The light dependent stage of photosynthesis in a suspension of isolated chloroplasts can be investigated using the Hill reaction. Dichlorophenolindophenol (DCPIP) can be used to follow the process. DCPIP is a blue dye which turns colourless when it is reduced by accepting hydrogen and electrons. oxidised DCPIP → reduced DCPIP blue colourless (a) (i) The natural hydrogen acceptor in chloroplasts that is replaced by DCPIP in the Hill reaction is NADP. Reduced NADP is then used in the light independent stage of photosynthesis. State the role of reduced NADP in the light independent stage. (ii) Predict and explain the effect on the concentration of RuBP in the chloroplasts if DCPIP becomes reduced instead of NADP. DCPIP can be used to investigate the effect of light wavelength on the rate of the light dependent stage of photosynthesis. One method is described here: • Dip a small glass capillary tube into a beaker containing a suspension of isolated chloroplasts. Some of the chloroplast suspension will be drawn into tube A, forming a colour standard. • Mix the rest of the chloroplast suspension with blue DCPIP solution. Dip a second capillary tube into this to draw up a sample, to form tube B. • Store the chloroplast-DCPIP mixture for future use by wrapping the beaker in foil and covering it with a removable foil lid, as shown in [Figure 2.1]. • Place tube A and tube B on a white tile under a bench lamp placed 15cm away and immediately cover them with a purple filter, as shown in [Figure 2.2]. • Measure the time for tube B to change colour to match that of tube A. This is the time taken for DCPIP to decolourise. • Repeat four more times using the same tube A and a fresh tube B, placing the tubes under a different coloured filter each time. (b) Explain the reasons for: the colour standard tube A covering the beaker containing the chloroplast-DCPIP mixture with foil. (c) [Table 2.1] shows the results of the experiment. (i) Complete Table 2.1 by calculating the rate of the light dependent stage of photosynthesis for the orange wavelength, using the formula: rate = 1000/t where t = time in seconds. (ii) With reference to Table 2.1, describe and explain the effect of light wavelength on the rate of the light dependent stage of photosynthesis.