Turn over 35 9 Auxins and cytokinins are plant growth regulators that control apical dominance and cell elongation. (a) Explain how auxins and cytokinins control the growth of lateral buds. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (b) Auxin is an uncharged molecule when it diffuses through the cell surface membrane. When inside the cytoplasm, auxin develops a negative charge. Auxin leaves cells by facilitated diffusion. Explain how the structure of the cell membrane affects the transport of auxin. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 36 (c) When auxin enters a cell, it causes hydrogen ions to be pumped out of the cell into the cell wall. The effect of auxin on the pH of the cell wall and cell elongation was investigated. Auxin was added to plant cells. The pH of the cell wall and cell elongation were measured over a period of 60 minutes. The results are shown in the graph. 240 200 160 120 80 40 0 Elongation of cell / μm 60 50 40 30 20 10 0 Time / min 6.5 6.0 5.5 5.0 4.5 pH of cell wall Key elongation of cell pH of cell wall (i) Calculate the maximum rate of elongation of the cell. (2) Answer.............................................................. 37 (ii) Expansin is a protein located in cell walls. Expansin disrupts hydrogen bonds and ionic bonds of molecules in the cell wall. Deduce a mechanism by which auxin and expansin cause cells to elongate. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 9 = 10 marks) TOTAL FOR PAPER = 90 MARKS 38 BLANK PAGE 39 BLANK PAGE 40 BLANK PAGE