Question 120 marks
Question
Anaerobic respiration in yeast causes the blue dye, methylene blue, to become colourless. You are going to investigate the effect of temperature on the rate of respiration in yeast. Read all the instructions but DO NOT CARRY THEM OUT until you have drawn a table for your results in the space provided in 1(a)(ii). You should wear the safety equipment provided during the practical work. Step 1 You are provided with a small beaker containing a yeast suspension. Stir the contents of this beaker with the glass rod. Step 2 Label one test-tube W, one test-tube WM, one test-tube C and the final test-tube CM. Place them in the test-tube rack provided. Step 3 Use a syringe to put 7 cm3 of yeast suspension into test-tube W and 7 cm3 of yeast suspension into test-tube C. Put test-tubes W and C back in the test-tube rack. Step 4 Raise your hand when you are ready for water to be added to the beaker labelled warm water. Step 5 Place test-tube W into the beaker labelled warm water and test-tube C into the beaker labelled cool water. Step 6 Use the thermometer to measure the temperatures of the warm and cool water. Record the temperatures in Table 1.1 in 1(a)(i). (a) (i) Table 1.1 beaker temperature at the start / °C temperature at the end / °C warm water cool water [1] Step 7 Start the stop-clock and wait for three minutes. Step 8 During this waiting time, use a pipette to add one drop of methylene blue dye to test-tube WM and one drop of methylene blue dye to test-tube CM. Step 9 After three minutes pour the contents of test-tube W into test-tube WM. Pour the contents of test-tube C into test-tube CM. Step 10 Use a pipette to slowly add a layer of oil to test-tube WM. The layer of oil should be approximately 1 cm thick. The layer of oil will float on top of the yeast suspension and methylene blue mixture, as shown in Fig. 1.1. Step 11 Place test-tube WM into the warm water beaker. layer of oil 1 cm to maintain anaerobic conditions test-tube containing yeast suspension and methylene blue dye not to scale Fig. 1.1 Step 12 Repeat step 10 for test-tube CM. Step 13 Place test-tube CM into the cool water beaker. Step 14 Restart the stop-clock. Step 15 Measure the time taken for the blue colour in test-tubes WM and CM to disappear. Record the times in seconds in your table in 1(a)(ii). If the time taken for the blue colour to disappear in each test-tube is more than 10 minutes stop timing and record >600 in your table in 1(a)(ii). Step 16 Measure the temperatures of the warm water and cool water beakers again and record these values in Table 1.1 in 1(a)(i). (ii) Prepare a table to record your results. [4] (iii) State a conclusion for the results in your table in 1(a)(ii). …… [1] (iv) Identify two variables that were kept constant in this investigation. 1 …… 2 …… [2] (b) (i) The data you have recorded in Table 1.1 may indicate that there is a source of error with the method used in this investigation. Identify the possible error and suggest an improvement to the method to reduce the effect of this error. error …… improvement …… [2] (ii) Identify one possible source of error in step 15 and suggest an improvement for this error. error …… improvement …… [2] (c) Anaerobic respiration in yeast produces a gas. State the name of an indicator which could be used to show that the gas is carbon dioxide and state the expected result. indicator …… expected result …… [2] (d) Anaerobic respiration in yeast cells also produces ethanol. In high concentrations ethanol can slow down the rate of respiration. Plan an investigation to determine the effect of different concentrations of ethanol on the rate of respiration in yeast cells. …… [6] [Total: 20]
Mark scheme
1(a)(i) four measurements in which the starting temperature for 1 warm water is higher than the starting temperature for cool water ; 1(a)(ii) table drawn with minimum two columns and a header line ; 4 appropriate column / row headings, with unit ; i.e. test-tube / temperature with warm and cool in the data cell OR temperature / °C, and time / s both times recorded in seconds ; suitable trend ; 1(a)(iii) at higher temperatures the methylene blue became, 1 colourless / disappeared, more quickly / AW ; 1(a)(iv) volume / amount / type of yeast (suspension) ; 2 number of drops / concentration / amount / type, of methylene blue / indicator / AW ; idea of equilibration time in water-bath / beaker ; depth / amount / type / volume / height / AW, of oil ; 1(b)(i) error from Table 1.1 2 temperature (of the water-bath) not controlled / temperature changes / AW ; improvement thermostatically controlled water-bath / maintain water-bath temperature / insulation of the beaker ; 1(b)(ii) error step 15 2 1 subjective / difficult to judge when the colour had disappeared / qualitative ; 2 idea that both tubes are observed at the same time ; 3 the indicator did not change within the ten minutes ; improvement 1 compare to, a test-tube containing yeast with no methylene blue / test-tube that had already changed colour / use a colorimeter / colour standard ; 2 idea that they should conduct each experiment at different times ; 3 measure for more than 10 minutes or dilute the methylene blue or use a different dye ; 1(c) hydrogencarbonate / bicarbonate ; 2 yellow or orange colour ; OR limewater ; goes cloudy / milky / white ; 1(d) independent variable: 6 A suitable alternative methods at least two different concentrations of ethanol ; dependent variable: measuring the time taken for methylene blue to decolourise ; constant variables: two from ;; • same temperature • same pH • same volume of ethanol (solution) • same volume / amount / concentration of sugar one from ; • same volume / amount / concentration / type, of yeast • same volume / amount / concentration, of methylene blue / indicator • same volume / depth of oil methodology idea of leaving to equilibrate at set temperature ; use of a water-bath / insulation to maintain one temperature ; detail of a comparison / control e.g. test with no ethanol / yeast, present ; two or more repeats / three or more trials ; safety relevant safety e.g. gloves / goggles / no flames ;