6 3 Phytochromes and IAA (indole acetic acid) are important substances that bring about growth responses in plants. (a) The diagram shows an oat seedling in part of a field. coleoptile of oat seedling oat seed surface of soil in the field Cells in the tip of the oat coleoptile release IAA. Explain how the IAA affects the growth of the coleoptile. (4) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 7 Turn over (b) Phytochrome is found in two different forms known as PR (or P660) and PFR (or P730). (i) Which row correctly describes the effects of sunlight on phytochrome? (1) Change in form of phytochrome Speed of change A PFR to PR slow B PFR to PR rapid C PR to PFR slow D PR to PFR rapid (ii) Phytochrome can be described as (1) A a form of opsin B a photosensitive pigment C an isomer of retinal D a type of cytochrome (iii) Give one example of a growth response of a plant that is affected by phytochrome. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 3 = 7 marks)