The Cell Membrane in Cells

This can be seen in living cells. The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration) water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid. If done with potato cells the cells would increase in length volume and mass because of the extra water.

If these potato cells were placed in a solution with a low water concentration, then the opposite would happen. Water would move out of the cell into the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmolysed. The potato cells will have decreased in length, volume and mass.

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.

However, there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred.

Using this information a graph and prediction can be made

At point A the graph suggests that no osmosis has occurred, suggesting that the concentration of water inside the cell is equal to the solution outside.

At point B (high water concentration), there is no indication that the cell is increasing further in size. This is because the cell is fully turgid and no more water can enter.

At point C (low water concentrations), there is no indication that the cell is decreasing further in size. This is because the cell is fully plasmolysed and no more water can leave the cell.

When making a prediction, you must say what you think will happen and why. It is also very important to have a numerical guess which can be tested in the analysis e.g. when you double the concentration the rate will double.

Method

Apparatus: 1 Molar sugar solution, distilled water, potato, cork borer, mass balance, boiling tubes, knife, labels, measuring cylinder

Plan: A range of sugar solutions will be prepared with concentrations from 0.2 M to 1.0, in intervals of 0.2, by adding varying amounts of distilled water. Sections of potato will be cut using a cork borer and knife to equal lengths, this will keep the surface area constant and their mass measured. 10 ml of each concentration of sugar will be placed in three test tubes and labeled. To each test tube a cut piece of potato will be added and its mass added to the label. These will be left for 24 hours. Then the potato pieces will be removed, the surface solution removed using paper towels and weighed. Three readings are taken for each concentration in order to get an average and reduce the effect of anomalous results. As