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AS Biology Practical - Planning Exercise

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Introduction

Biology Practical Examination 1 (Part A - Planning Exercise) Aim: To investigate the effect of the concentration of calcium ions on the coagulation of milk. Scientific Knowledge and Prediction[1][2][3][4][5] The major proteins in milk are the caseins; ?-caseins, �-caseins and ?-caseins which a group of small phosphoproteins that are able to easily form aggregates (called sub-micelles (see Fig 1.1)). In the presence of calcium ions, these sub-micelles can together form a larger aggregate called the casein micelles (see Figs 1.2 and 1.3). The enzyme rennin binds with ?-casein, forming an enzyme-substrate complex [3], hydrolysing a peptide bond and thus splitting it into two fragments (see Fig 2). The fragment that does not remain part of the sub-micelle (the macropeptide) carries the carbohydrate units[1]; the loss of the carbohydrate 'coats' means that strong cross-links between micelles can be formed which is essentially the process of coagulation [1][4] (see Fig 3). Notice how all bonds between sub-micelles in a micelle contain calcium (Fig 1.2). Therefore without calcium ions, no bonds can be made between sub-micelles meaning that micelles can not be formed. Even if rennin did act on the ?-casein, no effect would take place due to the lack of micelles in the first place. Fig 1: Fig 2: Fig 3: Hypothesis: As long as there is plenty of substrate available, if the calcium chloride solution concentration increases then the rate of coagulation also increases because there will be an increased number of micelles formed. ...read more.

Middle

3. Add to each beaker: Beaker Volume of Calcium Chloride solution (cm3) Volume of Water (cm3) Concentration of Calcium Chloride added altogether (mol dm-3) 1 3.0 0 1.0 2 2.7 0.3 0.9 3 2.4 0.6 0.8 4 2.1 0.9 0.7 5 1.8 1.2 0.6 6 1.5 1.5 0.5 7 1.2 1.8 0.4 By this method, a fixed final volume is maintained. Separate 5cm3 syringes are used for calcium chloride solution and water to prevent contamination. Mix each solution thoroughly with a glass rod, remembering to wash the rod with distilled water before use with each beaker so as to prevent cross-contamination. 4. Using a fresh 10cm3 syringe, add 10cm3 of solution 1 to a clean 50cm3 beaker. Using a 1cm3 syringe, add 1cm3 of the rennin solution, starting a stopwatch. Stir with a fresh microscope slide for roughly five seconds and take it out. A thin layer of milk should be now on the surface. Stop the stopwatch as soon as signs of coagulation first appear i.e. for 'flecks' of curd to appear on the slide. Record the time taken. The appearance of the coagulum is distinguishable by its whiter appearance to the liquid milk. Wash out the beaker tested in with distilled water, and repeat this step with the remaining 20cm3 of milk solution in beaker 1 a further two times. ...read more.

Conclusion

Also wash off any solution in contact with skin. Sodium Citrate solution Contains sodium hydroxide solution which is an irritant in such small concentrations. As above. General care to be taken with the apparatus; if glass is broken then immediate disposal required to prevent broken shards which can cause cuts. Calculations: The time taken for the test will be recorded in a table as following: Beaker Concentration of Calcium Chloride solution added (mol dm-3) Time taken for the first signs of coagulation (s) Mean time taken (s) Rate of Reaction (1/s) 1 1.0 2 0.9 3 0.8 4 0.7 5 0.6 6 0.5 7 0.4 A mean time of the three readings will be taken for further reliability. The rate of reaction is usually taken to be the initial rate of reaction, and is usually measured by taking the product produced per unit time. Unfortunately, there is no way in this experiment that one can measure accurately how much coagulation has occurred. As a result, the only distinguishably common moment, between tests of different concentrations of calcium ions, is the starting point of coagulation i.e. when 'flecks' of curd first begin to appear. The reciprocal of the time taken will then be proportional to the true rate; however this also means that the actual values and units are arbitrary. i.e. Rate of Coagulation= A graph would then be plotted of the rate against concentration of calcium chloride solution added, labelled as below, from which inferences could be made. ...read more.

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