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Extended experimental investigation: effects on membranes of environmental stress

Background

It is not possible to observe membranes directly, so this investigation is an indirect study of the effects of different substances and treatments on living beetroot cells. Beetroot cells have been chosen for this activity because each beetroot cell has a large central vacuole bounded by a membrane (Figure 1). Contained in the vacuole is the red pigment anthocyanin, which gives the beetroot its typical colour. The whole beetroot cell is also surrounded by the cell membrane. If the two membranes remain intact the anthocyanin cannot escape into the surrounding environment. If the membranes are stressed or damaged, the red colour can leak out. The cell wall surrounding plant cells provides a structure to the plant. It does not have a role in controlling the movement of substances into and out of cells.

The cell membrane is the same in both plants and animals, therefore you will be able to apply the knowledge you gain from this investigation to both plants and animals.

[pic]

Figure 1 Location of anthocyanin in a plant cell.

Part 1: Effects of temperature

The aim of this experiment is to investigate the effect of different temperatures on the cell membrane.

Materials

Bunsen burner, tripod and wire gauze
250 mL beaker
thermometer: 100–110ºC
10 mL measuring cylinder
marking pen or sticky labels
dissecting needle
fine forceps
8 test tubes
test-tube racks or 500 mL beaker to hold eight test tubes
access to freezer and refrigerator
washed beetroot slices: at least 40 in 100 mL beaker of water

Procedure

1Use forceps to collect at least 40 beetroot slices in a 100 mL beaker of water.

2Use a marking pen or sticky labels to mark eight test tubes with the temperatures listed in Table 1. Stand the test tubes in either test-tube racks or a 500 mL beaker.

3Put five beetroot slices in the test tube labelled ‘–5ºC’ and place it in a freezer for 30 min. At the end of this time, add 10 mL of tap water and stand the test tube in the rack.

4Put five beetroot slices in the test tube labelled ‘5ºC’ and place it in a refrigerator for 30 min. At the end of this time, add 10 mL of tap water and stand the test tube in the rack.

5Half fill a 250 mL beaker with water and stand it on a tripod. Heat the water gently until it reaches about 25ºC. (Use the thermometer to check.) Turn the burner off and let the temperature rise until it reaches 30ºC. Use more heat if necessary.

6Put five beetroot slices in a test tube labelled ‘30ºC’ and just cover the slices with water. When the water in the beaker reaches 30ºC, put the test tube with the slices into the beaker for exactly 90 s. Remove the test tube, pour off the warm water, add 10 mL of tap water and stand the test tube to one side.

7Heat the water in the beaker to about 35ºC, remove the burner and let the temperature rise to 40ºC. Use more heat if necessary.

8Repeat step 6.

9Repeat steps 6–8 for temperatures of 50ºC, 60ºC, 70ºC and 80ºC.

10Copy Table 1 into your workbook.

11After at least 30 min., shake each tube and hold it against a white background in a well-lit area.

12In Table 1, record the colour of each of the solutions. Use either coloured pencils or a star scale to show the intensity.

Discussion

1Which temperatures were found to cause damage to the membranes?

2Do you think the possible mechanism that caused the damage would be the same for the range of temperatures tested? Explain your answer by referring to the effect of heat on the different components of a cell membrane and also to one of the well-known properties of water.

3Look back at your predictions. How accurate were they? Why were the results not the same?

4Where in nature could plants be exposed to extremes of temperature? Think of examples other than the one provided in the introduction.

5Where in nature could...
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