Hydrogencarbonate indicator is a water-soluble solution that can act as a source of carbon dioxide for aquatic photosynthetic organisms. The solution changes colour depending on the concentration of carbon dioxide in the solution. These colours are related to different pH values, as shown in Table 1.1. Table 1.1 colour of hydrogencarbonate indicator solution | pH | concentration of carbon dioxide in the solution yellow | 7.6 | increasing carbon dioxide concentration yellow-orange | 7.8 | orange | 8.0 | orange-red | 8.2 | red | 8.4 | atmospheric concentration red-magenta | 8.6 | decreasing carbon dioxide concentration magenta | 8.8 | magenta-purple | 9.0 | purple | 9.2 | Chlorella vulgaris is a protoctist that is single-celled, aquatic and photosynthetic. It can be immobilised in alginate beads. Alginate beads with immobilised C. vulgaris can be used to measure the rate of photosynthesis. [Figure 1.1] [Figure 1.2] [Figure 1.3] [Figure 1.4] [Figure 1.5]

1. (a)A student noticed that a colour change occurred, from red to magenta, when the alginate beads with immobilised C. vulgaris were left in a container of hydrogencarbonate indicator solution and exposed to light. Explain why this colour change occurred.[2]
2. (b)(i)Identify the independent variable in this investigation.[1]
3. (b)(ii)The student was provided with a supply of alginate beads containing immobilised C. vulgaris. Describe a method the student could use to collect data to determine the effect of light intensity on the rate of photosynthesis of C. vulgaris using hydrogencarbonate indicator and the experimental set-up in Fig. 1.1. Your method should be set out in a logical order and be detailed enough to let another person follow.[8]
4. (c)The student set up a large test-tube containing alginate beads with immobilised C. vulgaris in hydrogencarbonate indicator solution at pH 8.4 (red). The student kept this set-up in the dark for 12 hours. Predict and explain the results that will be observed after 12 hours in the dark.[2]
5. (d)(i)When the scientists inserted the Secchi stick into a sample from their cell suspension, the circle (on the Secchi stick) was not able to be seen at a depth of 1.9 cm. Using the graph in Fig. 1.4, calculate the actual number of cells per cm³ of suspension. Show your working and give your answer to the nearest 1000 cells. number of cells per cm³ of suspension ..................................................[2]
6. (d)(ii)Count the number of cells in the 1 mm × 1 mm section of the counting chamber shown in Fig. 1.5. Use your answer to calculate the number of cells per cm³ of the suspension. Show all your working. number of cells per cm³ of suspension ..................................................[3]
7. (d)(iii)The scientists decided that using the Secchi stick was a less accurate method for determining the number of cells per cm³ of suspension. Give two reasons why using the Secchi stick is less accurate than using a counting chamber.[2]

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