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(a) Sketch a curve to show the expected results if the student's hypothesis is correct. [2]
(b) (i) Identify the independent and the dependent variables in this investigation.
independent variable............................................................
dependent variable............................................................ [2]
(ii) Describe a method the student could use to test the hypothesis. Your method should be detailed enough for another person to use.
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(c) To find the effect of GA on the growth of the young plants, the student calculated the percentage difference in growth of young plants from barley that had been soaked in water (untreated barley) and the young plants from each of the experimental groups of barley.
(i) State how percentage difference is calculated.
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(ii) Explain why the student used percentage difference instead of the actual difference between young barley plants.
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(d) The student used separate $t$-tests to find out if the percentage difference between the young plants grown from untreated barley and any of the young plants grown from the experimental barley, was significant.
(i) State one reason why the $t$-test is a suitable statistical test to use for the student's data.
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(ii) Suggest a null hypothesis for these tests.
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The effect of oxygen on the growth of a species of bacterium, Aerobacter aerogenes, was investigated. The procedure used is listed below.
• A container of sterile growth medium was inoculated with 2.5 × 10$^6$ cells per cm$^3$ of the bacterium.
• The growth medium was maintained at a constant temperature and pH.
• The culture was stirred constantly at a low speed.
• The bacteria were grown at first in anaerobic conditions.
• After a period of time a stream of air was passed through the culture at a constant rate.
• Samples of the culture were removed at intervals and the population of bacteria estimated in each sample.
(a) Identify the \textbf{independent} variable in this investigation.
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(b) Three of the precautions taken in this procedure were:
• the pH was kept constant
• a sterile medium was used
• the culture was stirred constantly at a low speed.
Explain why each of these precautions was necessary.
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Fig. 2.1 shows the surface view of a counting grid for a light microscope used to estimate the number of bacteria in each sample.
The depth of the grid is 0.1 mm.
(c) Suggest how this apparatus could be used to estimate the number of cells per cm$^3$ of culture.
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Table 2.1 shows the results of the investigation.
[Table_1]
Table 2.1
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|c|c|}
\hline
time since inoculation of growth medium / min & 100 & 130 & 160 & 190 & 220 & 250 & 280 & 310 & 340 & 370 \\ \hline
population of bacteria / 10$^6$ cells cm$^{-3}$ & 14 & 23 & 40 & 66 & 78 & 224 & 463 & 812 & 1122 & 1148 \\ \hline
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(d) Suggest why the results in Table 2.1 may not be reliable.
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(e) Suggest an explanation for the results of the investigation into the growth of Aerobacter aerogenes show in Table 2.1.
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