14   6 The nervous system is one of the features found only in animals. (a) The graph shows the change in potential difference across the cell surface membrane of a neurone when it is stimulated at point W. 0 2 4 6 8 12 10 W X Time / milliseconds 60 40 20 0 –20 –40 –60 –80 –100 Potential difference / millivolts (i) The threshold intensity is the minimum electrical stimulus that will cause a neurone to form an action potential. Which potential difference could be the threshold potential for this neurone? (1) A –80 millivolts B –70 millivolts C –60 millivolts D 40 millivolts Turn over 15   (ii) Explain the changes in the sodium and potassium ion concentrations in the cytoplasm of the neurone from point W to point X on the graph. 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(iii) Explain why the neurone becomes hyperpolarised after point X on the graph. (3) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 16   (b) The junction between the end of a motor neurone and a muscle myofibril is a modified synapse. The diagram shows this neuromuscular junction. muscle cell nucleus synaptic cleft acetylcholine vesicles motor neurone axon myofibril motor neurone endplate mitochondria Describe how the neurotransmitter crosses this synapse. 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(Total for Question 6 = 10 marks) Turn over 17   7 Athletes compete in a range of environmental conditions. (a) An investigation studied the effects of environmental conditions on athletes. A group of nine athletes ran for 50 minutes in two sets of environmental conditions: • warm temperature and high humidity • cool temperature and low humidity. All other variables were kept constant including the level of exercise. The core temperature of each athlete was recorded during the exercise and for 70 minutes following the exercise. The results are shown in the graph. 120 100 80 60 40 20 0 Time / minutes 40.0 39.5 39.0 38.5 38.0 37.5 37.0 36.5 Mean core temperature / °C Key warm temperature and high humidity cool temperature and low humidity (i) Explain why there was a change in core temperature during the first 20 minutes of exercise. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... 18   (ii) Using the results of this investigation, a student concluded that during exercise: • the human thermoregulatory system only works in cool environmental temperatures when the humidity is low. Comment on this conclusion. 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(iii) Compare and contrast the changes in core body temperature, in the two environments, after the exercise was completed. 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Turn over 19   (b) In another investigation, samples of blood were taken from athletes during exercise and after exercise. The concentration and activity of neutrophils, a type of phagocyte, in the blood were measured. The table shows the results. Measurements During exercise Immediately after exercise Mean concentration of neutrophils in blood / 106 cells cm–3 3.57 3.42 Mean activity of neutrophils / a.u. 28.4 22.0 (i) The mean body mass of the athletes was 70 kg and the mean volume of blood per athlete was 5 dm3. Calculate how many fewer neutrophils there are in the blood per kilogram of body mass immediately after exercise. (3) .............................................................. kg–1 (ii) State how these results support the statement that exercise increases the risk of infection. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 7 = 13 marks)