During the light dependent stage of photosynthesis, hydrogen ions and electrons are released by the photoactivation of chlorophyll and the photolysis of water. These are added to the carrier molecule, NADP, changing it from an oxidised state to a reduced state, as shown in Fig. 1.1. 2H⁺ + 2e⁻ + oxidised NADP → reduced NADP [Figure 1.1] DCPIP is a blue dye which becomes colourless when reduced by hydrogen ions and electrons, as shown in Fig. 1.2. oxidised DCPIP (blue) → reduced DCPIP (colourless) [Figure 1.2] A student used DCPIP to determine if the reactions in the light dependent stage had occurred in a suspension of chloroplasts. • The suspension of chloroplasts was prepared by grinding, filtering and centrifuging a sample of leaves with ice-cold extraction medium. • Four tubes covered in foil were set up containing the mixtures shown in Table 1.1. [Table 1.1] • The tubes were illuminated through a thin glass tank filled with water as shown in Fig. 1.3. [Figure 1.3] • The foil was removed from tubes 1, 2 and 3, but left on tube 4. • A timer was started immediately. • After 10 minutes the colour in each tube was observed and recorded. Table 1.2 shows the student's results. [Table 1.2]

1. (a)State why the results in Table 1.2 are qualitative.[1]
2. (b)Explain why tubes 2, 3 and 4 were included.[2]
3. (c)State the function of the thin tank of water.[1]
4. (d)(i)Identify the independent variable and the dependent variable in this investigation. independent dependent[2]
5. (d)(ii)Using the apparatus shown in Fig. 1.3, describe a method the student could use to collect the data needed to test their hypothesis. Your method should be set out in a logical way and be detailed enough to let another person follow it.[7]
6. (e)Light intensity (I) can be calculated using the formula shown in Fig. 1.4. I = 1 distance² [Figure 1.4] Use the formula shown in Fig. 1.4 to calculate the light intensity for the investigation shown in Fig. 1.3. I =[1]
7. (f)(i)The student concluded that the reaction reached completion at 8 minutes. State why the data in the graph do not support this conclusion.[1]
8. (f)(ii)Draw a tangent on Fig. 1.5 and use it to calculate the rate of change at 6 minutes. rate of change at 6 minutes =[2]
9. (f)(iii)On Fig. 1.5, sketch the curve that you would expect for the tube at low light intensity.[1]
10. (g)(i)Suggest a null hypothesis for the t-test used for tubes 1 and 2.[1]
11. (g)(ii)The t-tests were used to compare the means. State one feature of data that allows use of the t-test.[1]
12. (g)(iii)The student calculated that there were 22 degrees of freedom in each of these t-tests. An equal number of replicates for each tube was carried out. State the number of replicates of each tube that were carried out.[1]
13. (g)(iv)The critical value for t at p < 0.05 in this investigation was 2.07. State what the results in Table 1.4 tell you about the student's calculated value of t when comparing the effect between weed killers X and Y.[1]

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