In the light－dependent stage of photosynthesis，electrons and hydrogen ions are accepted by the coenzyme NADP，which becomes reduced． DCPIP is a dye that can act as an electron and hydrogen ion acceptor．The dye is blue when oxidised and colourless when reduced．In laboratory experiments，DCPIP can be used to follow the progress of the light－dependent stage because it can replace NADP as the acceptor molecule for electrons and hydrogen ions，as shown in Fig．1．1． Fig． 1.1 The effects of various factors on the light－dependent stage of photosynthesis can be investigated by using suspensions of isolated chloroplasts（chloroplast suspensions）and DCPIP． A student used DCPIP to investigate the effect of temperature on the rate of the light－dependent stage of photosynthesis in spinach，Spinacia oleracea． The student prepared a leaf extract to make a stock chloroplast suspension and then carried out a preliminary experiment to determine a suitable concentration of chloroplast suspension to use in the investigation． To carry out the preliminary experiment，the student followed a set of instructions，steps 1 to 11 ． 1 Cut spinach leaves into small pieces and place these pieces in a blender containing ice－cold $$\(10 \%\)$$ sucrose solution buffered at pH 7.0 ． 2 Turn on the blender for 15 seconds and then filter the extract to remove all the small pieces of leaf． 3 Place the leaf extract in a centrifuge and spin at low speed． 4 Pour off the supernatant that contains the chloroplasts．Keep this stock chloroplast suspension ice cold and in the dark． 5 Prepare 5 different concentrations of the chloroplast suspension using $$\(10 \%\)$$ sucrose solution． The percentage concentrations are $$\(10 \%, 20 \%, 30 \%, 40 \%\)$$ and $$\(50 \%\)$$ of the stock chloroplast suspension． 6 Wrap 5 flat－bottomed tubes in black plastic film to prevent light entering． 7 Put $$\(10 \mathrm{~cm}^{3}\)$$ of each concentration of stock chloroplast suspension into a flat－bottomed tube， and add $$\(1 \mathrm{~cm}^{3}\)$$ of DCPIP solution to each tube．The chloroplast suspension is now blue－green in colour． Fig. 1.2 9 Start a timer. Remove the black plastic film from the tube. Record the time taken for the DCPIP to decolourise so that the chloroplast suspension is green. 10 Calculate the rate of the light-dependent stage of photosynthesis by using the formula: $$\(t=\)$$ time taken in seconds for the chloroplast suspension to reach a green colour when all the DCPIP is decolourised. 11 Repeat step 8 to step 10 for the other tubes. ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ............................................................................................................................................... . ......................................................................................................................................... ［2］
Exam No：9700_w24_qp_52 Year：2024 Question No：(c)
Answer:
Knowledge points:
13.1.1 describe the relationship between the structure of chloroplasts, as shown in diagrams and electron micrographs, and their function
13.1.10.1 energetic electrons release energy as they pass through the electron transport chain (details of carriers are not expected)
13.1.10.2 the released energy is used to transfer protons across the thylakoid membrane
13.1.10.3 protons return to the stroma from the thylakoid space by facilitated diffusion through ATP synthase, providing energy for ATP synthesis (details of ATP synthase are not expected)
13.1.11.1 rubisco catalyses the fixation of carbon dioxide by combination with a molecule of
13.1.11.2 ribulose bisphosphate (RuBP), a 5C compound, to yield two molecules of glycerate 3-phosphate (GP), a 3C compound
13.1.11.3 GP is reduced to triose phosphate (TP) in reactions involving reduced NADP and ATP
13.1.11.4 RuBP is regenerated from TP in reactions that use ATP
13.1.12 state that Calvin cycle intermediates are used to produce other molecules, limited to GP to produce some amino acids and TP to produce carbohydrates, lipids and amino acids
13.1.2 explain that energy transferred as ATP and reduced NADP from the light-dependent stage is used during the light-independent stage (Calvin cycle) of photosynthesis to produce complex organic molecules
13.1.3 state that within a chloroplast, the thylakoids (thylakoid membranes and thylakoid spaces), which occur in stacks called grana, are the site of the light-dependent stage and the stroma is the site of the light-independent stage
13.1.4 describe the role of chloroplast pigments (chlorophyll a, chlorophyll b, carotene and xanthophyll) in light absorption in thylakoids
13.1.5 interpret absorption spectra of chloroplast pigments and action spectra for photosynthesis
13.1.6 describe and use chromatography to separate and identify chloroplast pigments (reference should be made to Rf values in identification of chloroplast pigments)
13.1.7 state that cyclic photophosphorylation and non-cyclic photophosphorylation occur during the light-dependent stage of photosynthesis
13.1.8.1 only photosystem I (PSI) is involved
13.1.8.2 photoactivation of chlorophyll occurs
13.1.8.3 ATP is synthesised
13.1.9.1 photosystem I (PSI) and photosystem II (PSII) are both involved
13.1.9.2 photoactivation of chlorophyll occurs
13.1.9.3 the oxygen-evolving complex catalyses the photolysis of water
13.1.9.4 ATP and reduced NADP are synthesised
Solution:
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