1. 1. A diagram was provided that showed part of a system designed to meet all basic needs of a
family of four living in the tropics. The system was based in part on the generation of fuel gas
by bacterial fermentation of urine and faeces.
(a) Candidates were asked to describe how processes which naturally formed part of the
nitrogen cycle could make nitrogen contained in urine and faeces available to crop plants.
There were many competent answers that showed detailed knowledge of the relevant
parts of the nitrogen cycle and scored full marks. There were also very weak accounts,
however, that included unselective descriptions of the whole of the nitrogen cycle
including inappropriate references to denitrification and nitrogen fixation. The average
type of response tended to include one or other of the following faults: the assertion that
ammonia was present in urine or faeces; use of the term “nitrogen” in a very loose way
with little reference to specific forms; errors in the sequence of nitrite and nitrate
production; confusion in the nomenclature of the microorganisms involved; lack of
precision concerning uptake of nitrate by plants.
(b) In the system, solid waste from the fermenter was used as fertiliser for crop plants and
candidates were asked to explain the advantage of growing leguminous plants such as
groundnuts or beans. It was evident that many candidates were unfamiliar with
leguminous plants. Answers were often quite weak, with scoring limited to mention of
root nodules (root “noodles” were held to be nutritious, but unacceptable here) and
nitrogen fixing bacteria (with the latter frequently misidentified). There was widespread
belief that nitrate was the product of nitrogen fixation and, in some cases, that the plant
itself fixed the nitrogen. Few candidates appreciated that remains of the legumes would
need to be decomposed before releasing material that could be converted to nitrates and
consequently there were many vague answers such as “legumes return goodness to the
soil”. Ironically, some of those who had discussed nitrogen fixation entirely out of
context in part (a) did not repeat this material which was now relevant.
The question concluded by asking candidates to explain the advantage in the system of
stocking the pond with fish that feed on algae rather than with carnivorous fish. Most
candidates chose to refer, sometimes at undue length, to prevention of eutrophication.
Very few, however, discussed trophic levels in relation to relative energy loss but some
did suggest that stocking with fish that feed on algae would produce a greater yield of
fish. A large number of obscure answers suggested that carnivorous fish would need to be
fed by “bringing meat in” and there was even the occasional idea that the cattle and
chickens would be fed to these fish.
2. In part (a) it was obvious that some candidates had actually done this sort of exercise as they
described in detail how random numbers are generated and used with a grid on the ground to
place the quadrats. A minority of students still described throwing random quadrats but this
method is prone to bias and is not random. The most common error was to omit the method of
generating random numbers.
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2. Many wrote at length in part (b)without focusing on the stages of heather growth given in the
diagram, or failed even to offer an indication of the heather’s age, and vaguely used the terms
‘early’ and ‘late’. The diversity of heather and not of the whole community was a popular
misunderstanding. Some candidates agreed that the diversity would change but failed to say
whether it would get greater or less. Although many did use the information in the table to
explain that increased cover offered by the heather, blocked light and affected diversity. Many
offered other factors such as more nutrients in the soil, but could not easily rationalise how that
occurred.
In part (c)(i) some candidates did manage to calculate the rate but this caused problems for
many. Some demonstrated misunderstandings with the correct notation of SI units. Few read the
question in part (c)(ii)so many did not relate age of the heather to the changing distribution of
green shoots and older woody ones. The most common incorrect response involved a discussion
of the death of parts of the plant. In part (iii) the availability of extra food for grouse was
frequently given, but few related the fact that burning heather eventually increased cover, which
provided the grouse with somewhere to hide. In part (d)(i) many students failed to show an
understanding of the term ‘organic’. This can be the only explanation for responses such as
ammonia, soil, humus, bone and root. Some however did offer protein and few even gave DNA.
Candidates really had a problem with (d)(ii) as phosphate, sodium, and hydrogen ions were
given, this suggested that the candidates read the question as “name an ion” rather than “name
an ion containing nitrogen”. Ammonia and the correct named ion, but with an incorrect
chemical formula, were common mistakes.
In part (e) there was still a great deal of confusion over the role of the bacteria and the sequence
of the nitrogen-containing compounds. The relative positions of ammonium, nitrite and nitrate
ions were regularly switched. Many candidates did, however, correctly name the bacteria
involved in the nitrogen cycle and gave complete and accurate accounts. Saprobiotic bacteria
were rarely mentioned and often the pathway began with the ammonium ions, but with no
mention as to how they were created from organic nitrogenous compounds. Inappropriate
bacteria such as the nitrogen-fixing bacteria were often mentioned, candidates being unable to
identify the relevant parts of the nitrogen cycle and so found it necessary to include everything
they knew.
In part (f) most candidates realised that the table showed changing amounts of nitrogen in the
soil and heather plants, but they were at a loss to explain the reason for burning heather after 8
rather than after 15 years. Few realised the significance of creating gaseous oxides of nitrogen
during burning and thought that burning would place nitrogen directly into the soil.
3. (a) There was a strong centre bias to this part of the question. Candidates were either familiar
with the ways in which organisms were classified or their answers appeared t6 be based
largely on guesses.
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3. (b) In part (i) most candidates appeared to be of the opinion that digestive enzymes were
present in worm casts so that digestion could continue in the soil. Some were of the
opinion that, as worms were primitive animals, they had short guts. They therefore
ingested the casts and passed them through the gut a second time. There were relatively
few references to the key points that the enzymes were not themselves digested and being
proteins they were too large to be absorbed through the gut wall. Some of the answers to
part (ii)showed a good understanding of the basic principles of experimental design, but
far too many simply assumed that demonstration of the presence of reducing sugar or the
absence of starch in worm casts proved the presence of amylase.
(c) Many candidates were content to take the lack of earthworms at face value and made no
attempt to link it in any sensible way to the use of fungicide. Some realised that the
information in the passage about breaking the leaves into smaller pieces was relevant, but
few went on to develop this point further and describe the effect of increased surface area
on the rate of microbial decomposition.
(d) The answers to this part of the question were generally poor. Although there were some
references in part (i) to nitrate and ammonium ions, mineralised nitrogen was rarely
described as being more than “nitrogen in mineral form”. The tendency to repeat the
question also characterised many of the answers to part (ii) with excretory nitrogen
generally described as being “nitrogen in excreted products”. Where the answer was
amplified, excretory nitrogen was usually identified as the nitrogen found in faeces.
Evidence from the answers to this part of the question, and elsewhere in the paper, point
to few candidates understanding the differences between the processes of secretion,
excretion and egestion.
(e) The number of times that ammonia appeared in the answer to part (i) suggested that either
few candidates understood the meaning of the word “organic” or that ammonia was
genuinely regarded as an example of an organic compound. Where both of the examples
offered were organic, lipids and carbohydrates often featured. There were, in addition,
many inappropriate references to cells and tissues. Although there were some excellent
answers to part (ii), others reflected a fundamental lack of knowledge on the part of the
candidates concerned or were totally unselective in the material they presented. The
production of ammonia was often omitted, the terms “nitrifying” and “nitrogen-fixing”
were frequently confused and nitrates were too often described as being converted into
nitrites.
(f) In part (i) most candidates showed understanding that the behaviour of earthworms in dry
conditions led to a smaller surface area over which water could be lost. Some,
unfortunately, linked this behaviour to saving energy through a lower rate of respiration,
while others referred to reduced rates of sweating. There were also many sound answers
to part (ii).
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4. (g) Many candidates understood the difficult concept that lowering the C : N ratio could be
achieved either by removing carbon or by adding nitrogen. Unfortunately, the
explanations of how these effects were achieved Were often insufficiently clear to gain
credit. Once again, far too much use was made of unqualified pronouns and many
members of the examining team commented on difficulties in deciding to what “it” was
referring.
4. (a) Weak answers followed the stem of the question and only referred to carbon rather than
considering the form it would take in the atmosphere, namely carbon dioxide. Unclear
responses proposed that ‘slash and burn’ would increase its concentration without
explaining whether it was the ‘slash’, the removal of trees, or ‘burn’, the combustion of
trees, that was responsible for the change. Not all candidates used the allocation of marks
to construct their answer and generally included only the increase due to release of carbon
dioxide from burning. Better candidates identified the reduced uptake of carbon dioxide
since there would be fewer photosynthesising organisms.
(b) The concept tested in this question was secondary succession. Where the role of pioneer
species was identified, clarification was needed since soil already existed from the earlier
presence of organisms. Weak responses produced unnecessary descriptions of primary
succession. Better candidates recognised that recolonisation would occur with seeds and
spores from neighbouring areas although this was not always well expressed. Many
achieved credit by explaining the process of succession and the resultant re-establishment
of a climax community, but there were also many who appeared unfamiliar with the
concept or who could not communicate the main ideas adequately.
(c) This question allowed candidates to demonstrate their ability to interpret information and
apply their knowledge. Thus, good candidates could identify the source of ammonium
compounds in manure, which soil bacteria would be relevant, and what was meant by
crop yield. Many showed a good knowledge of the nitrogen cycle but only the better
candidates considered the role of nitrogen-fixing bacteria. It was not necessary to name a
specific nitrifying bacterium, but where this was done, the bacterium had to be linked to
the appropriate reaction to secure credit.
(d) It was pleasing to see so many candidates familiar with environmental concerns and
conservation, although candidates often failed to express themselves clearly. A large
proportion considered the view of what would be lacking if a forest ecosystem was
removed, rather than the advantages of conserving one, but examiners sought to credit the
concepts. There was a limited appreciation of forests as sustainable resources but frequent
reference to habitats, carbon sinks, species diversity and food chains allowed maximum
credit to be obtained by many.
5. Once again, the nitrogen cycle proved a weak link in the understanding of a large number of
candidates.
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5. (a) More candidates were able to identify process P as denitrification than were able to
identify process Q as nitrogen fixation. Nitrification was a common wrong answer, as
was ammonification, since this refers to the conversion of nitrogen in organic compounds
to ammonia.
(b) Many candidates misread the questions and did not realise that they had to account for the
apparent discrepancy between the amount of ammonia converted to nitrates and the
amount formed by nitrogen fixation. Good candidates knew that ammonia is also formed
by the decomposition of proteins, amino acids, urea and other organic compounds in the
detritus that contain nitrogen.
(c) Most candidates realised that hydrogen is lost and oxygen is gained in the conversion of
ammonia to nitrate.
6. (a) Most candidates knew the mark-release-recapture technique, and were able to describe
the various steps. However, they did not always explain the reasons behind the steps. For
example, they did not always explain that the released insects should be left for a suitable
period of time to allow them to re-integrate with the rest of the population.
(b) (i) Nearly all candidates knew that there would be only one degree of freedom.
(ii) Most candidates knew that the 0.05 level of probability is that most commonly
used in biological analysis to judge statistical significance.
(iii) Responses to this section were generally disappointing. Most candidates were
unable to reason that, because the value for χ2 is greater than the critical value,
then there is a probability of less than one in one thousand that the results are due
to chance. They were uncertain as to whether the difference in values of χ2 implied
that the differences in results are due to chance or due to some biological cause.
They wrote about rejecting a null hypothesis which had not been stated and also
merely that ‘the results are statistically significant’. Candidates should be aware of
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the logic that, if χ is greater than the critical value, there is only a probability of
(usually one in twenty) that the results are due to chance as the basis for rejecting
any null hypothesis and accepting the experimental hypothesis.
(c) (i) A number of candidates realised that some of the biomass produced in
photosynthesis would be respired by the plant, but very few actually explained that
biomass is lost in the form of carbon dioxide. Most of those who involved
respiration in their answers suggested that energy is lost, which is true, but loss of
energy does not account for the difference in biomass between gross primary
production and net primary production.
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6. (ii) Nearly all knew that a higher net primary production would lead to more dead
plants and so more food for the detritivorous insects.
(iii) Most candidates realised that decomposers would respire compounds from the dead
plant remains, releasing carbon dioxide, which could be taken in by plants to be
used in photosynthesis.
However, only better candidates knew that the decomposers were saprobionts and
supplied details of extra-cellular digestion.
7. (i) Most candidates gained credit for the fact that the fertiliser had leached into the river, but
only the more able explained that it is excess fertiliser that leaches in this way.
(ii) The vast majority of candidates scored highly by correctly recounting the standard
sequence of events in eutrophication.
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