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(a) Identify the independent variable and the two dependent variables.
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dependent ............................................................
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(b) (i) Describe how the student could have measured the length of the pollen tubes using a microscope.
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(ii) Suggest why the percentage germination was calculated.
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(c) The student carried out \(t\)-tests on the results to compare the lengths of the pollen tubes when grown in different media.
To carry out a \(t\)-test the number in the sample must be counted. It is also necessary to carry out two calculations from the data.
(i) State the two calculations that must be carried out in order to complete a \(t\)-test.
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(ii) State why the student decided to use 18 degrees of freedom in each of the \(t\)-test calculations.
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The student carried out a number of \(t\)-tests to find out if, after 70 minutes, the difference in mean pollen tube length is significant:
1. between medium A and medium B \(t = 2.50\)
2. between medium A and medium C \(t = 3.56\)
3. between medium B and medium C \(t = 1.94\)
Table 1.3 shows the critical values for the \(t\)-test.
$$ \begin{array}{|c|c|c|c|c|c|c|c|c|c|c|c|c|} \hline \text{degrees of freedom} &10 &12 &14 &16 &18 &20 &22 &24 &26 &28 &30 &40 &50 &60 \\ \hline \text{probability 0.05} &2.23 &2.18 &2.14 &2.12 &2.10 &2.09 &2.07 &2.06 &2.06 &2.05 &2.04 &2.02 &2.01 &2.00 \\ \hline \text{probability 0.01} &3.17 &3.06 &2.98 &2.92 &2.88 &2.85 &2.82 &2.80 &2.78 &2.76 &2.75 &2.70 &2.68 &2.66 \\ \hline \end{array} $$
(iii) State what conclusions can be drawn about the significance of the differences in mean lengths from the three values of \(t\) given above.
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(d) From the results in Table 1.2 and the \(t\)-tests the student concluded that:
• calcium has no effect on the germination of pollen grains but increases the growth rate of pollen tubes
• sucrose is needed for pollen tube germination but has no effect on the growth rate of the pollen tube.
(i) With reference to Table 1.2 state the evidence that supports these conclusions and the evidence that does not support these conclusions.
evidence that supports these conclusions ..........................................................
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evidence that does not support these conclusions .........................................
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(ii) Outline a further investigation that the student might carry out to provide more support for the conclusions about the role of sucrose in the germination of pollen and in the growth of pollen tubes.
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(iii) Predict the results of this investigation if the student’s conclusion about the effects of sucrose is correct.
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569
522
502
In mammals, digested food is absorbed by active transport and diffusion. In an investigation into the absorption of sugars a short piece of intestine was turned inside out, so that the absorptive surface was on the outside. The ends of the intestine were tied to form a bag containing oxygenated buffer solution.
The apparatus used is shown in Fig. 2.1.
(a) (i) Outline a method that could have been used to oxygenate the buffer solution.
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(ii) Identify one important variable, other than those shown in Fig. 2.1, that should be standardised in this investigation.
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(b) Substances which are absorbed by the intestine collect in the centre of the bag. Samples from the contents of the bag can be removed for analysis.
The absorption of glucose was investigated using untreated pieces of intestine and pieces of intestine that had been treated with a respiratory inhibitor. The investigation was then repeated using fructose.
Table 2.1 shows the results of this investigation.
[Table_1]
(i) Outline a procedure by which the rate of absorption of glucose could be determined.
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(ii) Suggest how the relative rate of absorption of the sugars is calculated.
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(c) State the conclusions that can be drawn about the absorption of sugars from this investigation.
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583
545
591
