Determination of the water potential of potato tissue by a gravimetric method.
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Determination of the water potential of potato tissue by a gravimetric method. 1. Explain the theory behind the experiment. Water potential, w, is a measure of the ability of water molecules to move from one region to another. The more water molecules there are per volume of the cell the more likely that by random movement they will collide with the cell's plasma membrane, and travel out of it. Pure water has a w of 0. As all solutions have less water molecules per volume than pure water they have a lower w; therefore all solutions have negative water potentials. The net movement of water molecules is always from a region of high water potential to one of lower water potential, they move down a water potential gradient until equilibrium is reached it will be reached when the water potentials on both sides of the plasma membrane are the same. When the potato is placed in water or a hypotonic solution its cells will swell, although they will not burst due to the cell wall, as water molecules enter the cell down a water potential gradient. The cells become turgid, with the protoplast pushed up against the cell wall, therefore gaining in length and mass. When the potato is immersed in a hypertonic solution the protoplast shrinks and the plasma membrane pulls away from the cell wall.
Pipette: Very accurate, to the nearest 0.05cm3 but error none the less. The tissue sample was all from the same region of the potato but was still very varied. This will have rendered the results more inaccurate as the water potential of each tissue sample could have been different, depending on the proportions of each type of tissue it contained. I.e. the inner part is more dense suggesting there might be more solutes inside therefore affecting its water potential. The tissue sampled was also less dense than the solutions it was immersed in, (for the 1M and 0.8M). This meant that the potato chips floated in the solution and hence a small portion of each chip was not immersed in the solution (I estimate about 2%). These cells will not have been as affected as the immersed cells, although water will have diffused into or out of them through the immersed cells. So osmosis will have occurred at a slower rate, and thus to a lesser extent over the same time period. The mass of these cells will therefore have contributed to the initial mass and remained almost constant, making the final and change in mass less accurate. I used tap water to dilute sucrose solutions down to my selected values.
6. What suggestions can you make to account for any difference in water potential determined in disc and chips. Surface area of discs: + = = 0.51cm3 Surface area of chips: = 7.68 cm3 We can see that there is a 15 times difference in the surface area of the chip against the discs. And according to Fick's law which states that the rate of diffusion is proportional to the (surface area concentration difference) over the distance between the 2 areas. We can see that the concentration difference would be the same for a given concentration and the distance is roughly the same, so the only difference would be in the surface area. Which decides how fast the water molecules can diffuse out/into the cells. This would suggest that the chip would have a higher percentage change in mass against a disc for a given time and concentrations as diffusion rate is faster. And so we see a difference there. Also we see that the chip has a larger volume than the disc. This means that it has much more water in it than the disc, and also suggests that the chip can hold more water than the disc. This means that proportionally a small mass change in the chip can be detected more and so suggesting the true value for no mass change as it accentuates the other values. Whilst the disc shows less accentuated changes. PHILIP XIU
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