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(a) (i) Identify the independent and dependent variables in this investigation.
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dependent ............................................................ [1]
(ii) Identify two variables that the student controlled in this investigation.
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(a) (iii) There are other variables that the student could have controlled in this investigation.
Describe how two other variables could have been controlled.
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(b) (i) The student used the highest concentration (4000 mg dm-3) to prepare the other concentrations of sodium nitrate.
Describe a procedure that the student used to prepare the other concentrations stated in stage 1. Your description should be sufficiently detailed so that another person can easily follow your procedure.
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(ii) The student also prepared another sterile glass dish to use as a control.
Suggest a suitable solution to use as a control for this investigation.
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(c) The results of the student's investigation are shown in Table 1.1.
Table 1.1
[Table_1]
(i) State what standard deviation (s) shows about the data.
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(ii) The results show that the standard deviation increased with time.
Suggest a reason for this increase.
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(d) State the conclusions that can be drawn from the results in Table 1.1 about the effect of different concentrations of nitrate on the growth of this plant.
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(e) Suggest how the student’s investigation could be modified to find the optimum nitrate concentration for the growth of this plant.
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(f) Sewage and agricultural fertilisers contain phosphate as well as nitrate. In another investigation the student used sodium phosphate instead of sodium nitrate to test the effect of different concentrations of phosphate on the growth of the same plant.
Suggest a prediction that the student could make about the effect of different concentrations of phosphate on the growth of the plant.
Show your answer as a sketch graph on the axes below.
[2]
572
541
579
Bacteria can be modified to produce human or animal proteins, such as insulin. The production of these proteins is more efficient if carried out by animal cells, such as rat tumour cells, maintained in culture.
An investigation was carried out to test the suitability of two systems for culturing rat tumour cells:
• batch culture in which the cells are mixed gently in the culture medium
• a form of continuous culture, known as a perfusion system, in which new culture medium is added to the cells and the old medium removed throughout the fermentation. The cells are kept in the system and are not removed.
During the investigation 20 samples were taken from each system at intervals of ten hours. Four factors were measured in each sample:
1. the number of cells in the culture medium
2. the survival of the cells in culture
3. the concentration of the nutrient, glucose
4. the concentration of the waste, lactic acid
Table 2.1 shows the results for factors 1 and 2.
[Table 2.1]
Table 2.2 shows the results for factors 3 and 4.
[Table 2.2]
(a) Outline how the number of cells may have been estimated.
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(b) Living cells may be identified by using a staining technique. Suggest how the percentage of cell survival was calculated.
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(c) A $t$-test was carried out to see if there is a significant difference between the number of cells in the two culture systems at 10 hours.
The value of $t = 2.62$
(i) State a null hypothesis for this test.
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(ii) State the number of degrees of freedom for this $t$-test.
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Table 2.3 shows some probability values of $t$.
[Table 2.3]
(iii) State what the value of $t$ indicates about the difference in the number of cells in the two cultures systems at 10 hours.
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(d) The investigation showed that the perfusion system is a more efficient way to culture rat tumour cells.
Describe how the data in Table 2.1 and Table 2.2 support the statement that the perfusion system is more efficient.
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594
509
546
