Introduction The presence of solutes in water causes the osmotic potential to decrease and it reduces the free energy of water. This is because water molecules will tend to bind to the solutes rather than moving freely and away from the solution. If cells are surrounded by a solution that has an osmotic potential more negative (lower) than that inside the cell, water will move from inside the cell (higher water potential) to the outside solution (lower water potential) by osmosis. This water loss causes a decrease of the protoplast cell volume and eventually leads to plasmolysis (the separation of the plasma membrane from the cell wall).
The objective of this exercise is to estimate the osmotic potential of the external solution that causes 50% plasmolysis in red onion epidermal cells. Materials and Methods: Epidermal sections of red onion were placed in a series of sucrose solutions of increasing sucrose concentration. After a treatment period, pigmented cells in the sections were examined for plasmolysis under the microscope. The percentage of plasmolyzed cells in each solution was plotted against the sucrose concentration. The concentration that caused 50% plasmolysis was visually estimated from the graph. The osmotic potential of the solution that caused 50% plasmolysis was calculated using the van’t Hoff equation.
The Essay on Water Potential of a Potato
... incipient plasmolysis. The table below shows what happens to the cell when water moves in and out of a cell. Water potential of solution compared to cell Higher ... water potential through a partially permeable membrane” Definition of Water Potential. The potential for water to move across a selectively permeable membrane, where the osmotic potential of distilled water ...
Results: The osmotic potentials of the sucrose solutions are reported in Table 1. At low sucrose concentrations (less negative osmotic potential) plasmolysis of epidermal red onion cells was not apparent. However, as sucrose concentration increased (osmotic potential became more negative) plasmolysis increased reaching 50% of plasmolyzed cells at -1. 34 MPa (Figure 1).
Other students reported values from -0. 85 to -1. 5 MPa. Discussion: As expected, high osmotic potential (low sucrose concentration) did not induce plasmolysis suggesting that the external solution was in equilibrium with the protoplasm inside the cell. However, as sucrose concentration of the external solution increased plasmolysis became apparent indicating that the concentration of solutes in the cell protoplasm was lower than in the outside solution. The differences in the value at which 50% plasmolysis occurs observed by different students may have been due to several factors.
First, failure to obtain just one layer or so of cells from the sections would cause difficulty in seeing plasmolysis. Second, differences in the number of cell layers of the epidermal peel between students combined with variations in the osmotic potential of different layers of the onion may also result in variable results. Insufficient time for the tissue to reach equilibrium with the solution may also result in errors. Finally, variation may have been due to inherent differences between the onions used.
Highly negative (low) osmotic potentials in plants may be advantageous in certain environments where lowering total tissue water potential (as a result of low osmotic potential) allows roots to extract water from dry or saline soils. In other words, lowering the water potential allows plants to remain ‘drier’ than the soil and therefore continue extracting water from it.
