13. Photosynthesis
A section of Biology, 9700
Listing 10 of 183 questions
Maize, Zea mays, is a major cereal food crop. Unlike most crop plants, maize seed is produced by hybridisation between two different inbred parental strains. Explain why this is done. Suggest one disadvantage of producing seed in this way. In the light-independent stage of photosynthesis, the enzyme rubisco catalyses the combination of carbon dioxide with ribulose bisphosphate, RuBP. When the carbon dioxide concentration within the leaf is very low, rubisco tends to combine oxygen, rather than carbon dioxide, with RuBP. This process is called photorespiration. It reduces carbon dioxide assimilation and therefore reduces crop yields. Photorespiration is most likely to happen in hot, dry conditions. Suggest why photorespiration is most likely to take place in hot, dry conditions. Explain how the leaf anatomy of a maize plant reduces photorespiration, even in hot, dry conditions. It is expected that the carbon dioxide concentration in the atmosphere will increase in the future, which would be expected to increase rates of photosynthesis in many crop plants. Investigations were carried out into the effect of increased carbon dioxide concentration on the rate of photosynthesis in maize. • Maize plants were grown in open-air trials, in the same field and were exposed to the same changes in the weather. • 50% of the plants were exposed to a normal carbon dioxide concentration. • 50% of the plants were exposed to an increased carbon dioxide concentration. • The rate of photosynthesis was measured as the net assimilation rate of carbon dioxide. • Measurements were made at three-hourly intervals between 0700 hours and 1900 hours on three different days. The results are shown in . day 1 day 2 day 3 net assimilation rate of CO2 / arbitrary units time of day normal CO2 concentration increased CO2 concentration key Suggest an explanation for the lack of effect of carbon dioxide concentration on the rate of photosynthesis in maize plants, shown by these results. Suggest one explanation for the changes in the rate of photosynthesis between 0700 hours and 1900 hours on day 1.
9700_s11_qp_42
THEORY
2011
Paper 4, Variant 2
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Maize, Zea mays, is a major cereal food crop. Unlike most crop plants, maize seed is produced by hybridisation between two different inbred parental strains. Explain why this is done. Suggest one disadvantage of producing seed in this way. In the light-independent stage of photosynthesis, the enzyme rubisco catalyses the combination of carbon dioxide with ribulose bisphosphate, RuBP. When the carbon dioxide concentration within the leaf is very low, rubisco tends to combine oxygen, rather than carbon dioxide, with RuBP. This process is called photorespiration. It reduces carbon dioxide assimilation and therefore reduces crop yields. Photorespiration is most likely to happen in hot, dry conditions. Suggest why photorespiration is most likely to take place in hot, dry conditions. Explain how the leaf anatomy of a maize plant reduces photorespiration, even in hot, dry conditions. It is expected that the carbon dioxide concentration in the atmosphere will increase in the future, which would be expected to increase rates of photosynthesis in many crop plants. Investigations were carried out into the effect of increased carbon dioxide concentration on the rate of photosynthesis in maize. • Maize plants were grown in open-air trials, in the same field and were exposed to the same changes in the weather. • 50% of the plants were exposed to a normal carbon dioxide concentration. • 50% of the plants were exposed to an increased carbon dioxide concentration. • The rate of photosynthesis was measured as the net assimilation rate of carbon dioxide. • Measurements were made at three-hourly intervals between 0700 hours and 1900 hours on three different days. The results are shown in . day 1 day 2 day 3 net assimilation rate of CO2 / arbitrary units time of day normal CO2 concentration increased CO2 concentration key Suggest an explanation for the lack of effect of carbon dioxide concentration on the rate of photosynthesis in maize plants, shown by these results. Suggest one explanation for the changes in the rate of photosynthesis between 0700 hours and 1900 hours on day 1.
9700_s11_qp_43
THEORY
2011
Paper 4, Variant 3
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Photosynthesis is affected by many environmental factors. Explain why light intensity can be a limiting factor in photosynthesis. The concentration of carbon dioxide (CO2) can also be a limiting factor. It has an effect on the Calvin cycle in the light-independent stage of photosynthesis. Ribulose bisphosphate (RuBP), triose phosphate (TP) and glycerate 3-phosphate (GP) are three important molecules in the Calvin cycle. shows how the concentration of GP changes when the concentration of CO2 is reduced from 0.04% to 0.008%. Complete by sketching the lines for RuBP and TP when the concentration of CO2 is reduced from 0.04% to 0.008%. time GP GP TP RuBP relative concentration of Calvin cycle molecules atmospheric CO2 0.04% reduced CO2 concentration 0.008% In very dry conditions, CO2 concentration can become the main limiting factor of photosynthesis in plants. Explain how very dry conditions cause CO2 concentration to become the main limiting factor of photosynthesis in plants. A factor that can limit the rate of photosynthesis is the rate of regeneration of RuBP. Sedoheptulose-1,7-bisphosphatase (SBPase) is an enzyme in the Calvin cycle that controls the rate of regeneration of RuBP. SBPase is coded for by the gene SBPase. In an experiment, wheat plants were genetically modified to make more SBPase by introducing the SBPase gene from another grass species, Brachypodium distachyon. The resulting GM wheat plants were named Sox4. • Wild type plants (not GM) and Sox4 plants were grown in a greenhouse. • Light intensity, CO2 concentration and temperature were kept constant. • Mature plants were removed and dried to measure the biomass. shows the mean plant biomass for the wild type plants and GM Sox4 plants. mean plant biomass / g wheat plants wild type Sox4 Calculate the percentage change in mean plant biomass when Sox4 plants are grown compared to wild type plants. Show your working. percentage change = % Suggest and explain why Sox4 plants have a different mean plant biomass than wild type plants. Some soils may be deficient in nitrates. Suggest how nitrate deficiency could limit the quantity of SBPase made by Sox4 plants.
9700_s22_qp_42
THEORY
2022
Paper 4, Variant 2
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In plants such as rice, Oryza sativa, rubisco has a low rate of activity, which in turn affects the rate of photosynthesis. The cereal crop sorghum, Sorghum bicolor, has a high rate of activity of rubisco. A genetically modified (GM) variety of rice was produced. Parts of the quaternary structure of rubisco in rice were altered to be the same as the rubisco in sorghum. The rate of activity of rubisco in non-GM rice and GM rice was measured at different concentrations of atmospheric carbon dioxide (CO2). The results are shown in . rate of activity of rubisco / arbitrary units atmospheric CO2 concentration / mg dm–3 GM rice non-GM rice GM rice non-GM rice Compare the two curves shown in and explain why the curve for non-GM rice levels off. Suggest which part of the rubisco molecule was altered to produce the GM variety of rice. shows an outline of the Calvin cycle. CO2 GP RuBP TP other molecules ATP ADP + Pi ATP ADP + Pi NADP reduced NADP Name the process that involves the enzyme rubisco. State the type of reaction that occurs when GP is converted to TP. RuBP is regenerated from molecules of TP. State how many molecules of RuBP are produced from 10 molecules of TP. TP molecules that are not involved in the regeneration of RuBP can be used in the synthesis of other molecules. State two molecules that can be produced from these TP molecules.
9700_s23_qp_42
THEORY
2023
Paper 4, Variant 2
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Grass crops such as maize, sorghum and sugarcane are C4 plants. They are common grass crops of tropical regions. Oats and wheat, commonly grown in temperate regions, are C3 plants. Most plants are C3 plants. They are termed ‘C3’ because the first product of photosynthesis is a three carbon compound. Outline how the biochemistry of C4 plants differs from that of C3 plants. The C4 pathway for fixing carbon dioxide was worked out in 1966 by Hatch and Slack. During their investigation they measured the rates of fixation of carbon dioxide at high light intensities in leaves removed from both temperate and tropical grasses. They also measured the rates of activity of two carboxylase enzymes in the leaves, ribulose bisphosphate carboxylase and PEP carboxylase. All rates were measured at 30 °C. Some of their results are shown in Table 2.1. Table 2.1 grass crop rate of fixation of carbon dioxide / arbitrary units rate of activity of rubisco / arbitrary units rate of activity of PEP carboxylase / arbitrary units maize 3.5 0.62 17.50 sorghum 3.1 0.35 15.80 sugarcane 2.9 0.30 18.50 oats 1.6 4.50 0.33 wheat 1.7 4.70 0.29 With reference to Table 2.1, compare the rates of fixation of carbon dioxide in C3 and C4 grasses. Describe the role of rubisco in the Calvin cycle. With reference to Table 2.1, suggest reasons for the differences in activity of the two carboxylase enzymes in C3 and C4 grasses. It has been calculated that, to produce one molecule of glucose, the C3 pathway uses 18 molecules of ATP and the C4 pathway uses 30 molecules of ATP. Suggest why C4 plants can afford this high cost of ATP.
9700_w16_qp_41
THEORY
2016
Paper 4, Variant 1
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Grass crops such as maize, sorghum and sugarcane are C4 plants. They are common grass crops of tropical regions. Oats and wheat, commonly grown in temperate regions, are C3 plants. Most plants are C3 plants. They are termed ‘C3’ because the first product of photosynthesis is a three carbon compound. Outline how the biochemistry of C4 plants differs from that of C3 plants. The C4 pathway for fixing carbon dioxide was worked out in 1966 by Hatch and Slack. During their investigation they measured the rates of fixation of carbon dioxide at high light intensities in leaves removed from both temperate and tropical grasses. They also measured the rates of activity of two carboxylase enzymes in the leaves, ribulose bisphosphate carboxylase and PEP carboxylase. All rates were measured at 30 °C. Some of their results are shown in Table 2.1. Table 2.1 grass crop rate of fixation of carbon dioxide / arbitrary units rate of activity of rubisco / arbitrary units rate of activity of PEP carboxylase / arbitrary units maize 3.5 0.62 17.50 sorghum 3.1 0.35 15.80 sugarcane 2.9 0.30 18.50 oats 1.6 4.50 0.33 wheat 1.7 4.70 0.29 With reference to Table 2.1, compare the rates of fixation of carbon dioxide in C3 and C4 grasses. Describe the role of rubisco in the Calvin cycle. With reference to Table 2.1, suggest reasons for the differences in activity of the two carboxylase enzymes in C3 and C4 grasses. It has been calculated that, to produce one molecule of glucose, the C3 pathway uses 18 molecules of ATP and the C4 pathway uses 30 molecules of ATP. Suggest why C4 plants can afford this high cost of ATP.
9700_w16_qp_42
THEORY
2016
Paper 4, Variant 2
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Questions Discovered
183