Osmosis in Potatos

18. 11. 01 Osmosis Investigation

To investigate the effects of changing the sucrose concentration on osmosis in plant cells.

Water passes into cells through a special type of diffusion called osmosis. Water molecules diffuse through the membrane from a weak solution into a strong solution until the concentration is the same on both sides. A membrane that allows only certain molecules to pass through is called a semi-permeable membrane. In a plant, water passes from a weak cell sap solution to an adjoining cell with a stronger solution, as water passes in, the volume of the sap vacuole increases. When a full sap vacuole presses against the cell wall, it is said to be turgid. If water that is lost is not replaced the sap vacuole shrinks and pulls on the cell wall, the cell becomes flaccid; this is known as plasmolysis.

In the cells shown below, water molecules will diffuse from the turgid cell into the flaccid cell, until the cells contain equal concentrations of cell sap.

I intend to use potatoes for my investigation because these are sufficiently large, to enable all cores to be taken from the same potato, which will assist in ensuring a fair test.

The concentration of sap in the sap vacuole of a potato cell is approximately
10% - 15%. I intend to place a predefined weight of potato cells (0.15g) in varying concentrations of sucrose solution (0%, 20%, 40%, 60%, 80%), to see the effects of osmosis in the cells of a potato in varying levels of sucrose solution. The potato cores will be prepared, weighed and then placed in the solution and left for a certain amount of time, they will then be removed, re-weighed and the difference in weights calculated and plotted, and a conclusion reached.

I have done some preliminary work into osmosis in potato cells. I weighed six potato cores, and then put three into sucrose solution and three into distilled water. After 25 minutes, I removed the potato cores from the solutions and re-weighed them. I discovered that the three in water had increased in mass and the three in sucrose had decreased in mass. This decrease was due to osmosis.

I therefore predict that the potato cores in distilled water will increase in mass because the water molecules will diffuse from the solution into the sap vacuoles of the cells in the core. The water molecules will diffuse across the semi-permeable membrane, because the sucrose concentration is higher in the sap vacuole than in the distilled water. This is shown by the diagram (below left), water molecules enter the sap vacuole of the cell due to osmosis, and make the cell turgid.

However, the potato cores in sucrose solution will lose water molecules to the sucrose solution, causing the sap vacuole to shrink and the cell to become flaccid. The water molecules will diffuse because the sucrose solution has a higher concentration than the cell sap. As there are varying concentrations of sucrose solution, I think that the solution with the least concentration will have the cores which lose the least mass; and the solution with the strongest concentration will have the cells which lose the most mass. I think this will occur because the larger the difference in the osmotic pressure, the faster the osmotic diffusion will proceed.

A measuring cylinder was taken, and filled with 20ml of distilled water, distilled water was used, as it contained no impurities which could have caused anomalous readings. This water was transferred into a boiling tube, which was labelled and then placed in a boiling tube rack. The above steps were repeated twice, so there were three boiling tubes, each with 0% concentration of sucrose. The measuring cylinder was dried and 4ml of saturated sucrose solution was added, this was emptied into a boiling tube and then 16ml of distilled water was measured into the measuring cylinder and then added to the boiling tube to give a 20% concentration of sucrose. (It was decided that saturated sucrose solution could be taken as 100%, for the purposes of this experiment.) This boiling tube was then labelled, and placed in the rack, and the above steps repeated twice. The above steps were repeated until there were 15 boiling tubes; 3 with 0% sucrose concentration, 3 with