Coordination and response in plants 16w3gmz

References: to the ‘amount of light / water’ were accepted as equivalent to light intensity and volume of water, respectively, however students should always be encouraged to use appropriate scientific vocabulary whenever possible. Other creditworthy answers that were seen included age or size of shoots, temperature and soil type. Suggestions such as ‘time’, ‘direction of light’, ‘type of box’ and ‘black plastic used’ gained no marks. Students should always be as specific as possible in answers – responses such as ‘type of conditions’ and ‘surroundings they are grown in’ were clearly weak and gained no credit. (b)     Many answers to this part were excellent and displayed a sound understanding of tropisms. Explanations such as ‘more auxin is found on the dark side and this causes the cells here to grow’ were succinct and accurate, gaining all four marks in one sentence. Despite this students often then felt the need to elaborate or repeat in order to fill the space provided. Some students, however, were a little less certain of their ground. ‘Hormone’ was an acceptable alternative for ‘auxin’ but a variety of other, incorrect terms were used as well, including ‘enzyme’, ‘oestrogen’, ‘aphid’, ‘atom’, ‘axon’ and ‘toxin’. Many picked up the idea of unequal distribution of the hormone but then gave the wrong side or described it as ‘going down to the root’, often ‘sinking due to gravity’. Some hedged their bets and just said it was found on ‘one side’. When writing about the effect of the hormone, students gained credit for terms which clearly implied ‘growth’ such as elongation or cell division but not for weaker references like ‘bending’ (which was in the question stem), ‘leaning’, ‘tilting’ or ‘stretching’. Some students had clearly revised tropisms in terms of geotropisms of shoots placed horizontally. These students often discussed ‘auxin concentration increasing on the lower side’ or the effects of gravity on the movement of auxin, thus failing to apply their knowledge to the particular situation presented in the question. Students were asked to ‘explain how’ not to ‘explain why’ the plant responded in this way. Weaker students continue to misinterpret questions and, consequently, fail to gain credit. Quite a few, therefore, answered in terms of the shoot bending towards light ‘for more photosynthesis’ and failed to address how this had happened. (c)     (i)      Students were asked to draw conclusions from the results about the detection of the light stimulus. Quite a few, however, answered only in terms of the response by the shoot while some discussed the advantages of this response to the plant. A number of students also appeared to have misread “detection” and described the ‘direction’ of the light stimulus. Credit was given to any reference to the tip or top of the shoot detecting the light but not to ‘the part where the response occurs’ or ‘the side facing the light’. (ii)     Students were directed to identify a ‘part of the shoot’. Most did this and there were many correct answers. However, it was clear that having decided that ‘the tip’ was their answer here, many students went back to part (c)(i) and crossed out the correct answer, presumably thinking that the two could not be the same. The mark scheme to this question was quite generous, allowing virtually any reference to a region in the upper half of the shoot and many gained the mark here as a result. Incorrect ideas ranged through all other parts of a plant - even seed, stigma and stomata. Some students, again, misread the question - in this case the key word ‘where’ - and answered in terms of ‘when’ or ‘how’ instead.     E7.          Most candidates correctly sorted out the climatic factors but significant numbers failed to mention the effect of these on the rate of photosynthesis.