12. Energy and respiration
A section of Biology, 9700
Listing 10 of 192 questions
Yeast cells can respire in anaerobic conditions. Outline how yeast carries out respiration in anaerobic conditions. Explain why respiration in anaerobic conditions is an advantage to yeast. Bioethanol is a type of biofuel produced from maize starch on an industrial scale. In the high temperature method, heating to 120 °C is used to break apart starch molecules. The enzymes α-amylase and glucoamylase are added to the resulting starch suspension once it has cooled down. These enzymes hydrolyse the starch to glucose. Yeast cells are then added and maintained in anaerobic conditions to produce ethanol. The high temperature method is expensive to carry out, so a new method has been developed which heats starch to a lower temperature of 80 °C. In the lower temperature method, enzymes catalysing the hydrolysis of starch do not need to be added to the starch suspension. A genetically modified (GM) strain of the same yeast species is used. The GM strain of yeast has genes that allow the cell to produce α-amylase and glucoamylase and to attach these enzymes to the external surface of the cell surface membrane. The GM yeast cells are added to the starch that was heated to 80 °C and are maintained in anaerobic conditions to produce ethanol. An investigation was carried out to compare the GM strain of yeast with the yeast that had not been genetically modified (non-GM strain). In this investigation, the starch suspension produced after heating to 80 °C was allowed to cool to 30 °C before adding yeast cells. The results are shown in . The rate of ethanol production is similar for the lower temperature method and the higher temperature method. Suggest why using the lower temperature method has a similar rate of production of ethanol to the higher temperature method. Suggest one reason why the high temperature method is expensive to carry out.
9700_w22_qp_42
THEORY
2022
Paper 4, Variant 2
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Use Rice, Oryza sativa, is a grass that is grown as a cereal crop in many parts of the world. In most rice-growing regimes, the rice fields are flooded with water while the rice is actively growing. shows cultivation of rice. Describe one structural feature of the tissues in the submerged stems and leaves of rice that is an adaptation for growth in water. Explain the importance of the adaptation you have described in . Examiner’s Use An investigation was carried out into the effect of flooding on the growth of the submerged stems of rice plants. Young rice plants were grown in a container in which the level of water was increased in 10 cm steps, over a period of seven days. The mean length of the submerged internodes (lengths of stem between two leaves) and the concentration of ethene in the rice stems was measured each day. As a control, rice plants were grown in identical conditions but the water level was kept constant throughout the seven days. The results are shown in . 0.0 0.5 1.0 1.5 depth of water / cm mean length of internodes / cm mean concentration of ethene / mm3 dm–3 submerged submerged not submerged not submerged time / days time / days time / days Examiner’s Use With reference to , describe the effect of increasing water level on the length of the submerged internodes. Suggest advantages to the rice plants of the effect that you have described in . With reference to , describe the effect of increasing water level on the concentration of ethene in the rice stems. Examiner’s Use Application of gibberellin can also affect the growth of rice plants. In a further investigation, various concentrations of gibberellin were applied to submerged rice stems. The stems were placed, for three days in closed containers, in which the air supply either contained pure air or contained ethene. Ethene is a gas that is secreted by plant tissues and acts as a plant growth regulator. The results are shown in . no ethene with ethene mean increase in length of stems / mm concentration of gibberellin / nmol dm–3 State the meaning of the term plant growth regulator. Using your knowledge of the effects of gibberellin, and the results shown in , suggest an explanation for the results shown in .
9700_s07_qp_4
THEORY
2007
Paper 4, Variant 0
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Microorganisms play an important role in the cycling of nitrogen in ecosystems. is a diagram of a nitrogen cycle. A B C D E F G H nitrate nitrite ammonia primary consumer secondary consumer primary producer 1 primary producer 2 organic nitrogen detritus nitrogen gas Read the information below about four different species of soil bacteria. In the box provided, write the appropriate letter that matches each microorganism to its corresponding stage in the nitrogen cycle in . • Nitrosomonas europaea is an ammonia-oxidising bacterium. • Bacillus cereus is a denitrifying bacterium. • Azospirillum lipoferum lives in the roots of some cereals and grasses and supplies fixed nitrogen to plants. • Streptomyces coelicolor is a bacterium that secretes powerful hydrolases to break down compounds such as proteins and cellulose. Some fungi form beneficial associations with plant roots. They enable the plant to increase the uptake of nutrients, such as phosphates, which are not readily available to the plants from the soil. Suggest how increasing phosphate ion uptake will lead to increased plant growth.
9700_s12_qp_22
THEORY
2012
Paper 2, Variant 2
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Golden Rice™ is a genetically modified form of rice that produces relatively large amounts of β carotene in the endosperm. β carotene is metabolised in the human body to produce vitamin A. Explain why rice has been genetically modified to produce extra β carotene. The first types of Golden Rice™ produced only a very low mass of β carotene per gram of rice. Research continued to try to increase this. shows the metabolic pathway by which β carotene is synthesised in plants, and the enzymes that catalyse each step of the pathway. GGDP phytoene Ȣ carotene lycopene ȕ carotene phytoene synthase phytoene desaturase Ȣ carotene desaturase lycopene ȕ cyclase The first types of Golden Rice™ contained a phytoene synthase gene, psy, from daffodils and a gene crtl, which produced the two desaturase enzymes, from the bacterium Erwinia uredovora. Measurements of the quantities of intermediates in this metabolic pathway in rice endosperm showed that there was always a large amount of GGDP present, and that no phytoene accumulated in the tissues. Explain how this suggests it was not the enzymes produced by the crtl gene that were limiting the production of β carotene. Investigations were carried out to see if psy genes taken from species other than daffodils would enable rice endosperm to produce greater quantities of β carotene than the first types of Golden Rice™. • Psy genes were isolated from the DNA of maize, tomatoes, peppers and daffodils. The genes were inserted into different plasmids. • The promoter Ubi1, and crtl genes from E. uredovora, were also inserted into all of the plasmids. • The four types of genetically modified plasmids were then inserted into different cultures of rice cells. • The quantity of β carotene produced by these rice cells was measured. The results are shown in Table 4.1. Table 4.1 source of psy gene total β carotene content of rice cells / arbitrary units maize pepper tomato daffodil Name the type of enzyme that would have been used to cut the psy gene out of the DNA of the plant cells. Explain why a promoter was inserted into the plasmids. Explain whether or not these results support the hypothesis that the psy gene, not the crtl gene, was limiting the production of β carotene in genetically modified rice. The original choice of a psy gene from daffodils was made because daffodils produce large amounts of β carotene in their yellow petals, and because they are monocotyledonous plants, like rice. Suggest explanations for the much lower production of β carotene in rice containing the psy gene from daffodils than in rice containing the psy gene from maize. Describe the possible disadvantages of growing Golden Rice™.
9700_s12_qp_42
THEORY
2012
Paper 4, Variant 2
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The filamentous fungus, Fusarium venenatum, can be grown in a fermenter and harvested as mycoprotein. It is sold as a food in a number of different countries. The fungus is grown in continuous culture in 150 000 dm3 airlift fermenters, in which the introduction of bubbles of compressed air both oxygenates and stirs the contents. The fungus grows as narrow, branched filaments, giving the harvested mycoprotein a naturally chewy, fibrous texture. Approximately 300 kg of fungus can be harvested per hour. Explain what is meant by the term continuous culture. After about six weeks, mutants may appear in the fungal population, for example, a more highly-branched form of the fungus. The fermenter is emptied, cleaned and repopulated with the original strain of F. venenatum every six weeks. Explain why the fermentation process should be stopped before mutants appear. Approximately 12% of the harvested fungus is protein. Calculate the approximate mass of protein harvested in one day during continuous culture. Show your working. answer
9700_s13_qp_41
THEORY
2013
Paper 4, Variant 1
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The filamentous fungus, Fusarium venenatum, is grown in continuous culture in large fermenters to provide mycoprotein for human consumption. Explain what is meant by the term continuous culture. In an investigation into the growth of the fungus in culture, several factors were varied including: • temperature • concentration of the carbon source • concentration of the nitrogen source. Some of the results are shown in Table 3.1. Table 3.1 temperature / °C concentration of carbon source / g dm−3 concentration of nitrogen source / g dm−3 dry mass of fungus / g dm−3 7.0 2.9 3.1 14.0 3.5 4.3 7.0 3.5 4.8 14.0 2.9 4.2 Describe the effect of temperature on the growth of the fungus at the different concentrations of the carbon source. Explain why the fungus needs sources of carbon and nitrogen. carbon nitrogen
9700_s13_qp_42
THEORY
2013
Paper 4, Variant 2
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The filamentous fungus, Fusarium venenatum, can be grown in a fermenter and harvested as mycoprotein. It is sold as a food in a number of different countries. The fungus is grown in continuous culture in 150 000 dm3 airlift fermenters, in which the introduction of bubbles of compressed air both oxygenates and stirs the contents. The fungus grows as narrow, branched filaments, giving the harvested mycoprotein a naturally chewy, fibrous texture. Approximately 300 kg of fungus can be harvested per hour. Explain what is meant by the term continuous culture. After about six weeks, mutants may appear in the fungal population, for example, a more highly-branched form of the fungus. The fermenter is emptied, cleaned and repopulated with the original strain of F. venenatum every six weeks. Explain why the fermentation process should be stopped before mutants appear. Approximately 12% of the harvested fungus is protein. Calculate the approximate mass of protein harvested in one day during continuous culture. Show your working. answer
9700_s13_qp_43
THEORY
2013
Paper 4, Variant 3
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A group of membrane proteins which transport sugars out of cells have been identified and called SWEETs. They are found in the cell surface membranes of both animal and plant cells, including mammalian liver cells and rice mesophyll cells. Each SWEET is a protein with seven coiled regions which together make a pore through a membrane bilayer as shown in . coiled region Explain why, to enter or leave a cell, sugars need molecules such as SWEETS. Suggest how a SWEET is held within the membrane bilayer. The bacterium, Xanthomonas oryzae (Xoo), causes the disease known as bacterial leaf blight in rice plants. It infects the intercellular spaces of the leaves of the host plant. Then, by switching on SWEET genes in the mesophyll cells, it stimulates the secretion of glucose into the intercellular spaces. Several different recessive alleles have been found, in rice plants from different countries, which give resistance to bacterial leaf blight. All these alleles have a mutation in the promoter of the SWEET gene. The effect of Xoo on wild type and resistant rice plants is compared in . wild type rice Xoo secretes a chemical into the rice mesophyll cells chemical binds with promoter region of SWEET gene SWEET protein produced and inserted into cell surface membrane switches on SWEET gene glucose secreted into intercellular spaces Xoo multiplies resistant rice Xoo secretes a chemical into the rice mesophyll cells chemical cannot bind with mutated promoter region of SWEET gene number of Xoo cells remains low Using the information in , explain this resistance of rice plants to Xoo. Explain why it would be difficult to transfer this resistance into susceptible rice plants by genetic engineering. Explain why the presence of large numbers of Xoo in the intercellular air spaces of rice plants affects the ability of the plants to grow with their roots submerged in water.
9700_s14_qp_42
THEORY
2014
Paper 4, Variant 2
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Questions Discovered
192