Comparing The Water Potential Of Plants

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Comparing The Water Potential Of Plants

Aim

The aim of this experiment is to compare the water potential of two different plant tissues, potato and sweet potato tubers, by measuring the gain or loss of water when samples of the tissue are placed in a range of concentrations of sucrose solutions.

Background information

Osmosis is the movement of water molecules from a region of their higher concentration to a region of their lower concentration through a semi permeably membrane.  If the cell is surrounded by pure water (which has a water potential of zero), or by a solution that has a lower concentration and therefore a higher water potential, than that of the cell’s contents water flows into the cell by osmosis and the cell would swell up (become turgid). The external solution is said to be hypotonic to the solution in the cell (hypo meaning lower than and applies to the solute concentration).

If the reverse was to occur and the cell was surrounded by a solution whose concentration is higher and whose water potential is lower then that of the cell, the water would flow out of the cell into the solution. In this case the external solution would be known as hypertonic to the solution in the cell.  If the cell has the same solute concentration and water potential as the surrounding solution, there would be no flow of water into or out of the cell so the cell stays the same size. In this case the external solution is called isotonic, meaning it is the same as.

We have seen evidence of using hypo and hypertonic solutions during an experiment using onion cells. Both onion cells were placed onto a microscope slide, a drop of water (hypotonic solution) was then added to slide A, and a salt solute (hypertonic solution) to slide B, below are photos that were taken of the cells during this experiment.

Slide A                                                Slide B

As you can see from the images above, slide A has become turgid. This is due to the fact that the water we placed on the slide has a higher water potential then the cells. There fore the flow of water is from the external solution into the cell causing it to swell, unlike animal cells, plants cell have a rigid cell wall and this stops them from bursting. As the water flows into the vacuole, the turgid cell begins to oppose the uptake of water, called the pressure potential.

Slide B is the opposite to slide A and the cells have begun to shrink         (plasmolysis). This time the cells had the higher water potential then the salt solute that we put on them was hypertonic. As the cell had the higher water potential, the water from within the cell diffuses into the solute.  When the onion cell loses water, the pressure of the water within it is reduced and this is the reason that the cells membrane begins to shrink and the cell would begin to feel flaccid.  The point where the membrane just    starts to leave the cell wall is called incipient plasmolysis.

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To test the results of this onion investigation further I am going to conduct another experiment this time using sweet potato and normal potato. During this time I am going to place both the sweet Potato and normal potato into solutions of different sucrose concentrations and also one with distilled water. From here I hope to draw conclusions to the water potential of plant tissue and to see if any of my predictions are correct.

Predictions

  • From what I have learned about water potential I would expect distilled water to have to highest ...

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