The light-dependent stage of photosynthesis occurs within chloroplasts. In this stage, electrons are emitted from the chlorophyll a molecules and passed to electron acceptors. If a redox indicator, such as DCPIP, is added to a suspension of illuminated chloroplasts, electrons will be transferred to DCPIP, causing the colour of the DCPIP to change from blue to colourless. A student investigated the effect of the wavelength of light (colour of light) on the rate of photosynthesis. • DCPIP was added to three colorimeter tubes, each containing a suspension of chloroplasts. The chloroplast suspensions were kept in the dark until required. • The colorimeter tubes were each exposed to light of a different colour: red, blue or green. The intensity of light was the same for all tubes, and each was exposed to light for four minutes. All other conditions were kept the same. • The absorbance of each chloroplast suspension was measured at one-minute intervals using a colorimeter. The results are shown in Fig. 7.1. [Figure 7.1]

1. (a)(i)Explain why the chloroplast suspensions were kept in the dark until required.[1]
2. (a)(ii)Describe the results shown in Fig. 7.1.[3]
3. (a)(iii)With reference to the light-dependent stage of photosynthesis, explain the differences between the results shown in Fig. 7.1 for red light and for green light.[3]
4. (b)Changes in the atmospheric carbon dioxide concentration, light intensity and temperature can affect the rate of photosynthesis. These three factors directly affect different processes of photosynthesis. Complete Table 7.1 using a tick (✔) to identify the processes that can be directly affected by each factor or a cross (x) to identify the processes that are not directly affected by each factor. Indirect effects where a change in the rate of one process affects the rate of a different process should not be considered. A tick or a cross must be placed in the final column of every row. Table 7.1 factor: carbon dioxide concentration, light intensity, temperature. process: Calvin cycle, photophosphorylation. ✓ or x[3]

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