6 3 Single-celled algae can be trapped in gel beads and used to study photosynthesis. The beads are placed in a test tube of hydrogencarbonate indicator. The table shows the colour of the indicator when it contains different concentrations of carbon dioxide. Colour of indicator Relative carbon dioxide concentration yellow highest orange higher than atmospheric air red same as atmospheric air magenta lower than atmospheric air purple lowest A student used the following method to investigate the effect of light intensity on the rate of photosynthesis. 1. Set up five test tubes, each half-filled with red hydrogencarbonate indicator. 2. Add a teaspoon of gel beads containing single-celled algae to each test tube and close with a bung. 3. Place each test tube at a different distance from a lamp in a dark room. 4. Leave the tubes for 30 minutes. 5. Record the colour of the hydrogencarbonate indicator in each tube and the position of the gel beads. The table shows the results. Distance from lamp / cm 5 15 25 35 45 Colour of indicator after 30 minutes purple purple red orange orange Position of gel beads in tube after 30 minutes half‑way up less than half‑way up bottom bottom bottom Turn over 7 (a) (i) Describe two control tubes that should be used in this investigation. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (ii) Explain the changes in colour of the hydrogencarbonate indicator in this investigation. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 8 (b) The colour of the indicator and the position of the beads can be used to give a quantitative measure of the effect of light intensity. Describe how the method could be modified to give valid, quantitative results. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 3 = 8 marks)