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Investigating Some Properties of Cell Membranes
All cells are bounded by a plasma membrane, but many internal cell structures are also bounded by membrane. These include the tonoplast, the membrane surrounding a plant cell vacuole, the double membrane (envelope) surrounding the nucleus, mitochondria and chloroplasts, and the membranes of other organelles such as vacuoles, vesicles, endoplasmic reticulum, and Golgi apparatus. All these membranes have basically the same structure.
One of the roles of membranes is to control what passes across them. Membranes are partially permeable, ie. they do not normally allow large molecules to pass across them. Many plant cells contain pigment molecules in the cytoplasm. In beetroot cells these molecules are large and reflect red light, so that a solution of them turns a shade of pink. Many pigment molecules are also affected by pH and change colour at different pH values. Litmus, for example, is a plant pigment and is blue in alkali and red in acid.
Ethanol is an organic solvent.
Proteins are stable at a particular pH and become denatured outside this. The normal pH of cells 7.2.
You are going to find out what happens when you treat beetroot tissue in different ways. Using the above information, plus your knowledge of membranes, you will have to explain your observations.
Procedure:
1. Label 8 McCartney bottles 1 – 8 and measure into them 10cm3 as listed below.
Tube 1 distilled water
Tube 2 distilled water
Tube 3 ethanol
Tube 4 0.1M hydrochloric acid
Tube 5 0.1M sodium hydroxide solution
Tube 6 distilled water
Tube 7 distilled water
Tube 8 distilled water
2. Cut 3 slices from the beetroot chip, each about 2mm thick and place them into bottle 1.
3. In the beaker are a number of similar slices from the same sized chip, but after being cut they have been rinsed to remove any pink colour. If they still show some pink rinse them again. Into each of bottles 2 – 8 place 3 of these slices.
4. Set up a water bath by adding 100cm3 of tap water to a 400cm3 beaker and place in it bottles 6 – 8. DO NOT PUT THE CAPS ON THE BOTTLES.
5. Heat until it reaches 400C, then remove bottle 6 using the tongs.
6. Continue heating until it reaches 650C, and then remove bottle 7. Take care.
7. Continue heating until it reaches 1000C (or near – record the temperature) and then remove bottle 8. Take great care.
8. Allow the hot bottles to cool.
9. Fit the caps to all the bottles and gently shake them.
10. Set up the colorimeter and insert the blue filter slide. Do NOT touch the filter with your fingers. The colorimeter measures the % transmission of light through a solution so it is set to read maximum, 100%, with pure solvent. Thus the darker the colour of the solution the lower the % transmission. The colour filter used is always the opposite end of the spectrum to the colour of the solution under investigation.
11. Handle the cuvettes by the corrugated sides and do NOT touch the clear sides.
12. Use the 5cm3 syringe to measure 3cm3 of distilled water into a colorimeter cuvette. Place it in the colorimeter, making sure that the clear sides are facing front/back. Close the rubber cap.
13. Adjust the colorimeter to read as close as possible to 100%. Record this reading. Do not throw away this cuvette as it is the standardising one.
14. Into a second cuvette measure 3cm3 of the water from bottle 1. Place it in the colorimeter, close the cap and take a reading.
15. Wash AND DRY the second cuvette.
16. Wash the syringe to prevent contamination.
17. Repeat for bottle 2.
18. Insert the standardising cuvette and record the reading. Re-adjust back to 100%.
19. Repeat steps 15 to 18 for bottles 3, 4, 6, 7 and 8. (You have not done bottle 5 as the colour of the solution requires a different filter and so the results cannot be compared.)
20. Collect all the data from at least 3 other students.
21. The changes in the readings for the standardising cuvette give an indication of the error for the colorimeter. These are not expensive instruments and have a tendency to ‘drift’.
Technician’s notes and equipment:
1. Personal protective equipment (gloves, laboratory gown, mask, goggles)
2. 8 McCartney bottles
3. 50 cm3
4. 10 cm3 ethanol
5. 0.1M HCl
6. 0.1M NaOH solution
7. 2mm thick beetroot chip
8. water bath
9. 100 cm3 tap water
10. 400 cm3 beaker
11. tongs
12. colorimeter and colorimeter cuvettes
13. 5cm3 syringe
14. 3cm3 distilled water
15. data recording materials
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