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Crossing MembranesAim: To observe the processes and responses of osmosis in plant cells to explain the effects of different concentrations of sucrose solutions on the movement of water across membranes.

Hypothesis:The higher the concentration of sucrose solution that the plant cell is immersed in; the plant cell is more likely to lose mass because the water is transferring from a place of higher concentration (inside the cell) to lesser concentration (outside the cell) which therefore means that the vacuole will appear smaller than usual due to loss of water. Vice versa with lower concentrations of sucrose solution.

Materials:Part A

Rhubarb petioles Microscope slide Cover slips Distilled water 1.0M sucrose solution Forceps Compound microscope

Part B Fresh potato Cork borer Razor blade Petri dishes x10 Distilled water Sucrose solutions – 0.2M

– 0.4M – 0.6M

– 0.8M– 1.0M

Paper towels Scales Ruler Chopping board

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Method:Part A – Osmosis in action

1. Obtain a small section of the rhubarb petiole and rip off a small section of the red-coloured outer epidermal layer with the forceps.

2. Place it on a clean microscope slide and add a drop of distilled water. Record the time. Place a cover slip over the specimen and view under low power (x100) on the microscope.

3. Draw and label a diagram of the cells showing the cell wall, vacuole and the position of the plasma membrane and vacuolar membrane.

4. Add more water if specimen is drying out. 5. Remove your slide from the microscope and remove the cover slip. Carefully dry the rhubarb

epidermis with a paper towel. It is essential to use the same piece of rhubarb. 6. Add 0.1M to the rhubarb and replace the cover slip. View it under low power (x100) under

the microscope. Draw and label a diagram of the cells after 10 minutes.

Part B – Osmosis in potato cellsDay 1

1. Using the cork borer, cut a core from the potato and remove the cylinder of potato. Trim the core so that there is no skin present on the ends.

2. Slice across the core to make smaller cylinders; each 1mm think. Obtain 20 slices. 3. Place the petri dish on the set of scales and ‘zero’ it. Then place all 20 slices of potato into

the petri dish and weigh the mass. Record the total mass. 4. Repeat steps 1-3 until you have 7 sets. One for each concentration and two for the control. 5. At approximately the same time, cover the slices of potato in the petri dishes with various

sucrose solutions and the controls. 6. Place the lid, with the corresponding sucrose solution written on it, onto the petri dish. 7. Leave overnight at room temperature.

Day 21. Carefully remove the slices from the solution and place them on paper towels. Fold the

paper towel over the slices and gently press down onto the top of the slices. 2. Place the 20 slices of potato on the set of scales and weigh the total mass. Record. 3. Repeat with each set of potato slices in each concentration of sucrose solution. 4. Calculate the change in mass as a percentage of the original mass for each sucrose

concentration. 5. Produce a graph of the concentration of the sucrose VS the percentage change in the mass

of the potato slices.

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Results Concentrations of sucrose solutions

(M)Original mass

(±0.01)Mass after

(±0.01) ∆ mass

(±0.01)g% ∆

massH₂O 1. 3.36 4.24 0.88 26.19

H₂O 2. 3.72 4.54 0.82 22.04

0.2 3.05 2.96 -0.09 -2.95

0.4 3.68 3.04 -0.64 -17.39

0.6 3.62 2.93 -0.69 -19.06

0.8 3.59 3.04 -0.55 -15.32

1 4.56 3.11 -1.34 -29.39

Discussion Part A

1. The pink pigment of the rhubarb epidermis cell allows you to determine the position of the vacuolar membrane.

2. The size and shape of the vacuole before the addition of the sucrose solution was not clear due to the low power of the microscope however the shape and size of the vacuole after the addition of the 0.1M sucrose had become clear and seemed to have shrunk away from the cell wall. The cell immersed in sucrose solution would have a shrunken vacuole due to osmosis. The concentration of water is less outside the cell, so therefore the water within the vacuole would move out of the membrane to satisfy equilibrium.

3. The appearance of the cells in the distilled water was unclear. The only features easily distinguished were the cell wall and cytoplasm. However the vacuoles were able to be seen as it had a pink pigment though it was not possible to distinguish each individual vacuole.

4. Of the three boundaries that a substance can cross to enter the vacuole of a rhubarb cell, water is permeable through the cell wall, plasma membrane and the vacuole membrane. The cell wall is permeable to both sucrose and water however, the plasma membrane and the vacuole membrane is impermeable to sucrose.

5. The evidence that supports the fact that the vacuolar membrane is partially permeable is the fact that when immersed in a sucrose solution, the vacuole was able to lose water which passes through the membrane.

6. Water is able to cross a partially permeable membrane when it is moving from a higher concentration area to a lower concentration area.

7. If this experiment were to be repeated on an animal cell, we would expect the cell to burst due to the excessive water moving into the cell because it has no cell wall or we could expect the entire cell to shrink due to the cell losing water because the cell membrane is not turgid.

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Part B

0 1 2 3 4 5 6 7 8

-40

-30

-20

-10

0

10

20

30

Concentrations of sucrose solutions VS % change in mass

% ∆ mass