Selective permeable membranes

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Aim

This practical was to investigate the effects of temperature on the selective permeable membranes of red cabbage.

Introduction

As Allaby (1999) said, the permeable membrane is a membrane that can select the passage of certain substances in and out of cells based on the size and usefulness of molecules. For instance, water and ions can pass in and out by simple osmosis diffusion (high to low concentration), but some macromolecules like pigments cannot. Inside the structure of the cell membrane, there are many different types of protein to select and allow some macromolecules to get through (Peter, 2003). The selectivity of these proteins is decided by the structures themselves. Therefore, the cell membrane can keep supporting some macromolecules inside the cell.

The structure of protein can be affected by physical causes, such as high temperature. This process is called the denaturation of protein. According to Charles (2003), the protein structure turns into a random shape after denaturation, and the properties of protein would be lost permanently. When the permeability of the cell is changed, those macromolecules will be transported by diffusion, (Wurts, 1987) because the concentration of pigment inside the cell is much higher than the environment. As a method, the effect of different temperatures on the permeable membrane of red cabbage can be determined by observing the colour of the environment liquid.

The pigment inside red cabbage is anthocyanin (Dr. Stephen, 2009). According to The President and Fellows of Harvard College (2008), it is soluble in vacuole and can reflect the pink-red colour of most red fruits. Anthocyanin has a molecule of glucose and four aldehyde groups as function groups (seeing figure 1 from The President and Fellows of Harvard College, 2008).

Steps (from Second Biology Practical IFY Class Handout, Lane, R. 2009)

This practical was done follow according to the steps. Firstly, a cork borer was used to cut discs of fresh red cabbage into 42 pieces and each 3mm diameter discs. All the discs were placed in a small beaker and washed under running water for 5 minutes. Meanwhile, the test tubes was labeled 30?, 40?, 50?, 60?, 70?, 80? and 88?, and a cylinder was used to add about 6cm3cold water to each. Thirdly, a water bath using a large beaker, tripod and gauze, and a alcohol burner was prepared. When the water was been heating, 6 red cabbage discs were impaled on a mounted needle and placed into the test tube labeled 30?. As soon as the water was 30?, the burner was removed and the test tube was placed in the large beaker for exactly one minute. Then, the test tube was taken out and cooled down. The water was then heated to 40? and the same steps for 30? were repeated. However, when the water was heated to 50?, the burner was removed and the 6 red cabbage discs on a mounted needle for 50? were placed into the water for exactly one minute. Then the discs were pushed off and put into the test tube labeled 50? to cool down. All the steps for 50? were done precisely for the temperatures on 60?, 70? and 88?. Finally, after the discs were left in the test tubes for at least 20 minutes, the tubes were shaken and held to the white paper to compare the colour of each tube.

Results

The results of this practical were shown in Figure 2.

In the first test tube, there are four discs floated on the surface while three discs in the next two tubes. The one for 60? was having two discs and more water than other. Colour changed apparently in the left three tubes. The discs were sinking and colour changed into violet.

Discussion

During this practical, many errors occurred. When the part of 30? and 40? was being done, the 6 red cabbage discs were placed into the test tube then a tube was placed into the large beaker. By doing this wrong step, the temperature of water environment that the discs experienced was probably lower than the desired one, and it took time for the cold water in the tube. Low temperature affected the results in the first two tubes. Another error was made by not using pipette to measure the volume of water in each tube. In fact, a graduated pipette was provided. It is much more accurate than a measuring cylinder. This error affected all test tubes for about 10%. The volume of water in the 60? tube was more than others. It led to the colour of the liquid being lighter than expected. To improve this, the graduated pipette should be used to avoid this as instructed. The results cannot be compared with the effected concentrations.

Referring to the introduction, the colour became more apparent. The colourless observed in the first three tubes (30? - 50?) shows that the temperature was not high enough to denature the proteins in the selectively permeable membrane. The purple liquid in the last three tubes indicate that the permeability of membrane was increased by the denaturation of protein which led to the releasing anthocyanin. It is because that the pigment concentration in water and cell causing the pigment to diffuse into water.

Another phenomenon which is worth considering is that only some discs in the first four tubes floated. It is possible that high temperature did not only denature the protein also destroyed the structure of discs. It is possible that the high temperature affected both on the structure of membrane and the equilibrium of diffusion of anthocyanin.

Conclusion

The results showed that the permeability of selectively permeable membrane of red cabbage increases with the higher temperature (70? - 88?) and those discs became violet.

Reference

  • Charles, E. O. (2003) [online] Denaturation of Proteins Available at: http://www.elmhurst.edu/~chm/vchembook/568denaturation.html [Accessed time: 01-03-2010]
  • Dr. Stephen G. (2009) [online] Introduction to the Protoplast Lab Available at: http://employees.csbsju.edu/ssaupe/biol327/Lab/protoplast/protoplast_introduction.htm
  • Farlex, Inc. (2010) [online] Semi Permeable Membrane Available at: http://medical-dictionary.thefreedictionary.com/Selectively+permeable+membrane [Accessed time: 01-03-2010]
  • Lane, R. (2009) Second Biology Practical IFY Class Handout
  • Peter, V. S. (2003) [online] Membranes: Diffusion, Permeability, Osmosis, Turgor, Active and Passive Transport Available at: http://www.biologie.uni-hamburg.de/b-online/e22/22.htm [Accessed time: 01-03-2010]
  • The President and Fellows of Harvard College (2008) [online] Leaf Pigments Available at: http://harvardforest.fas.harvard.edu/research/leaves/leaf_pigments.html [Accessed time: 01-03-2010]
  • Wurts, W. A. (1987) [online] Membrane Permeability, Calcium, and Osmotic Pressure Available at: http://www.ca.uky.edu/wkrec/MembranePhysiologyCalcium.htm [Accessed time: 01-03-2010]

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