these results for my graph.

Conclusion

The results shown in my graph and my results table indicate to me that the potato cylinders placed in the lower concentrations of sugar solution increase in mass, whereas the potato cylinders placed in the higher concentrations of sugar solution decrease in mass. Therefore, the lower the concentration of the sugar solution, the higher the increase in the mass of the potato cylinder, and the higher the concentration of the sugar solution, the more the potato cylinder will decrease in mass.

As I have earlier stated, when a cell is placed in a solution with higher water potential than the cell, it increases in mass due to osmosis. A cell that is placed in a solution with lower water potential than itself will decrease in mass, due to osmosis. This is what has happened to the potato cylinders when they were placed in different concentrations of sugar solution:

The potato cylinder placed in distilled water (0 Mole concentration) increased the most in mass (3.56 % more than its initial mass). As the distilled water had the highest water potential out of all the concentrations, more water molecules could diffuse through the cell wall of the potato cells, through osmosis, therefore causing it to gain mass and to become turgid.
The potato cylinder placed in the 1.0 Mole concentration, decreased the most in mass. As the 1.0 M sugar solution had the lowest water potential out of all the concentrations of sugar solution, and the cell had higher water potential, the water molecules from inside the cells passed through into the sugar solution. This happened due to osmosis. The water molecules travelled from an area of high water potential to an area of low water potential, therefore decreasing in mass and becoming flaccid.
The potato cylinder placed in the 0.7 Mole concentration, decreased by 3.53% of its original mass. Therefore it lost mass through osmosis, as the sugar solution had a lower water potential than the potato cells
The potato cylinders placed in the 0.2 and 0.5 concentrations, decreased and increased in mass in equal proportion to each other. The cylinder in the 0.2 M concentration increased by 1/31% of its original mass, therefore the water potential was higher in the sugar solution than in the cell. In the 0.5 solution, however, the cylinder decreased in mass by 1.31% of its initial mass; therefore the water potential was lower in the sugar solution than in the cell. Therefore, the point of equilibrium is about halfway between those two points.

When I drew my line of best fit, the point of equilibrium was at 0.365 Mole sugar solution. This is about halfway between 0.2M and 0.5M.

I had predicted that the lower the concentration of the sugar solution in the test tube, the higher the increase in mass of the potato would be. This agrees with what I have found out. However, using my pilot study graph, I had also predicted that the point of equilibrium would be at when the concentration of sugar solution was 0.2M, whereas my actual point of equilibrium is at 0.365M sugar solution. This does not agree with what I have found out from this investigation.

Considering my investigation as a whole, I think that my data is quite reliable. This is because, although for one of the potato cylinders, the starting mass differed quite a lot from the others, I overcame this problem by converting the change in mass to the percent change in mass of the original mass of each potato cylinder. I did the experiment twice, to ensure I had enough repeats to make my results reliable, and I used the average of these results to draw up my graph. The rest of my starting and ending masses were very similar to each other. Therefore my results are reliable, and if I repeated the experiment again, I think I would get about the same point of equilibrium, give or take 0.05M.

I did not get the same point of equilibrium in my investigation that I got in my pilot study. However, the point of equilibrium I found in my investigation is more reliable than that from my pilot study. This is because I used more concentrations of sugar solution, therefore the average results from these five concentrations, compared to the average results from the three concentrations in my pilot study, will be more reliable, and my line of best fit can show where the point of equilibrium is more efficiently.

The average percentage change in mass for the potato cylinder that was placed in the 1.0M sugar solution may be an anomalous result. This is because it is far away from the line of best fit, whereas all the other points are very close to it. I think that this result is anomalous because on my repeat, the starting mass for the potato cylinder was a bit less than the others. Cutting the length of the cylinder slightly slanted probably caused this. If I could do this investigation again, I would take more care when cutting out the cylinders.

The only thing I would change in my method would be to do two repeats instead of just one. This would make my results even more reliable, however, I did not have enough time to two repeats. I had initially decided on doing two repeats, but I realised I would not have enough time to weigh the mass of each potato cylinder at the end, so I decided to do only one repeat.

I think that my experiment was a fair test because I kept all the non-variables constant:

I used the same potato for all the potato cylinders.
I cut all the potato cylinders uniformly with an apple corer, therefore all the side were the same length.
I left all of the potato cylinders in their test tubes of sugar solution for exactly 25 minutes each; therefore they all had the same amount of time for osmosis to take place.
I used the same volume of solution for each test tube.

Some further work I could do to help me find out more exactly what the point of equilibrium is, is to do this experiment again, however instead of measuring the increase or decrease in mass of the potato, I would measure the change in length of the potato cylinder. Then I would plot the results for that experiment on the same graph I drew for this experiment. Hopefully, the point of equilibrium would pass at the same point on the x-axis, however if it didn’t, I would average out the two points of equilibrium and use that answer for my point of equilibrium.