18   6 The red flour beetle (Tribolium castaneum) is a food pest found in flour. The photograph shows a red flour beetle. A B (Source: © Nigel Cattlin/Alamy Stock Photo) magnification ×15 (a) Calculate the length of the red flour beetle between A and B. (1) .............................................................. mm (b) The main food source for adult red flour beetles is starch. Red flour beetles lay eggs that contain all the nutrients required for the development of the embryo. The main food source in the eggs is glycogen. Describe how starch can be used to make glycogen. 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Turn over 19   (c) The graph shows the changes in mass of glucose and glycogen in the eggs of the red flour beetle during embryo development. 80 70 60 50 40 30 20 10 0 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 Time after egg laid / hours Mass per egg / μg glucose glycogen (i) Calculate the percentage of glycogen used from 4 to 8 hours after the eggs were laid. (2) .............................................................% 20   (ii) A student suggested that between 20 and 24 hours the cells in the developing embryo switched from anaerobic to aerobic respiration. Describe how a respirometer could be used to test this suggestion. 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(Total for Question 6 = 9 marks) Turn over 21   *7 The human genome codes for approximately 20 000 different proteins. The pie chart shows the proportion of proteins carrying out different functions. transcription factors and other nucleic acid binding molecules 20% receptors and control of receptor activity 11% carrier and transporter 17% communication and signalling inside cells 10% enzymes 41% other less than 1% defence 1% The 20 amino acids used to make proteins can be classified according to the properties of their side chains (R groups). Table 1 shows the number of amino acids with these properties. Property of the side chain group Number of amino acids with the property Non-polar 9 Polar, uncharged 6 Negatively charged 3 Positively charged 2 Table 1 22   Table 2 shows three amino acids, used to synthesise proteins, that have unique properties. Amino acid Comment on structure Cysteine The side chain contains a thiol group (-S-H) that is chemically reactive. Glycine The side chain is a hydrogen atom which is much smaller than any other side chain. This allows tight coiling of polypeptide chains. Proline The side chain forms a peptide bond with the nitrogen in the amino group. This makes a polypeptide chain more rigid. Table 2 Discuss the importance of the amino acid side chain to the structure, function and location of proteins. 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(Total for Question 7 = 9 marks)