Algae are a diverse group of photosynthetic eukaryotes in the kingdom Protoctista. Rhodomonas salina and Skeletonema costatum are two species of alga that live in seawater. Fig. 1.1 shows a scanning electron micrograph of R. salina, which is a unicellular organism. R. salina is red in colour. [Figure 1.1] Fig. 1.2 shows a scanning electron micrograph of S. costatum, which is also a unicellular organism. The individual cells can group together in a chain. S. costatum is yellow-brown in colour. [Figure 1.2]

1. (a)A student prepared extracts of the photosynthetic pigments from R. salina and S. costatum. The student carried out chromatography to identify and compare the pigments in the two species. The chromatograms are shown in Fig. 1.3. [Figure 1.3]
2. (a)(i)Use Fig. 1.3 to calculate the Rᶠ value of alloxanthin from R. salina. Rᶠ of alloxanthin = .............................................................. [1][1]
3. (a)(ii)Table 1.1 shows the Rᶠ values of some photosynthetic pigments. [Table 1.1] R. salina lacks two photosynthetic pigments that are present in S. costatum. Use Fig. 1.3 and Table 1.1 to identify the two photosynthetic pigments that are present in only S. costatum. [2][2]
4. (b)The extract of the photosynthetic pigments from each of the two species was used to obtain absorption spectra as shown in Fig. 1.4. [Figure 1.4] The student planned to investigate the effect of light wavelength on the rate of photosynthesis in R. salina and S. costatum. The student planned to determine the rate of photosynthesis in the two species when they were exposed to three different colours of light: • blue light, peak wavelength of 455nm • green light, peak wavelength of 550 nm • red light, peak wavelength of 670 nm. Use Fig. 1.4 to compare the expected rates of photosynthesis for the two species in blue, green and red light. blue light .......................................................................................................................................................................................................................................................................................................................................................... green light .......................................................................................................................................................................................................................................................................................................................................................... red light .......................................................................................................................................................................................................................................................................................................................................................... [3][3]
5. (c)In the investigation, the student used three different filters, blue, green and red, to expose the two species to the three different colours of light. The student immobilised the algae in sodium alginate to form two sets of algal beads, one set for R. salina and one set for S. costatum. The algal beads were the same size. Photosynthesis of the algae is not affected when they are immobilised in the beads. The student placed the algal beads in small bottles for the experiment as shown in Fig. 1.5. [Figure 1.5] The student used hydrogencarbonate indicator solution to estimate the rate of photosynthesis when the algal beads were exposed to each colour of light. The indicator solution changes colour with pH as shown in Table 1.2. [Table 1.2] The student used pH change as an indirect measure of the rate of photosynthesis. A greater decrease in CO₂ concentration indicates a higher rate of photosynthesis. For each bottle tested, the student: • added algal beads to a small bottle containing indicator solution • removed a sample of the indicator solution from the small bottle after some time • used a colorimeter to measure the absorbance of the sample of the indicator solution. The student used a calibration curve to estimate the pH of the samples of the indicator solution.
6. (c)(i)Suggest one reason why using algal beads, instead of placing the algal cells directly into the indicator solution in the bottles, improves the validity of the investigation. [1][1]
7. (c)(ii)Identify the two independent variables and the dependent variable in this investigation. independent variable 1 .......................................................................................................................... independent variable 2 .......................................................................................................................... dependent variable .......................................................................................................................... [3][3]
8. (c)(iii)In addition to the colour filters, algal beads, small bottles, indicator solution and a colorimeter, the student had access to standard laboratory equipment. The student carried out the investigation in a temperature-controlled room. Describe a method the student could use to collect sufficient data so that the rates of photosynthesis of both species of alga in the three colours of light can be compared. The description of your method should be set out in a logical way and be detailed enough for another person to follow. You should not include a risk assessment or details of how to prepare the algal beads. [8][8]
9. (c)(iv)Identify a hazard in this investigation and state a risk associated with the hazard and state one precaution that the student should take. [1][1]
10. (d)For each colour of light, the student carried out a statistical test to compare the pH of the samples taken from the small bottles that contained R. salina and S. costatum. The student decided that a t-test was the most appropriate test to use for these data.
11. (d)(i)Suggest one reason why a t-test was the most appropriate test to use for these data. [1][1]
12. (d)(ii)State a null hypothesis for this test. [1][1]

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