Ce Biology 1994 Paper1

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CE BIO 1994
1. (a) (i) The formation of red precipitate indicates the presence of reducing sugar (glucose) in the water ------------------------------------------------------------------- 1 This shows that the dialysis tubing is permeable to glucose / permits the diffusion of glucose into the external solution ------------------------------------------ 1 (ii) (1) Increased amount of precipitates in set-up a indicates the presence of more reducing sugar ------------------------------------------------------------- 1 which comes from the hydrolysis / break down of starch ------------------------------------ 1 catalysed by the amylase / enzyme in digestive juice X -------------------------------------- 1 (2) * saliva / salivary juice -------------------------------------------------------------------------------- 1 * pancreatic juice --------------------------------------------------------------------------------------- 1 (iii) The knot of the dialysis tubing should be tied tightly ------------------------------------------------- 1 The outside of the dialysis tubing should be rinsed with distilled water before immersing into the water. Any ONE of the following : - after putting in the dialysis tubing, the water in the boiling tube should be tested for the presence of reducing sugar immediately. - the digestive juice X should be tested for the presence of reducing sugar. - the dialysis tubing should be examined for any damage. - the volume of distilled water in the boiling tube should be the same. ) ) ) ) ) any ONE------------------------------------- 1 ) ) ) ) Total : 10 marks 1. (a) (i) In the Benedict’s test, the formation of red precipitate indicates the presence of reducing sugar. However, many candidates incorrectly deduced that glucose was present. Some even came to the wrong conclusion that starch could not pass through the dialysis tubing. A considerable number of candidates stated wrongly that glucose passed out of the dialysis tubing by osmosis. These answers showed that some candidates were rather weak in their logical thinking and they tended to rely on factual recall to explain experimental results. (ii) (1) Many candidates wrongly concluded that starch was digested into glucose. The result of the Benedict’s test only suggested that reducing sugar was formed from the breakdown of starch. In fact, the two digestive juices in humans or other mammals that can break down starch, i.e. saliva and pancreatic juice, produce maltose instead of glucose through the action of amylase. (2) (iii) Many candidates gave the name of the enzymes involved, i.e. salivary and pancreatic amylase, instead of the names of the digestive juices. Some candidates stated the purpose of the precautions instead of indicating the precautions themselves, which was required by the question. This was probably due to a

Paper I Marking Scheme 1994 Year Report

P.1

CE BIO 1994
misinterpretation of the question, which could have been avoided by more careful reading.

Paper I Marking Scheme 1994 Year Report

P.2

CE BIO 1994
1. (b) (i) The difference in no. of yeast cells between flask A and B at hour 20 = 675 - 150 --------------------------------------------------------------------- 1 = 525 ------------------------------------------------------------------------------------------------------------- 1 EITHER in flask A, the yeast cells undergo aerobic respiration ----------------------------------------------- 1 whereas in flask B, the yeast cells undergo anaerobic respiration. ------------------------------- 1 Anaerobic respiration releases less energy for cell division / growth of the yeast ------------ 1 OR In flask A, the yeast cells undergo aerobic respiration whereas in flask B, the yeast cells undergo anaerobic respiration ------------------------------- 1 Anaerobic respiration produce ethanol / alcohol ------------------------------------------------------ 1 which inhibits the cell division / growth of the yeast...
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