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Investigate the water potential of potato tissue and compare this with the water potential of apple tissue.

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Introduction

Josie Richards 12NB Investigating Osmosis in plant tissue Aim: To investigate the water potential of potato tissue and compare this with the water potential of apple tissue. I am going to find out the water potential of both apple tissue and potato tissue by immersing samples of each type into different concentrations of sucrose solutions. From this I will be able to draw a graph and discover at which concentration of sucrose solutions the two samples of tissue will reach equilibrium. (i.e. the water potential inside the cells will equal the water potential of the solution.) From this, I will be able to find out the solute potential of the concentration of sucrose solution in which the cells reached equilibrium. This in turn will enable me to work out the water potential of the solution and therefore, the plant tissue. Hypothesis: I predict that both types of plant tissue will gain in mass when put in solutions with a higher water potential than them. The water potential of the solution outside the plant cells will be higher than the water potential inside the plant cells, therefore water will move into the cells by osmosis through the partially permeable plasma membrane. The plant tissue immersed in concentrations of sucrose with a lower water potential than the plant cells will lose mass as water is lost by osmosis. The plant cells will have a higher water potential than the surrounding solution so water will move out of the cells through the partially permeable plasma membrane by osmosis. ...read more.

Middle

This could perhaps be to do with the pressure potential of the potato cells. (In the 0M solution the potato cells gained water as the ? was higher outside the cells.) As water moves into the cell, pressure is exerted on the cell wall which has a great tensile strength. This pushes the water back into the surrounding sucrose solution, therefore decreasing the mass. The other anomalous result was the apple sample in 1M solution at 3.5 hours. Here, the mass of the apple also firstly increased and then decreased again suggesting that 3.5 hours is too long for the plant matter to be left in the sucrose solutions. I therefore thought of leaving my experiment for 3 hours. After this I check if my modification was suitable. The results still showed enough of an increase/ decrease between the mass of the plant matter at the start and at the end, and therefore I decided that in my actual experiment I would leave my experiment for 3 hours. From other preliminary work, I discovered that the concentrations of sucrose solutions would be the following: 0M, 0.2M, 0.4M, 0.6M, 0.8M, and 1M. This range and these intervals of concentrations will give me sufficient points to plot on my graph at the end to construct an accurate line of best fit. I also discovered that using Petri dishes to immerse the plant material into sucrose solution in is sufficient only if 40cm3 of solution is used to cover the plant samples completely. ...read more.

Conclusion

The potato cells also lost more water than the apple cells by osmosis. Again this is due to the greater difference of water potential between the sucrose solution and the potato cells. From my graphs I can conclude that, as I predicted, the water potential of the apple (-700kPa) was much higher than that of the potato (-1000kPa.) This is because apples have much more soluble solutes in their parenchyma i.e. sucrose. This means there are less water molecules relative to the solute molecules making the water potential low. The potato had a higher water potential as its stores starch rather than sucrose. Starch is insoluble and therefore is not a solute in the cytosol of the potato parenchyma cells. This means that the water potential is not brought down and so it is higher than the water potential of the apple cells. I can also see that, due to the above reason, the apple cells were in equilibrium with the sucrose solution at 0.26 M. This is lower than the potato, which was in equilibrium with the sucrose solution at 0.36 M. In a 0.26M solution, there is less water relative to a 0.36M solution (i.e. the sucrose is more concentrated.) This means that a 0.26M solution has a lower water potential than a 0.36M. As the plant cells' water potential is equal to the water potential of the sucrose solutions in equilibrium, we can conclude that the water potential of the potato was higher than that of the apple by looking at these above figures. ...read more.

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