Centre Number Surname Candidate Number For Examinerʼs Use Tota Task 1 Other Names Candidate Signature Genera Certificate of Education Advanced Leve Examination June 2013 Human Bioogy HBI6X/PM1 Unit 6X A2 Externay Marked Practica Assignment Task Sheet 1 To be competed before Task Sheet 2 For submission by 15 May 2013 For this paper you must have: a ruer with miimetre measurements a cacuator. HBI6X/PM1
2 Investigating the effect of an antiseptic soution on the rate of respiration in yeast Introduction Yeast are microorganisms. Some species of yeast ive on human skin. Many skin ceansing products contain antiseptics that are caimed to reduce the popuations of microorganisms on skin. You wi investigate the effect of an antiseptic on the rate of respiration in yeast. You wi measure the rate of respiration by measuring the rate of carbon dioxide production by a yeast suspension. Task 1 In this task, you wi famiiarise yoursef with the technique invoved and record preiminary resuts. You wi use this information in Task 2. Materias: You are provided with: yeast suspended in a soution of gucose antiseptic soution distied water 2 arge tubes 2 sma tubes pastic pipettes gass rod thermometer boiing-tube rack timer marker pen 250 cm 3 beaker to use as a water bath access to hot and cod water a ruer with miimetre measurements. You may ask for any other apparatus you require.
3 Do not write outside the box Outine method Read these instructions carefuy before you start your investigation. Practise setting up the apparatus by foowing steps 1 to 4. 1. Competey fi a sma tube with water; hod this in one hand. 2. With your other hand turn a arge tube upside down. 3. Push the sma tube up inside the arge tube unti it reaches the end. You may use your finger or the gass rod to do this. 4. Keeping your finger or the gass rod in pace, quicky turn the arge tube the right way up. Most of the water shoud stay in the sma tube as shown in the diagram. Large tube Line drawn on tube Bubbe Yeast suspension Sma tube 5. Practise unti you fee confident in your technique then move on to step 6. 6. Set up a water bath at 35 ºC. Maintain the water at 35 ºC throughout the experiment by adding warm water to the beaker if the temperature drops. 7. Labe the arge tubes A and B. 8. Add 10 drops of antiseptic to one of the sma tubes. 9. Use a pastic pipette to fi up competey the sma tube with yeast suspension. 10. Set up the apparatus as you did in your practice using this tube instead of water. Your equipment shoud be set up as you did in step 4. 11. Draw a ine on tube A to show the eve of the bottom of the bubbe in the smaer tube. 12. Put tube A into the water bath and eave for 10 minutes. 13. Take tube A out of the water bath and draw a ine on tube A to show the new eve of the bottom of the bubbe. 14. Record the distance between the ines on tube A. 15. Add 10 drops of distied water to the second sma tube. 16. Repeat steps 9 to 14 using this tube and tube B. You wi need to decide for yoursef: how many repeats to carry out. Turn over
4 Do not write outside the box Recording your resuts Record your resuts in a suitabe tabe beow.
5 Do not write outside the box Questions on Task 1 Answer a questions in the spaces provided. 1(a) Tube B was the contro experiment for this investigation. Expain how it acted as the contro experiment. (1 mark) 1(b) You measured the gas produced in the sma tubes by recording the difference in the eve of iquid. Give two assumptions made when using this method of measuring gas produced. 1... 2... (2 marks) 2 Some of the yeast suspension was dispaced by the gas formed. Suggest how this woud have affected the resuts. (2 marks) Turn over
6 Do not write outside the box 3 The distance between the two ines on tube A is a measure of the rate of respiration of yeast. Expain why. (2 marks) 4 A student carried out a simiar investigation to you. She measured bubbe ength and recorded the ph of the contents of the sma tube. She found that the ph in tube A became more acidic at the end of each experiment. 4(a) Suggest why the ph became more acidic. (2 marks) 4(b) What coud the student do to prevent this ph change? (1 mark) 10 END OF TASK 1
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