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.