Turn over 17 5 The diagram shows a section of a human retina. X (a) Which of the following is the name of the cell labelled X? (1) A bipolar neurone B ganglion neurone C motor neurone D optic neurone (b) Explain the role of rhodopsin in the generation of a nerve impulse in cell X when light falls on the retina. 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This is the method used: • the volunteer sat in a room illuminated with white light for twenty minutes • the lights were switched off and the room was placed into total darkness for one minute • an LED emitting light of wavelength 500 nm at low light intensity was switched on • the light intensity was gradually increased until the volunteer was able to see the LED and this minimum light intensity was recorded • this was repeated with increasing lengths of time that the volunteer was kept in total darkness • the investigation was then repeated with an LED that emitted red light with a wavelength of 620 nm. The graph shows the results of this investigation. Time spent in darkness Minimum light intensity at which LED was seen 620 nm light 500 nm light The graph shows the relative sensitivity of rods and three types of cone to light of different wavelengths. 350 400 450 500 550 600 650 700 Wavelength / nm Relative sensitivity / a.u. Blue cones Green cones Rods Red cones Turn over 19 Analyse the graphs to explain the results of this investigation. 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(Total for Question 5 = 10 marks)