The Effect Of Concentration On Osmosis In Plant Cells

Hypothesis:

The following factors all effect the rate of osmosis:

* Surface area of the tuber

* Concentration of sucrose solution

* Amount of sucrose solution

* Temperature

* Time tubers are left in solution

-The surface area of the potato tuber will effect the rate of osmosis because The more the surface area, the more osmosis will occur because more water can diffuse because there will be more area for it to pass through. I will control the surface area by making all my tubers weigh the same. This should ensure that they have roughly the same surface area.

-The amount of sucrose solution will effect the rate of osmosis because there more solution there is, the more water there will be to pass through e.g. If the solution does not immerse the tuber, less osmosis will occur.

-The temperature will effect the rate of osmosis because the particles will move faster in a higher temperature so more will pass through.

-The time the tubers are left in the solution will effect the total amount of osmosis because the longer the tubers are left in the solution the more time the reaction has to occur, and therefore the greater the tubers change in weight.

In this investigation I will be looking at the effect of concentration of the bathing solution on osmosis in plant cells. I will be using sucrose solution and potato tubers to conduct this experiment. Potato cells have a partially permeable membrane. The partially permeable membrane is similar to a tea strainer in that it will let small things, such as the tea, pass through, but not big things, such as the tealeaves. Similarly, the potato’s partially permeable membrane will let small molecules, such as water, pass through, but not big molecules, such as sugar. When the potato tubers are placed in water, diffusion occurs. This is the movement of particles from a high concentration to a low concentration until they are spread out evenly. In this investigation I will look at a particular type of diffusion called osmosis. Osmosis only involves water molecules. Osmosis is water diffusion.

The diagram above shows osmosis taking place. As it shows, sugar molecules are too big to pass through the membrane, as it is only partially permeable. The water passes both ways through the membrane until the internal and external solutions are of the same strength.

When water passes into the cell from the outer solution the cell swells. It will then be turgid. When the opposite happens and water is passed out, causing the cell to shrink, the cell is plasmolysed. This can also be called flaccid.

When the tuber is placed in the 0 Molar solution (pure water) I think the tuber will become turgid. This is because the solution inside the tuber will be stronger than the solution outside the tuber. In order for the internal solution of the tuber to be equal with the external solution, the water from outside the tuber has to diffuse inwards through the partially permeable membrane to dilute the inner solution. The tuber’s gain of water will cause it to expand, and therefore become turgid.

When the tuber is placed in the 1 Molar sucrose solution I think the cells will become plasmolysed. This is because the solution outside the tuber will be stronger than the solution inside the tuber. In order for osmosis to occur and the solutions to be of equal strengths, the water from inside the tuber will mainly diffuse outwards through the partially permeable membrane to dilute the outer solution. The tuber’s loss of water will cause it to shrink, or plasmolyse.

The greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater the amount of water that leaves the cell. Therefore I predict that the lower the concentration of sucrose of the external solution, the more the potato tuber will become turgid, and the higher the concentration of sucrose in the external solution, the more the potato tuber will become plasmolysed. As the concentration increases from 0 M to 1 M I expect the tubers size and weight to increase.

Method:

In this investigation I am looking into the effect of osmosis on potato tubers when they are placed in different strength sucrose solutions. It is easier to ensure a fair test when only using one variable. So all the other variables will be kept as constant as possible to ensure the test is fair. I will keep the size of the tubers constant by weighing each tuber before I use it in the investigation. I will also measure the amount of sucrose solution before using it in the investigation. I can also measure the length of time the tubers are left in the solution.

I will conduct all the experiments at room temperature, which may change slightly, but should stay approximately the same, or only change by a few degrees, which would not have a big effect on the investigation. We will move the tubers around using tweezers to make sure they are not contaminated with sugar solution this, again, will ensure a fair test.

1. I will measure the tubers and, using the scalpel on the tiles, I will cut them down to the same weight as my lightest tuber. This will ensure they will all be the same size. I found out in a preliminary experiment, that my smallest tuber was 8.85g, so this is how much I will make all my tubers weigh. I will lift the tubers on and off the scale using the tweezers to make sure they are not contaminated

2. I will label my test tubes A, B, C etc so I can recognise them.

3. I will mix up my solutions before starting to put the tubers in, so I can put them all in at the same time. This will ensure they are all left in for the same amount of time. I will create my solutions by diluting the 1 Molar solution with the appropriate amount of distilled water. For example, when creating a 0.5 Molar solution we would mix 10ml of distilled water with 10ml of 1 Molar sucrose solution.

4. I will then place my tubers in 20ml of sucrose solution, ranging from 0M (pure water) to 1M (0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1M) and leave them for 20 minutes. This is enough time for osmosis to occur. I will again move the tubers around using tweezers.

5. After 20 minutes I will remove the tubers from their solutions. I will remove any excess solution from them with blotting paper before re-weighing them and recording my results. I know that 20 minutes is long enough, because I found this out in my preliminary experiment. (the results of which I used in my final results table).

6. I will carry out each test 3 times and take an average result, this will help make my results more accurate and also help me to spot any results that do not fit.