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To investigate one of the factors that affects the permeability of cell membranes.

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Permeability of Cell Membranes Practical Aim: To investigate one of the factors that affects the permeability of cell membranes. Prediction: There are many factors which can affect the permeability of a cell membrane. These factors include temperature, alcohol, detergents, salt (sodium chloride), chloroform, pH levels and paraffin oil. Cell membranes are important for allowing nutritive, respiratory and excretory processes in plants, animals and single-celled organisms. To allow this to happen correctly the membranes are semi permeable, so that they allow small molecules such as sugars and salts to pass through, but not large molecules like proteins. Beetroot cells contain a red pigment called anthocyanin, a substance which cannot pass through the membrane when it is intact. The concentration of the pigment in different parts of the plant is quite homogenous. This means that beetroot is good to use to measure the permeability of cell membranes because there is a brightly coloured pigment that is easy to measure, spread evenly throughout the plant so the experiment will be fair. As the pigment cannot pass through the membrane when it is intact, the factors that I have found affect the permeability of cell membranes, must somehow destroy part of the cell membrane to allow the pigment molecules to get through and make the surrounding water turn red. (Sources of information: www.fed.cuhk.edu.hk/~johnson/tas/investigation/membrane_teacher.htm C.A.D Discussion on beetroot cells website.) The diagram on the next page shows how the membrane is made of a bilayer of phospholipids with the hydrophilic heads pointing outwards and the hydrophobic tails inside. There are also proteins embedded in the membrane. When the membrane is intact like this the pigment molecules cannot pass through. Here is a diagram of a cell membrane: Effect of temperature: When the temperature of beetroot cells is increased, the proteins and phospholipids of the cell membrane, which are sensitive to temperature variations, are damaged. The proteins are denatured and the lipids are liquefied, so holes are created in the cell membrane, allowing the pigment to come out. ...read more.


Another factor which I will have to keep the same is the amount of time that each experiment is left for. If one experiment is left for longer than the others then it will leak out more of the pigment simply because it has more time for it to leak out, not because the permeability has increased. To make sure the results are as accurate as possible I will change only the concentration of alcohol each time, and make sure that every other variable is kept constant for each experiment. Reliability: To make the results as reliable as possible so that valid conclusions can be made I will try to repeat the experiment as many times as possible. I will try to get three sets of results, so that an average can be taken. This will help to make the results more reliable and make the possibility that the results happened by chance less likely. Safety: To ensure that I carry out the experiment safely I will take a number of safety precautions. I will wear goggles, and I will be careful when using the alcohol as it is flammable. As I will be using glassware, I will be careful not to break it, and will be careful when using sharp instruments such as the cork borer and the knife. Implementing: While carrying out the practical I ensured that I did the following for safety reasons: * Wear goggles because I was using glass equipment and alcohol. * Work away from people using naked flames, because alcohol is flammable. * Put bungs in all test tubes because alcohol is flammable. When making measurements, the accuracy of the measurements depended on the equipment that I used, so I tried to use equipment that would give me as accurate results as possible. For measuring out the distilled water and the alcohol I used a 10cm� pipette, and made measurements to ?0.05cm�. ...read more.


Although this would not have a great effect on the results, it may have given the one left for longer the chance to lose a bit more pigment. To minimise this error you could do the experiments at different times, so that you could concentrate on just one tube at a time. However this would be very time consuming, and was not possible for me to do. * Accuracy of measurements of alcohol and water: I used a fairly accurate pipette to measure out the quantities of alcohol and water which I needed to use. The accuracy was ?0.05cm�. Therefore the measurements should have been fairly accurate. The only way that this could really become a source of error in the procedure and possibly have created the anomalous results is if I made a mistake in measuring out the quantities. However repeating the experiment would show if this was the case. The only way to minimise human error such as this is to repeat the experiment a number of times. For reliability of my results I repeated the experiment 3 times. If there were any anomalous results during one experiment due to human error this could be discarded if I could see that during the other two experiments a completely different result was obtained. Although it would not help if the problems encountered were due to the limitations of the procedure itself, I think that by repeating my experiment this number of times and taking an average, I have been able to gain results that I can presume are fairly accurate. I think that because of the number of sources of error I have found within my practical procedure, and the fact that results which could be anomalous were obtained, I cannot really draw valid conclusions from my practical. With the greater sources of error such as the size and shape of the beetroot pieces and time that they were washed for potentially affecting my results to a significant extent, I do not think I can say that my final conclusions drawn are entirely valid. ...read more.

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