Linsey Belford U5E
Aim: To find the concentration of cell sap in potato and to investigate the behaviour of potato in salt solution
Aim: The aim of my experiment is to use salt solution to find the concentration of the cell sap in a potato and to see how a piece of potato behaves when placed in different concentrations of salt solutions. In my experiment this will be measuring the mass of the potato before and after the experiment to see if it has gained or loss any mass during osmosis.
Prediction: I predict that the higher the concentration of the salt solution, the more water will move out of the cell and the mass will decrease. I think that mass will only decrease up to a certain point.
The Science: Osmosis is the diffusion of water through a selectively permeable membrane from a region of high water concentration to a region of low water concentration. Selectively permeable means that the cell membrane allows only some things to pass through it. That is gases and water. It does not allow other things to pass through it. For example salt, sucrose, starch or proteins as their molecules are too big.
A region of high concentration water is where there is pure water or a very dilute solution of something like salt. In both cases there is a lot of water so therefore a high concentration of water. A region of low water concentration is where there is a very strong solution of something such as salt and where there is much less water present. Osmosis occurs into the cell if the medium surrounding the cell has a higher water concentration than in the cell. This is where the cell will gain water. The water molecules are still free to pass both ways across the cell membrane, though more will enter the cell than leave. This means that the cell becomes turgid (hard structure as the cell will have lots of water in it). As the water molecules diffuse in and out of the membrane, a hydrostatic pressure builds up in the lower concentration. This pressure is known as osmotic pressure. Whereas if the medium surrounding the cell has a lower concentration of water than inside the cell, then the cell will lose water by osmosis. In this case the size of the cell will decrease and the cell will become flaccid (a softer structure as water has diffused out of the cell). However, if the concentration of water inside the cell is equal to the concentration of water outside of the cell then the cell remains the same size. I will find the concentration of cell sap by putting the results from my experiment into a graph (x axes = concentration of salt solution, y axes = percentage change in mass) and seeing the point where there is no loss or gain of mass to the potato. This means that the concentration of water will be equal to the concentration of cell sap as the same amount of water that diffuses into the cell diffuses out again.
