13.2. Investigation of limiting factors
A subsection of Biology, 9700, through 13. Photosynthesis
Listing 10 of 75 questions
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 . 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 absorbance time / min green light blue light red light Explain why the chloroplast suspensions were kept in the dark until required. Describe the results shown in . With reference to the light-dependent stage of photosynthesis, explain the differences between the results shown in for red light and for green light. 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 (3) to identify the processes that can be directly affected by each factor or a cross (7) 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 process 3 or 7 carbon dioxide concentration Calvin cycle photophosphorylation light intensity Calvin cycle photophosphorylation temperature Calvin cycle photophosphorylation
9700_m24_qp_42
THEORY
2024
Paper 4, Variant 2
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is a photomicrograph of a transverse section through the leaf of a C4 plant. J K L Identify structures J to L. J K L Outline how this leaf anatomy adapts the plant for high rates of carbon fixation at high temperatures. Sorghum is a C4 plant and Sorghum bicolor is a major food crop in dry tropical regions. The leaves of S. bicolor are covered with a layer of wax made up of a mixture of esters and free fatty acids, with a melting point of 77– 85 °C. Waxes from the leaves of non- tropical plants tend to have melting points lower than this. For example, wax from the bayberry, Myrica sp., has a melting point of 45 °C. Suggest how the wax on sorghum leaves helps the plant to survive in dry, tropical regions. An investigation was carried out into the response of sorghum to being kept at a low temperature for a short period of time. Soybean plants, which are better adapted than sorghum for growth in subtropical and temperate climates, were used for comparison. Plants of sorghum and soybean were kept at 25 °C for several weeks and then at 10 °C for three days. The temperature was then increased to 25 °C again for seven days. Day length, light intensity and carbon dioxide concentration were kept constant throughout. The uptake of carbon dioxide, as mg CO2 absorbed per gram of leaf dry mass, was measured • at 25 °C before cooling • on each of the three days at 10 °C • for seven days at 25 °C. The results are shown in Table 4.1. Table 4.1 plant carbon dioxide uptake / mg CO2 g–1 at 25 °C, before cooling at 10 °C at 25 °C (mean over days 4 to 10) day 1 day 2 day 3 sorghum 48.2 5.5 2.9 1.2 1.5 soybean 23.2 5.2 3.1 1.6 6.4 Compare the changes in carbon dioxide uptake in sorghum and soybean during the three days at 10 °C. During the cooling period, the ultrastructure of the sorghum chloroplasts changed. The membranes of the thylakoids moved closer together, eliminating the spaces between them. The size and number of grana became reduced. Explain how these changes could be responsible for the low rate of carbon dioxide uptake by sorghum even when returned to a temperature of 25 °C.
9700_s10_qp_41
THEORY
2010
Paper 4, Variant 1
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Leaves of tobacco plants, Nicotiana spp, have a high concentration of nicotine, the addictive component of tobacco smoke. Scientists are continually seeking ways to produce tobacco plants that have reduced nicotine content. Describe and explain the effects of nicotine on the cardiovascular system that can contribute to a person developing coronary heart disease. The production of low-nicotine cigarettes and cigars is considered a strategy that may reduce the harmful effects of smoking. Explain whether or not you agree with this statement.
9700_s11_qp_22
THEORY
2011
Paper 2, 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_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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shows a diagram of a stoma, its guard cells and adjacent epidermal cells. stoma epidermal cell guard cells nucleus cytoplasm nucleus Guard cells have chloroplasts while epidermal cells do not have chloroplasts. State one other difference, visible in , between guard cells and epidermal cells. During stomatal closure: state precisely where abscisic acid (ABA) binds identify the ion that diffuses from the guard cells to epidermal cells compare the relative water potential of the guard cells with that of epidermal cells describe the change in volume of the guard cells. The following experiment was carried out to investigate the effect of light intensity on the rate of photosynthesis of a water plant, Elodea. • Elodea was cut into three pieces, each 10 cm long. • Each piece of Elodea was placed in a glass tube, containing 0.5% sodium hydrogencarbonate solution, which was then sealed with a bung. • Tube A was placed 10 cm away from a lamp. • Tube B was placed 5 cm away from a lamp. • Tube C was placed in a dark room. • An oxygen sensor was used to measure the percentage of oxygen in the solutions at the start of the experiment and again at 5, 10 and 20 minutes. The results are shown in . time / minutes percentage of oxygen in solution A B C State why sodium hydrogencarbonate solution was used. Calculate the mean rate of oxygen production for tube A for the 20 minutes of the experiment. Show your working. answer Compare the results for tubes A and B. Explain the results for tube C. Suggest what factor, which may have an effect on the rate of photosynthesis, was not taken into account in this experiment. shows the relationship between the light-dependent and light-independent reactions in a chloroplast. Calvin cycle light-dependent reactions X H2O CO2 O2 sugar Y Name the substances X and Y in . X Y
9700_s12_qp_42
THEORY
2012
Paper 4, Variant 2
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The unicellular green alga, Chlorella, a photosynthetic protoctist, was originally studied for its potential as a food source. Although large-scale production proved to be uneconomic, the many health benefits provided by Chlorella mean that it is now mass produced and harvested for use as a health food supplement. shows cells of Chlorella. Chlorella In one study into the productivity of Chlorella, carbon dioxide concentration was altered to investigate its effects on the light-independent stage of photosynthesis. • A cell suspension of Chlorella was illuminated using a bench lamp. • The suspension was supplied with carbon dioxide at a concentration of 1% for 200 seconds. • The concentration of carbon dioxide was then reduced to 0.03% for a further 200 seconds. • The concentrations of RuBP and GP (PGA) were measured at regular intervals. • Throughout the investigation the temperature of the suspension was maintained at 25 °C. The results are shown in . time / s concentration / arbitrary units 1 % CO2 0.03 % CO2 GP RuBP State precisely where in the chloroplast RuBP and GP are located. Explain why the concentration of RuBP changed between 200 and 275 seconds. Calculate the rate of decrease per second in the concentration of GP between 200 and 350 seconds. Show your working and give your answer to two decimal places. answer arbitrary units per second Explain how the decrease in the concentration of GP leads to a decreased harvest for commercial suppliers of Chlorella.
9700_s14_qp_41
THEORY
2014
Paper 4, Variant 1
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Questions Discovered
75