Ukraine Population Crisis

Biology Coursework - Osmosis

Aim:

The aim of this investigation is to discover the concentration of sugar within the cells of potatoes.

Prediction:

Osmosis is the net movement of water molecules in cells from a high to low water potential through a partially permeable membrane as shown below.

In plants, when a cell has a high water potential and contains a lot of water is known as turgid. Cells with low water potential and less water are known as plasmolysed. They have the appearance of being sunken and smaller where as turgid cells are swollen and fat as shown below.

In this investigation I will be using osmosis in the cells of potatoes and attempting to discover the concentration of sugar in the cells of the potatoes. I hope to achieve this by placing the potatoes into solutions of sugar and water of known concentration. This will enable me to view how the potato cells change while they carry out osmosis, wether they lose, gain or retain the same mass. I will be using 5 different sugar solutions of 0% (pure water) 5%, 10%, 15% and 20% sugar. Because I know that the solution inside potato cells is not pure water and contains at least some sugar I expect the potato placed into the pure water to gain mass because the pure water has a higher water potential and will travel through the potatoes partial permeable membrane by osmosis and the mass of the potato will increase. This is shown in the diagram below.

However, I doubt that the unknown solution inside the potatoes is stronger than 20% because potatoes are not sweet tasting and they would be if the cells had a 20% sugar concentration. Therefore I expect that the potato placed in the 20% sugar solution will decrease in mass because it will have a higher water potential than the strong sugar solution it is placed into therefore the net movement will be from potato to solution and osmosis will occur through the partially permeable membrane. This is shown in the diagram below.

In order to discover the concentration of potato cells it will be necessary to match this unknown concentration with the solution it is placed in. If this is achieved then there will be no net movement because there will be no difference in the concentration gradient from difference in concentration of sugar in the solutions and as a result there is no change in mass. This is shown in the diagram below.

However it is extremely unlikely that the concentration will be exactly 0% 5% 10% 15% or 20% which I am testing. It will almost definitely be in between 2 of these values. Therefore one accurate way to discover what the concentration is will be to draw a graph and determine from it the point at which the change in mass of the potato was 0. When it is zero it shows that the concentration in the outside solution must have been the same as the potato because there will be no slope in the concentration in the gradient and no change net movement of water molecules. I expect this to appear on the graph roughly as shown below.

Variables:

In this investigation all the variables apart from 1 must stay the same throughout all experiments. If this happens, then it will be clear which factor causes any changes in mass in the potato chips. Below are listed the variables which will stay constant.

1. Number of potato chips placed into each beaker must be kept the same because that way every chip is surrounded by the same volume of solution and submerged in the same way to avoid loads of chips being put into 1 beaker which could mean they are in contact during the experiment which would decrease the surface area on which osmosis could take place.

2. Containers used to hold potato chips will stay the same because this way the chips will always be exposed to a certain amount of solution and can be the same distance apart from each other reducing the risk of changing the area which osmosis can take place on.

3. Volume of sugar solution will stay the same otherwise it would be possible to plot discover the sugar concentration because the readings would change a lot more if the volume is increased.

4. Length of time the experiment is left running each time will stay the same to ensure that all chips have time to change in mass for the same time otherwise some chips would change more if left for longer therefore making it impossible to plot a graph.

5. Method used to dry chips making sure that only external solution on the outside of them is dried off will stay the same so that the same amount of solution is dried off on each chip because if dried on paper towel for longer it would absorb water from inside a potato chip, and not another if dried for a much shorter amount of time meaning that one would have decreased in mass more making results inaccurate.

The variable which will change is the sugar concentration of solution which the potato chips are placed into.

Preliminary work:

Before carrying out the real experiments I carried out preliminary tests so that I would be able to make sure the real experiment would achieve more accurate and reliable results that I will need. I carried out 3 repeats for each solution to increase the reliability of the results. Below are the results from the preliminary work.

Sugar concentration (%) Start masses of potato chips (g): Final masses of potato chips (g): Average change in mass (g): Time left for (minutes):

0 (pure water) 3.75 3.62 3.66 3.91 3.72 3.84 + 0.18 30

20 3.63 3.65 3.56 3.26 3.27 3.31 - 0.33 30

The results of the preliminary work do support my prediction and show that