Chlorella is a photosynthetic, single-celled protoctist that lives in ponds. A group of students decided to investigate the effect of temperature on respiration in Chlorella. The students were provided with: • test-tubes • metal foil • flat-bottomed tubes with lids • glass beakers • hot water supply • thermometers • timer • pH probe and meter • hydrogencarbonate indicator solution in a closed container • a pump connected to an oxygen supply • a pump connected to a carbon dioxide supply • a suspension of Chlorella with a known cell density. Carbon dioxide is an acidic gas so it causes the pH of the hydrogencarbonate indicator solution to change. The indicator solution changes colour as pH changes. [Figure 1.1] shows the range of colours seen in hydrogencarbonate indicator exposed to different carbon dioxide concentrations. atmospheric carbon dioxide (0.04%) pH 8.4 yellow orange red magenta purple increasing carbon dioxide decreasing carbon dioxide concentration in indicator solution concentration in indicator solution Fig. 1.1 The students made a series of colour standards from yellow to purple by bubbling different volumes of carbon dioxide gas into the indicator solution. The pH was measured with a pH meter for each colour and the solutions were sealed in flat-bottomed tubes labelled with their pH. The students used the indicator solution to investigate the effect of temperature on carbon dioxide production during respiration in Chlorella.

1. (a)(i)Suggest a hypothesis that the students could test in this investigation.[1]
2. (a)(ii)State the independent variable and the dependent variable in this investigation. independent variable dependent variable[2]
3. (a)(iii)Describe how the students could use the apparatus provided to investigate the effect of temperature on carbon dioxide production during respiration in Chlorella. Your method should be set out in a logical order and be detailed enough to let another person follow it. Details of making the colour standards should not be included in your method.[7]
4. (b)(i)State how the students could calculate the rate of respiration from their results.[1]
5. (b)(ii)Sketch a graph on [Fig. 1.2] to show the expected result of the effect of temperature on the rate of respiration of Chlorella. Include suitable axes labels and units on your graph.[3]
6. (c)(i)Complete Table 1.1 by calculating the rate of carbon dioxide production for the cell density of 40 arbitrary units. The students used Pearson's linear correlation to test the hypothesis: As cell density increases the rate of carbon dioxide production increases.[1]
7. (c)(ii)Suggest a null hypothesis for this test.[1]
8. (c)(iii)The Pearson's linear correlation coefficient (r) was calculated as 0.85. State what the calculated value, r = 0.85, indicates about the results.[2]
9. (c)(iv)The students concluded that the results showed that their hypothesis is correct. Explain why this conclusion may not be valid.[2]

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