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(a) A group of students was given the task of planning a method to find the effects of the growth regulator gibberellin (GA) on the germination of maize grains.
The students looked up information about investigations into the germination of maize. The information that the students found is listed below.
• Maize needs to be soaked in water for 24 hours to stimulate germination.
• Maize starts to germinate 48–72 hours after soaking.
• GA promotes germination by activating a gene that causes the synthesis of the enzyme amylase.
• The range of concentrations at which GA is found in plants is between $1−10\mu mol.dm^{-3}$.
• Amylase is released by the aleurone layer into the endosperm of the maize grain to hydrolyse the starch reserves.
• The activity of amylase can be estimated by cutting the maize grains lengthways into two halves and placing the cut sides onto agar containing starch in Petri dishes.
• After incubation at a constant temperature, iodine solution is used to test the agar for the presence of starch.
The students carried out a preliminary investigation using maize grains that had been soaked in a $3mmol.dm^{-3}$ solution of GA.
Fig. 1.1 shows the arrangement of cut maize grains that the students decided to use for their main investigation and the results of testing the agar for starch using iodine solution after incubation.
The students thought that the area stained brown was proportional to the activity of the amylase and could be used to test the hypothesis:
The greater the concentration of gibberellin (GA) to which the maize is exposed, the greater the activity of amylase.
(i) Identify the independent and the dependent variables in the students’ preliminary investigation.
independent variable ...........................................................................................................................
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dependent variable ..............................................................................................................................
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(ii) The students were provided with a $3mmol.dm^{-3}$ GA solution. Describe how the students could use the method from their preliminary investigation to test their hypothesis. Your method should be detailed enough for another person to use.
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(b) The students decided to plot a graph of their results.
Suggest labels, including units, for the axes of this graph.
x-axis .....................................................................................................................................................
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y-axis .....................................................................................................................................................
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(c) (i) The students thought that the area stained brown was proportional to the activity of the amylase.
Suggest three limitations of using this way to estimate amylase activity.
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2. ..............................................................................................................................................................
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3. ..............................................................................................................................................................
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(ii) For one of these limitations, suggest how the estimation of amylase activity could be improved.
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569
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591
(a) Describe the trends shown by the distribution of the two types of bacteria in the stored soil after 6 months and suggest reasons for these trends.
[Table_1]
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(b) In a further study, soil samples were taken at two depths, A and B, in the soil store. The samples were taken at intervals over six years. Soil samples of equal mass were used to determine the activity of dehydrogenase enzymes in the Krebs cycle of the aerobic bacteria.
Fig. 2.1 shows the mean dehydrogenase activity of the bacteria in these samples.
(i) State the evidence from Fig. 2.1 that samples in B were taken at a greater depth than samples in A. ....................................................... [1]
(ii) Suggest two variables that should be standardised when the dehydrogenase activity is determined.
1. ..............................................................
2. .............................................................. [2]
(iii) Suggest a control for the determination of dehydrogenase activity in the bacteria in the soil samples. ............................................... [1]
(c) The error bars on Fig. 2.1 are two standard error units above and below the mean.
(i) Using evidence from Fig. 2.1 state what these error bars show about the reliability of the data.
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(ii) A t-test was carried out between each of the following sets of data from the samples at depth A:
6 months and 2 years; 6 months and 3 years; 6 months and 6 years;
2 years and 3 years; 2 years and 6 years; 3 years and 6 years.
Identify two t-tests in which the difference in dehydrogenase activity is likely to be significant. Show your answer by underlining the tests.
Give a reason for your answer.
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(d) Table 2.2 shows the dehydrogenase activity and the number of aerobic bacteria present in six soil samples.
[Table_2]
The dehydrogenase activity of 1 g of another soil sample was measured. Explain how the data in Table 2.2 could be used to predict the number of aerobic bacteria in this soil sample.
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535
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524
