Seaweeds are photosynthetic multicellular organisms found on the coast of many parts of the world. There are green, brown and red seaweeds. During a field study, some students observed that the differently coloured seaweeds grew at different distances from the highest part of the shore reached by the sea (the high tide line). Fig. 1.1 shows the distribution of these differently coloured seaweeds. [Figure 1.1] The students were asked to compare the action spectra of the differently coloured seaweeds. The students decided to: • make a suspension of chloroplasts from each of the differently coloured seaweeds • for each suspension of chloroplasts, find the time for methylene blue to change colour at different wavelengths of light. Methylene blue is a redox indicator that changes colour from blue to colourless as a result of the reactions of the light dependent stage of photosynthesis.

1. (a)State the independent variable and the dependent variable in the experiment to find the action spectrum of green seaweed. independent variable dependent variable[2]
2. (b)(i)From the internet, the students found that: • a chloroplast suspension can be obtained from a suspension of crushed seaweed tissue by filtering out and discarding the large fragments • chloroplasts in suspension are easily destroyed by enzyme activity • chloroplasts in suspension can burst by absorbing too much water by osmosis • light of different wavelengths can be obtained from a light source using light filters of different colours, e.g. a red filter only transmits (allows through) red light from a light source. The students were provided with the materials listed. • samples of green seaweed, brown seaweed and red seaweed in sea water • bench lamp • light filters of different colours that transmit light at known wavelengths • ice cold phosphate buffer solution at pH 7 • 5 cm³ dilute methylene blue solution with a dropping pipette • squares of small mesh cloth for filtering suspensions • crushed ice in a shallow plastic bowl • balance • scissors • mortar and pestle • blender • 1 cm³, 5 cm³ and 10 cm³ syringes • 50 cm³ measuring cylinder • metal foil • 100 cm³ beakers • test-tubes • test-tube rack • stop-clock or timer • white tile or white card Describe how the students could use this apparatus to: • prepare a suspension of chloroplasts from each of the differently coloured seaweeds • find the time for methylene blue to change colour at different wavelengths of light for each of the chloroplast suspensions. Your method should be set out in a logical order and be detailed enough to let another person follow it.[10]
3. (b)(ii)Describe how the students could use their results to find the action spectrum for each of the differently coloured seaweeds.[2]
4. (c)(i)In another experiment, the students used thin layer chromatography to separate the chloroplast pigments from each of the differently coloured seaweeds. Fig. 1.2 shows the chromatogram obtained and the colours of the pigments. [Figure 1.2] Compare the chloroplast pigments present in the three differently coloured seaweeds, as shown in Fig. 1.2.[2]
5. (c)(ii)The students suggested that brown seaweed and red seaweed can photosynthesise in deeper water than green seaweed, due to differences in their pigments. The students based this suggestion on their observations shown in Fig. 1.1 and their results shown in Fig. 1.2. The students found the wavelengths of light absorbed by each of the different chloroplast pigments that are shown in Fig. 1.2. Table 1.1 shows the range of wavelengths at which light is absorbed by each of these pigments. [Table 1.1] The depth that light reaches into water is dependent on wavelength. The shorter the wavelength, the further light can travel into water. Discuss whether or not the data in Table 1.1 support the suggestion that brown seaweed and red seaweed can photosynthesise in deeper water than green seaweed, due to differences in their pigments.[2]

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