Osmosis is the movement of water from a high concentration to a low concentration through a partially permeable membrane. This lowers the concentration of the strong solution, and evens out the concentrations on either side. Biological knowledge The water molecules can actually pass through the membrane in two directions, but as there are more molecules on one side than on the other, there is a net flow of water to the side with less water molecules.
The sucrose molecules are too large to pass through the partially permeable membrane (also known as a cell surface membrane), so can not pass the other way, and the partially permeable membrane does not allow dissolved substances to pass through. The completely permeable cell wall in plant cells acts as a resistance against the partially permeable cell surface membrane. The cell wall is very strong- strong enough to stop the cell bursting, even when it is full. Even when the cell loses water, the cell wall stays reasonably firm, although the cell becomes a lot less rigid.
Pure water has a water potential of zero, so solute makes the water potential lower- or more negative. This means that water moves from a less negative water potential to a more negative water potential. At the end of the experiment, I will look at the net mass difference of the potato cylinders, that is, the overall movement of water which has taken place as a result of osmosis happening. Osmosis is the process by which plant cells become turgid- that is, when the osmotic pressure builds up and the cell swells. Plant cells are not pure water, they are solutions, o they have a lower water potential than pure water.
This means that when the cell is placed in pure water, water molecules move into the cell, causing the cell to swell. However, plant cells do have a higher water potential than a strong sugar solution. This means that water passes out of the cells when they are placed in a strong sugar solution. As water passes out, the vacuole of the plant cell starts to shrink and the cells become flaccid (or limp). When the cytoplasm starts to peel away from the cell wall due to so much water leaving the cell, then the cell is plasmolysed.
Variables An independent variable is something that I can change and in this experiment, my independent variable will be the concentration of the sucrose solution in which the potato cylinders are placed. The dependant variable of this experiment will be the percentage difference of the net mass of the potato cylinders as a result of water moving in or out by osmosis. The control variables, which I must keep the same in order to be able to compare results at the end of the experiment, will be the volume of sucrose solution and the size of the potato cylinders.
If either of these variables is different at the start of the experiment, then my results will be unreliable as I may measure the results to be larger or smaller than they actually are. One variable that I can not control is the temperature of the room when I do the experiment, and this may have an effect on my results, as the warmer the room, the more kinetic energy means that the molecules move faster and so diffuse faster. Precision To increase the accuracy of my results, I will use burettes to measure the volume of sucrose solutions as, although this method takes longer than using a measuring cylinder, it is likely to be more precise.
Another piece of equipment that is likely to be precise is a scientific balance measure, which measures to 0. 01grams, which is far more accurate than a normal scale reading so I will use this to weigh my potato cylinders. To make sure the potato cylinders are the same size I will use a cork borer, as they will then all have the same diameter, and by using a scalpel and cutting the ends straight so that the cylinders are the same length, they will all be the same volume.
Vernier callipers, which measure to 0. 1mm, can give me a precise length. I will use mass to compare all the cylinders, so I will weigh the cylinders before and after, and I will look at the percentage difference. I will also have to remove the potato skin, if there is any, from the cylinders, as the skin is less permeable and leaving it on could affect the net movement. For safety, and to protect the work surface, I will do all the cutting of potatoes on a white tile. Preliminary work
I am going to place 2 potato cylinders in each boiling tube, but each of these two cylinders will be left in for a different amount of time. By doing this I can see how time affects osmosis and I can then compare these results to the results I get from other boiling tubes, and from this I will also see how concentration affects osmosis. Another advantage of having the two cylinders in the same boiling tube is that it cuts down on the amount of equipment I need to use in this experiment.