Volume 27, Number 2, November 2014
Activity
A laboratory-based investigation
Bill Indge
In a laboratory-based investigation, sexually mature fathead minnows were randomly divided into two
groups — an experimental group and a control group.

A solution of synthetic oestrogen was added to the tank containing the experimental group of
fish. The resulting concentration of oestrogen reflected environmental concentrations.

A control solution was added to the tank containing the control group.
1 Explain why it was important that the fish were randomly divided into the two groups.
(2 marks)
2 A control solution was added to the tank containing the control group of fish. What would you expect
to be in this control solution? Explain your answer.
(2 marks)
Measurements were made to assess the effects of exposure to the oestrogen on some aspects of the
biology of the fish. The table shows some of the results obtained for the male fish.
Treated with synthetic
oestrogen
Control
Body condition factor
total mass/g × 100 000
total length3
1.07
1.10*
0.802
0.761
1.55
1.35*
2
3*
Gonadosomatic index
testis mass/g × 100
body mass/g
Hepatosomatic index
liver mass/g × 100
body mass/g
Secondary sexual characters
index
* Significantly different, p<0.01
3 One of the control fish had a hepatosomatic index of 1.35. The body mass of this fish was 2.73 g.
Calculate the mass of its liver. Show your working.
(2 marks)
4 The testis mass is given as an index. Explain why it was given as an index, not the actual mass.
(2 marks)
Philip Allan Updates © 2014
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5 The scientists who carried out this investigation reported that large protein-containing vesicles had
accumulated in the cytoplasm of the liver cells in a significant number of the experimental group of
fish.
(a) Explain how the data in the table support this observation.
(2 marks)
(b) Use your knowledge of the way in which oestrogen affects vitellogenin production to explain the
presence of the protein-containing vesicles in the liver cells of the experimental group of fish.
(3 marks)
6 Using only data from the table, explain why synthetic oestrogens may affect the reproductive
success of male fathead minnows.
(4 marks)
Philip Allan Updates © 2014
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Suggested marking guidelines
1
The idea of preventing conscious choice;
This might influence results;
(2)
2
Everything that was added to the fish in the experimental tank other than the synthetic
oestrogen;
This ensures that any difference would result from the oestrogen/would not be the result of
other substances in the solution;
(2)
Students need to think carefully about the nature of the control here. With the information provided,
‘nothing’ would be an inappropriate response. ‘Water’ makes assumptions about the composition of
the synthetic oestrogen solution.
3
2 marks for correct answer of 0.04 g;
1 mark for incorrect answer but with the candidate clearly appreciating the need to calculate
the answer from the formula:
hepatosomatic index = liver mass × 100;
body mass
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(2)
Allows results to be compared/standardised/body size to be taken into consideration;
Because testis mass will vary with body mass/larger fish will have larger testes;
5 (a)
(2)
The experimental/oestrogen-treated group have a (significantly) higher hepatosomatic index;
(Larger liver) caused by the protein-containing vesicles; (2)
(b)
Binds with oestrogen receptors present in liver cells;
Stimulates transcription of VTG mRNA;
VTG protein synthesised;
VTG accumulates in vesicles in liver cells;
(max 3)
The 3 marks allocated to this question should provide an indication of the depth of answer required.
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Mating success is likely to depend on body condition/Males with poorer body condition likely to
have lower mating success;
The difference in gonadosomatic index is not significant/oestrogen-treated fish have
significantly lower body condition and secondary sexual characters index;
Larger liver in oestrogen-treated males likely to be linked to high vitellogenin production;
Secondary sexual character important in courtship/defence of territory;
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The key to answering questions such as this is to look at the number of items in the table. Here four
features are compared and each should receive a comment.
This resource is part of BIOLOGICAL SCIENCES REVIEW , a magazine written for A-level students by subject experts.
To subscribe to the full magazine go to www.hoddereducation.co.uk/biologicalsciencesreview
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