Osmosis - Estimating the water concentration of red onion epidermal cells

Question

An investigation was carried out to estimate the water concentration of red onion epidermal cells. Pieces of epidermis were covered with a drop of salt solution on a microscope slide. After 5 minutes, the number of plasmolysed cells in a count of 50, in three different areas of the slide, were counted.

This was repeated with other salt concentrations.

The results are shown in the table.

Concentration of Salt Solution (M) Average Percentage of Plasmolysed Cells (%)

0 0

0.05 20

0.1 40

0.15 60

0.2 80

0.25 100

Give evidence that supports the statement that water concentration varies amongst the red onion epidermal cells.

Answer 1

Salt lowers water potential. At a solution salt concentration of 0 M (.25 M), the water potential in the cell is lower (higher) than the solution. Water moves from high to low water potential. Hence as salt is added to the solution we observe an increasing degree of plasmolysis.