22 8 The photograph shows a ghost shrimp that lives in fresh water. heart © Nicholas Toh/Alamy Stock Photo The effect of caffeine concentration on the heart rate of ghost shrimps was investigated. A shrimp was placed in a caffeine solution and observed using a microscope. The number of heartbeats in one minute was counted and the heart rate was recorded. This was repeated for other concentrations of caffeine solution. The investigation was repeated using two more ghost shrimps. The table shows the results of this investigation. Concentration of caffeine / mg cm–3 Heart rate / beats min–1 Shrimp 1 Shrimp 2 Shrimp 3 3 272 298 304 7 268 284 298 10 260 270 266 13 152 242 292 (a) (i) State and justify a suitable control for this investigation. 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Turn over 23 (ii) Variables relating to the caffeine solution and the ghost shrimps should have been controlled. State and justify two variables that should have been controlled in this investigation. (4) Variable relating to the caffeine solution........................................................................................................................................................................ ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... Variable relating to the ghost shrimps............................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... (b) (i) Calculate the percentage change in the heart rate of shrimp 2 when the caffeine concentration of 10 mg cm–3 was replaced with 13 mg cm–3. Give your answer to one decimal place. (2) Answer ................................................................................ % 24 (ii) Comment on the results of this investigation. 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(c) Explain one advantage of using ghost shrimps for this investigation. 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(Total for Question 8 = 14 marks) TOTAL FOR PAPER = 80 MARKS Every effort has been made to contact copyright holders to obtain their permission for the use of copyright material. Pearson Education Ltd. will, if notified, be happy to rectify any errors or omissions and include any such rectifications in future editions.