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To determine the effect of Detergent on the Permeability of Cell Membranes.

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To determine the effect of Detergent on the Permeability of Cell Membranes Background All living cells are surrounded by a plasma membrane, made up of a bi-layer of phospholipids. A cell membrane is very thin, on average it is 7nm. Unlike ordinary (neutral) lipids, each phospholipid molecule has a hydrophilic, water miscible head. This occurs because one of the fatty acid molecules of the tryglyceride is replaced by a phosphate group. This phosphate group, with the glycerol, forms the head of the molecule and because it is a polar molecule i.e. has an uneven distribution of electrical charge, it is hydrophilic. The other end, with the two tails, however, is still hydrophobic and non-polar. In the cell membrane the phospholipids molecules are arranged in a bilayer with the polar heads on the outside and the hydrophobic tails directed inwards. (See diagram below). As well as phospholipids, cell membranes also contain cholesterol, proteins, glycoproteins, and glycolipids. Cholesterol has hydrophilic heads and hydrophobic tails, and is important for maintaining the fluidity of the cell membrane. The main function of proteins is to act as transport proteins, which are hydrophilic channels, which only allow certain ions or molecule through, with each channel being specific to a certain one. The cell membrane is said to be partially permeable and acts as a barrier to the diffusion of large polar molecules either into or out of the cell. This is vital for living cells. Substances can pass through the cell membrane by diffusion, osmosis, active transport or facilitated diffusion. Glycoproteins and Glycolipids are proteins and lipids, with various carbohydrate molecules attached to them. The carbohydrate groups project out of the cell, and one of their main functions is to act as receptor molecules for certain substances such as hormones. Detergents are made of a long hydrophobic side chain and a charged polar head. They can be cationic or anionic. ...read more.


will be drawn on two individual pieces of masking tape. The scales should go up to 80mm (8cm). This is so the height of the columns can be measured later on. These scales will then be stuck onto the two boiling tubes. The scales will be stuck on after they have been drawn, so that they are being drawn on a flat surface - not whilst on the side of the boiling tube. The scales must be stuck on straight up (i.e. at 90 degrees to the work surface), and must be drawn accurately. The scales will start from just above the curvature of the boiling tubes. A letter 'S', using another piece of masking tape, will also added to one of the tubes, that tube being used for the standard. After 30 minutes on the stop clock, the liquid from the 10% detergent concentration will be decanted, so that no further leakage can take place. This will be done by taking the final new boiling tube, and gently pouring the liquid into it, discarding the beetroot from the original tube, and then pouring the liquid back into the original tube. This procedure will be repeated for the 9%, when the stopwatch reads (start time for 9% + 30 minutes). This method of decanting will be repeated for all the other tubes, right down to 1%. Once all the liquid in each of the ten tubes has been decanted, enough of the 10% will be transferred into the empty boiling tube labelled S, so that the height of liquid in this boiling tube is 1cm. (This is why the scale was needed). This will be the standard, as indicated by a letter S on the tube. The liquid will be transferred by pouring, so that the flow of liquid from to the standard can easily be controlled. Using a syringe would create extra bubbles which would affect the accuracy of the results, as more light would be absorbed by more bubbles. ...read more.


During the 30 minutes, however, changes in room temperature could have changed the water temperature. Again, this is very insignificant, as even if there were small changes it would affect all the tubes equally. Another problem was that the liquid each tube, after decanting had bubbles on the surface. This may have distorted the light when looking up into the tube from underneath. This may have distorted the light meaning less was able to reach and pass through the solution, making it appear to have a higher colour intensity. A more serious problem is concerned with the curvature in the boiling tubes. The scale was stuck on above the curvature on each boiling tube. It was difficult to ensure the scale was in the same place on each of the two boiling tubes. This meant that the scale would be slightly inaccurate, meaning the heights in mm were not correct. Using test tubes would overcome this problem, as these are flat bottomed, and therefore the scale could be more accurately applied. Measuring cylinders with pre-printed scales could also be used. However, neither of these pieces of apparatus was available during the investigation. Finally, a potential minor problem is that if any of the pieces of beetroot were not washed thoroughly, some pigment would have leaked into the detergent solution immediately. Again, this did not happen, as the beetroot was left to soak overnight. Overall, the main problem was that the investigation was reliant on human eyesight in order to compare the colour intensities. This meant that the results may have been slightly inaccurate. The other problems, such as temperature and whether the beetroot had been washed were much less significant. The other significant problem was the scale on the boiling tube. Overall, despite the problems with the human eyesight, and with the scale, a reasonable set of fairly accurate results were obtained. Repeating the entire experiment would have been desirable in order to obtain more accurate results, but there was simply not time. John Cummins ...read more.

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Here's what a teacher thought of this essay

3 star(s)

*** An unusual methodology for a standard experiment which has produced some usable data. Some of the sections normally expected in A level coursework have been omitted.
To improve;
Research and rationale
The rationale for the experiment was clearly justified and appropriate biological principles were discussed. The chemical composition in different types of detergent should be explored since these may affect the predicted result.
There is evidence of thought and ingenuity in the design of the experiment and a good attention to detail in the way in which most variables are controlled. The use of particular apparatus or methodologies needs to be discussed and justified. There needs to be a more thorough risk assessment included.
Observations were carried out over a suitable range of values but insufficient observations were made to allow a valid conclusion. Replicates or class data could have been used to ensure greater reliability.
Analysis and Evaluation
No graph was included. The trend in the data was described but greater use of background biological material would be useful to explain the conclusion fully. The anomalies were described and explored but the lack of data made interpretation difficult. The candidate is aware of the errors inherent in the method and that the results may not be entirely reliable. The use of the colorimeter suggested would give more valid results.

Marked by teacher Stevie Fleming 26/07/2013

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