Experiment to investigate the water potential of potato tissues when immersed in various sucrose solutions of different concentrations

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Experiment to investigate the water potential of potato tissues when immersed in various sucrose solutions of different concentrations

Aim:

The aim of this experiment is to determine the water potential of potato tissues when immersed in different concentrations of sucrose solution and find the point at which the cells in the tissue are incipiently plamolysed. The immersion of the tissue in different concentrated solutions will enable an understanding to be developed of which concentrations the water potential is greater in the cell in comparison to the external solution; lower in the cell than in the external solution and ultimately, equal in both solutions thereby identifying the normal sucrose solution of the cells and the point at which the cells within the potato tissue are incipiently plasmolysed.  

Background Theory:

Incipient plasmolysis is the point at which the water potential within a cell and the water potential of its external solution are equal. In order to understand this, the concept of water potential must first be grasped. Water potential, represented by the Greek notation psi (Ψ), is the tendency of water molecules to move from one place to another as a result of the pressure created by these molecules. Water molecules will always move down a water potential gradient, which is from a region of higher water potential to a region of lower water potential. Equilibrium is reached when the water potential of both regions are equal and there is no longer any net movement of water molecules. This net movement of water molecules between two different systems with varying water potentials across a partially permeable membrane is the process of osmosis. Water potential is affected by two factors: solute potential (Ψs) and pressure potential (Ψp). Solute potential is the amount by which the presence of solute molecules lowers the water potential. The greater the amount of solute molecules present (and thus the more concentrated the solution), the greater the amount by which the water potential is lowered. This is because the water molecules in a solution have formed bonds with the solute molecules causing them to move more slowly relative to the free water molecules in pure water thus lowering their water potential. Pressure potential can be defined as the pressure that is applied to a solution that resists entry of water molecules from the solution with a higher water potential thereby increasing the water potential of the solution that has a lower water potential. Increasing the pressure exerted on a solution will consequently increase the water potential of the solution. It is evident to realise that water potential is dependent on both the solute and pressure potential of a solution. The relationship between each of these three factors can therefore be expressed by the equation:

Water potential = solute potential + pressure potential

Ψ = Ψs + Ψp

This equation therefore enables the net movement of water molecules between different systems with varying water potential to be established.

Plant cells such as the potato tissue used in the experiment contain a variety of solutes, largely dissolved in the water of the large cell vacuole that each possesses. As a result, when such tissues are immersed in a aqueous solution they will either absorb water from the external solution or water will leave the tissue, both by osmotic means. The net movement of water molecules is dependent on the water potential of both the potato tissue and the external solution. When immersed in pure water (which has a water potential of 0kPa, the highest value possible), the potato tissue will absorb a large amount of water from the external solution through osmotic means due to its relatively lower water potential. However, unlike animal cells, the plant tissue will not burst when placed in a solution of high water potential. This is solely due to the structure in which it is composed. Plant tissues possess a cellulose cell wall, which as a result of the arrangement of the β-glucose molecules within the cellulose, gives each individual cell great strength. When water molecules enter the plant tissue in osmosis, the portion of the cell known as the protoplast (which is the cell surface membrane, tonoplast and cytoplasm) exerts a pressure against the cell wall, which, due to its limited extension, will resist the entry of further water molecules. This is the aforementioned pressure potential. As the pressure potential prevents further water molecules from entering, it increases the water potential within the cell. In this situation, the protoplast is positioned pushed up against the cellulose cell wall and the cell shape is described as turgid. In the case of the potato tissue being immersed in a solution of lower water potential (i.e. 1M sucrose solution), water molecules will leave the cell by osmosis as the tissue will now have a higher water potential relative to its external solution. As this occurs the volume of the cell will decrease and therefore the pressure potential will also decrease as the protoplast exerts less of a force against the cell wall. A stage will eventually be reached where the protoplast no longer exerts any force on the cell wall. At this point the pressure potential is equal to zero and therefore the water potential is equal to the solute potential:

Water potential = solute potential

Ψ = Ψs

This precise stage in which the Ψ = Ψs is referred to as the point of incipient plasmolysis and the cell is said to have been incipiently plasmolysed. Further movement of water molecules from the potato tissues after the point of incipient plasmolysis will cause the cell to shrink further and the protoplast to pull away from the cell wall. This condition is referred to as plasmolysis and the shape of the cell at this stage is described as flaccid.

The following figure summarises the stages involved in the osmosis that occurs when potato tissues are immersed in solutions of different concentrations (varying water potentials):

Figure 1: Summary of osmosis in a plant cell:

Preliminary Study:

The preliminary study involved investigating the water potential of onion tissue when immersed in varying concentrations of sucrose solution. The experiment was carried out through the means of finding the number of plasmolysed cells (out of any 20 through the use of a light microscope) present for each concentration of sucrose solution and thus calculating the percentage of plasmolysed cells at each concentration.  The objective of the preliminary study was to identify at which strength of sucrose solution 50 percent of the cells within the tissue were plasmolysed thereby identifying the point of incipient plasmolysis. Figure 2 is a graph presenting the results obtained. The results give evidence to suggest that for increasing concentrations of sucrose solution, the percentage of cells showing signs of plasmolysis increased in relation. The results therefore appear to be valid as the predicted trends (that is for higher concentrations of sucrose solution, there will be a net movement of water from the onion cells as they will have higher water potential and thus the number of plasmolysed cells will increase) are reflected in the results. It is evident from the graph that the point at which 50 percent of the cells within the tissue are plasmolysed is at the sucrose concentration of 0.45 M, where Ψs = -1280 kPa. At the point of incipient plasmolysis, water potential is equal to solute potential and thus, the water potential of the solution = -1280 kPa.

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The results do not appear to have produced any anomalous values thereby suggesting that the method used to carry out the experiment was sound. Valid results are also likely to have been produced due to the number of concentrations used. This amount of concentrations enabled the trend of water potential to be recognised clearly thereby allowing any possible anomalous results to be easily identified. A further positive aspect of this study was that the equipment required was easily set up and was in no need of any specialist skills and thus the results were obtained with no evident difficulty. ...

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