Agriculture reports

1. (a) A common error here was to describe environmental problems associated with
deforestation rather than explaining how deforestation actually caused them.

(b) While most candidates appeared to understand how species conservation could result
from the given pattern of logging, the ability to express these ideas with sufficient clarity
was limited and simple details often omitted. Many candidates seemed to be somewhat
optimistic in imagining that conservation of minerals in the soil of unlogged areas could
later result in their natural redistribution, to logged areas, over distances of several
kilometres. Most candidates did appreciate the importance of the time scale involved as
being sufficient for the regrowth of the trees.

2. (a) Some candidates were confused as to the differences between fertilizers and pesticides.
As a result this question proved difficult for many. Most failed to describe how the
greenflies acquired the insecticide though most explained that the ladybirds eat many
greenflies, resulting in accumulation of the insecticide. A surprising number suggested the
insecticide would be increased as the greenflies reproduced.
(b) The distinction between description and explanation was often not observed. Many failed
to set out patterns suitably and even more could not explain both the rises and falls.
(c) Chemical control attracted many correct answers but the second part was rarely
completed accurately, perhaps because of uncertainty over the nature of biological
control. The idea of a balance was, however, explained well by many candidates.

3. The vast majority of candidates correctly named interspecific competition in part (a), and in part
(b) could give two resources for which spruce and heather were likely to be competing. Part (c)
proved to be more difficult with very few candidates gaining maximum marks. In part (c)(i)
many candidates obtained one mark for identifying the spruce and heather as the control.
However, only a minority of candidates correctly calculated the difference in rate of growth. A
significant number of candidates were not awarded the second mark as the units were omitted in
their answer. Part (c)(ii) produced vet-v few correct responses, with many candidates referring
to competition or nutrients supplied by the decay of leaves.

Hyde Clarendon Sixth Form College 1

4. There was a wide range of marks for this question. Part (a) was relatively straightforward with
most candidates gaining both marks for the idea of interbreeding producing fertile offspring. In
part (b)(i), as in previous years, the candidates found it difficult to explain the process of natural
selection. A significant number of candidates answered this question in terms of succession.
Many of the descriptions of natural selection were poorly expressed or very vague. However,
some excellent answers were obtained which included all the mark points and much more.
Generally, common scoring points were: references to isolation, mutations and the different
environmental factors on the island compared to the mainland. Only better candidates
appreciated that isolation would prevent gene flow between the populations and that a change in
allele frequency would occur. In part (b)(ii) several candidates misinterpreted the question and
continued to write about natural selection. Candidates who correctly suggested that the number
of animal species would increase often gained a mark for referring to more habitats being
available. Only a few candidates gained a second mark. usually for appreciating the greater
range of food available for herbivores. In part (c) a significant number of candidates started
describing colonisation of bare rock but were still able to gain maximum marks for a detailed
description of succession. However, although many candidates mentioned secondary
succession, there was often confusion over the sequence of the colonising plant communities.
The herbaceous or shrub layer was often omitted. A significant minority of candidates answered
this question in terms of man’s activities in replanting the forest.

5. The answers to part (a) contained many unlikely possibilities ranging from industrial effluent to
slurry spillage. The most frequent correct response involved weathering of rock, although there
were some references to shells and others which suggested an appropriate marine origin. Once
again it was disappointing that so many candidates ignored the wording of the question and
described the action of microorganisms on organic molecules in the soil. Relatively few
candidates were able to apply their knowledge of succession to part (b). In part (b)(i), for
example, point 4 was frequently identified as the point where a climax community would be
expected but supporting evidence was usually worded in general terms such as that this was
‘where there would be oak trees’ or ‘the environment would be most favourable’. Some
candidates did attempt to select evidence from the diagram but the choice was not always
appropriate and there were numerous unsuccessful attempts to relate the decrease in wind-
blown sand to the location of a climax community. Although rather more candidates appreciated
that water drained rapidly’ through sand and that humus played a role in retaining soil moisture,
very few could link wind dispersal with pioneer plants.

6. Although ‘dry ice’ and ‘paraffin stove’ were common (acceptable) answers to part (a), they were
rarely seen together. Candidates should realise that statements such as ‘pipe it in’ will not
receive credit at Advanced level. In part (b), although many candidates realised that carbon
dioxide is probably not a limiting factor at concentrations above 0.1%, they did not often
complete their explanations by referring to economic reasons. Most answers to part (c) were in
terms of ‘carbon dioxide escaping through open vents’ rather than the required reference to the
crops fetching a higher price in winter, therefore covering enrichment costs.


7. In part (a), most candidates gained a mark for defining biological control. The most common
example was the use of the parasitic wasp Encarsia to control whiteflies. Although a vast range
of examples were credited, several predator / prey relationships described have not been used as
a method of biological control. In part (b), although most candidates gained half the marks the
answers were often too vague or unqualified. Common responses referred to biological control
being safer’, ‘cheaper’, ‘better’ or ‘more natural’. A large number of candidates referred to
organisms becoming immune’ to chemicals rather than to the population developing a
resistance. However, there were some excellent answers with all the points on the mark scheme
being represented. Better candidates often gained maximum marks in part (c) by providing an
accurate description of the process and the advantages of genetic engineering. Many good
candidates referred to the use of restriction enzymes but did not mention their specificity in
cutting the DNA at specific base sequences. Most candidates understood the role of the enzyme
ligase but there was considerable confusion over the use of a vector. Candidates who gained the
mark for giving one advantage of genetic engineering usually suggested it was quicker than
selective breeding.

8. Part (a) was answered correctly in terms of percentages by a pleasing number of candidates.
Whilst most candidates suggested using insecticides in part (b)(i), few could suggest a
reasonable second alternative, being content with ‘Use biological control’. Only a few
candidates gave explanations in terms of reduced reliance on pesticides in part (b)(ii). The level
of the vast majority of answers to part (c) was ‘The predator might get out of control’ or ‘When
it has eaten all the pest the predator will start on the crop’. Many candidates answered in terms
of ‘The beetles will build up immunity to the virus

9. Candidates found the first parts of the question straightforward. In part (b)(ii). A majority of
candidates simply stated that tile law of diminishing returns means that high concentrations of
fertilisers inhibit growth. In part (d), considerable numbers of candidates gave a direct causal
relationship between algal blooms and reduction in dissolved oxygen.

10. (a) This was well answered with most candidates gaining four of the five possible marks. The
majority of candidates gave two correct answers to part (i) of the question. The most
frequent were light intensity related to lower levels of photosynthesis and temperature
related to lower levels of enzyme activity. Some candidates also correctly related light
duration to lower levels of photosynthesis. The most common incorrect answers were
reduced carbon dioxide level and reduced water level. The most common correct answers
to part (ii) were carbon dioxide and light. Almost all candidates enhanced carbon dioxide
by burning fossil fuels. Candidates usually enhanced light availability by extending the
light period with artificial lights. A few candidates used an ultraviolet source.

(b) Relatively few candidates gave a correct answer to this part of the question. One common
error was to confuse net assimilation rate with leaf area index, so candidates tried to
answer in terms of leaf surface area. The other common errors were either to give the
maximum gross assimilation rate or the compensation point.

11. (a) This part was generally well answered with the majority of candidates using parasitic
wasp/whitefly as their example. There is still a significant number of candidates using


inappropriate examples of biological control such as ladybird/greenfly, cat/mouse and
fox/rabbit.
(b) Most candidates did not gain more than half marks because they only made four points,
even though the question carried 8 marks. Poor expression was common with many
candidates repeating the information given in the question in relation to the effect on
other organisms or making vague statements about not ‘harming’ other species or “to see
the effect on food webs”. Only a minority of candidates referred to parasitising, killing or
competing with other species. For many candidates there is no distinction between the
terms ‘predation’ and ‘parasitism’, as a large number of them described control of the pest
solely in terms of predation, even though the question referred to a parasite. Answers
were often incomplete, and not developed sufficiently to score high marks.
Misconceptions included “so that the parasite doesn’t damage the crop” (point 2) and “to
prevent the pest becoming resistant to the parasite” (point 3).

(c) This part was generally well answered , but again poor expression was common. There
were many references to biological control not affecting or damaging the environment,
not causing pollution or not harming other species. Cost without a valid reason was
frequently given, and pesticides were sometimes confused with fertilisers as there were
many references to eutrophication. “Immunity” rather than “resistance” to pesticides was
quite common.
A significant number of candidates could not be awarded the quality of language mark because
their answers to Section B were not written in continuous prose. Notes and abbreviations were
frequently used.

12. This question revealed very poor understanding of fundamental principles by many candidates,
and few gained high marks.
(a) Most gained one mark for linking light energy to photosynthesis, but few referred to
chemical energy being incorporated into the structural compounds of the producers.
Many were triggered into describing energy flow though consumers. Far too many
described energy as being “created”, “made” or “converted into sugar” during
photosynthesis.

(b) In part (i) many referred to limited light penetration or shortage of nutrients, but only the
best candidates linked the two and pointed out the shortage of nutrients near the surface
where there was enough light. Part (ii) was another question where candidates were often
unable to settle on one reason and then give a corresponding explanation. As a result it
was often impossible to give a mark for an explanation that related to a specific factor.
There was also much unclear thinking about how productivity, as given in the
introduction to the question, would be affected. A common suggestion was reduced
diversity, but this was rarely related to differences in sizes of plants or absence of
structuring into layers. It was quite often suggested that use of fertilisers or pesticides
would reduce productivity. The most successful candidates were those who pointed out
that crops would be harvested and that this would remove nutrients from the soil.


(c) Most candidates realised that the desert would have the lowest productivity because of
the hostile conditions, but often they did not go on to explain how this would affect the
number of animal species as opposed simply to the number of animals.

13. Candidates generally gained over half marks for this question.
(a) In part (i) candidates always referred to light energy, but rarely to heat energy, entering
the greenhouse. However, the candidates always described how heat was retained within
the greenhouse. Some candidates did not gain credit as their answers were too imprecise
for example, “the glass lets in the sun”. In (ii) and (iii) almost all candidates gave a
correct answer. Again some candidates’ answers were too vague for credit, for example,
for (ii), “to keep out the sun” and for (iii), “to let air circulate”.
(b) Relatively few candidates gave a correct answer. The most common answer, indicating a
general misconception, was that shortage of water would limit the rate of photosynthesis.

14. With a few exceptions answers to this question were poor.
(a) Examiners were looking for an understanding of limiting factors in relation to nitrate and
eventual nitrate toxicity. Many of the candidates described the shape of the curves,
without any attempt at an explanation. Another common error was to quote the size of the
yield at the points A, B and C. Candidates who did attempt an explanation rarely
mentioned limiting factors for either curve A or curve B. Candidates who made a correct
link between yield and amount of fertiliser were awarded one of the two possible marks.
Many candidates stated incorrectly that curve C showed the law of diminishing returns.
However, some candidates did make a correct reference to nitrate toxicity. The majority
carried out a correct calculation in part (ii). The only common error was to omit the cost
of the fertiliser.
(b) The majority of candidates gave at least one correct answer. Some candidates misread the
question and gave advantages and disadvantages of organic fertilisers.

15. Overall this question was answered well, with many candidates gaining at least eight of the
possible twelve marks.
Most candidates gave an acceptable answer to part (a). Answers supporting either of the two
methods were acceptable. Most candidates opted for method 2 and supported their answer by
referring to the number of aphids after 35 days. In part (b) most candidates were able to give a
least one appropriate advantage and disadvantage. The most popular advantages given were
related to the reduced use of chemicals and the reduced chance of pests developing resistance to
pesticides. The most popular disadvantages given were related to the need to retain some of the
pest as a food source for the biological control and to the possibility of the biological control
agent dying or needing replacement.


16. This question was poorly done. Understanding of this area of the syllabus appears to be weak.
(a) It was anticipated that candidates would identify the energy involved in the production of,
for example, fertilisers. Very few were able to do this.

(b) Some candidates could clearly link the mechanisation of farming in the United States to
higher energy input and gained full marks in part (i). Many, however, gave answers that
were confused or even contradicted the information given in the question. In part (ii) only
a small number correctly indicated that plants selectively bred for efficient energy
conversion were likely to be grown in modem intensive agricultural systems.
(c) There were many vague and rambling responses. Some candidates did correctly identify
pests/diseases as important and specific therefore less likely to eradicate all crops in a
mixed system.

17. No Report available for this question.




21. (a) Many candidates referred to loss of hedgerows, soil erosion and increased numbers of
pests, but attempts at explanations were often weak. Candidates should realise that
expressions such as ‘many animals lose their homes’ do not gain credit at A level.


(b) (i) Most candidates referred to bioaccumulation, but fewer related this to gulls eating
large numbers of fish.

(b) (ii) This was the only weakly answered synoptic question on the paper. A majority of
candidates gave answers in terms of ‘strong bonds’. Better candidates often
realised that the explanation had something to do with enzymes, but only rarely did
they go on to refer to the shapes of insecticide molecules and active sites.

(b) (iii) A majority of candidates seem to have grasped the basic idea of natural selection,
but many gave a prepared answer which did not refer to this situation.

22. (a) Most recognised that light would be the limiting factor but few were able to explain this
clearly. The answer that increasing light intensity leads to increase in the rate of
photosynthesis was expected but infrequently seen.

(b) Many candidates recognised that increasing carbon dioxide concentration to 0.12% would
result in the greater increase in growth but few could use their knowledge of
photosynthesis to explain why this should be so. Those candidates who tried to offer an
explanation often produced answers lacking in detail or precision.

(c) Many candidates failed to read this question in full and took no notice of the information
given in the stem that it was a dull winter’s day. In these conditions both carbon dioxide
and light intensity would be limiting factors. Though some had realised that an increase in
temperature would not be cost effective, they could not give a reason for this. Few
candidates realised that, as an increase in temperature would increase both photosynthesis
and respiration, there would be no net gain.

(d) Many gave two correct responses, with the two most popular being that the light would
be of the wrong wavelength and that it would miss the chloroplasts.

23. (a) Candidates often failed to gain full marks because their answers were very vague and
lacked precision. Common incorrect answers included, ‘chemical control allows pest
numbers to go up and down’ and ‘biological control has low numbers’.
(b) Again too many candidates made vague statements and their answers lacked precision.
Few candidates were able to suggest that the control agent should be able to survive in the
abiotic conditions of the environment. Many suggested that the control agent must be able
to reproduce without emphasising that it must be able to reproduce in the environment of
the pest.

24. The responses to this question was generally disappointing. Very few candidates gained more
than one or two marks.
(a) (i) Most candidates incorrectly wrote that clover gain ‘nitrates’ from their symbiotic
relationship, rather than ammonium compounds or amino acids.


(ii) Many candidates were able to recall that the bacteria receive sugar or carbohydrate
from the clover. Of concern was the number of candidates who gave glucose as
their named sugar, as this sugar is not made available by plants. However this was
not penalised.

(b) The responses to this question were very disappointing. The majority of candidates failed
to compare the use of clover with the application of fertiliser. Many simply gave two
disadvantages of using inorganic fertiliser, usually in terms of their cost and the risk of
leaching. If candidates gave differences between the two systems of adding nitrogen
compounds, very few offered any explanation of the advantage.
Many candidates incorrectly referred to pest control, differential uptake of mineral ions
and effect on soil structure by the rotation of crops which would include clover in the
cycle but made no comment on the advantages of growing clover itself.

25. (a) Most recognised that the rate of photosynthesis would increase, and quite a few
mentioned that carbon dioxide concentration would normally be limiting. Very few gave a
clear link to increased yield, often only making a vague statement, such as ‘there would
be more food for growth’
(b) Several noted that respiration would increase, but few pointed out that faster use of
respiratory substrate would reduce the amount available for growth or that the rate of
photosynthesis would be too low to replace the respiratory loss.

(c) Most candidates appreciated that the sodium chloride would lower the water potential in
the solution, although some assumed that it would be taken up to the fruits. Only the
better candidates who had thought about the information given suggested that the rate of
water uptake would be reduced; most assumed a net loss, which would not only reduce
yield but also kill the plants.

26. (a) Surprisingly few could give two reasons for the farmed salmon being cheaper. Many
answers were too vague, e. g. ‘more salmon produced’. The commonest correct answers
referred to fishing effort. None mentioned such factors as the reduced movement of the
fish and hence the lower respiratory loss.
(b) This part was generally well answered, although few candidates explained the reduction
in diversity as resulting from there being few species able to tolerate low oxygen
concentration, rather than just few organisms.


(c) Again, there were few precise answers referring to respiratory losses, indigestible parts of
the fish meal or overall inefficiency. There was considerable confusion about energy in
relation to trophic levels, with statements such as ‘salmon need more energy because they
are near the top of the food chain’ and ‘there’s not much energy left in the small fish’.

27. (a) It was encouraging that over half coped with the use of indices. Weaker candidates often
gave absurd answers, either absolutely minute or in the multi-millions.
(b) The majority showed that they understood the process of bioaccumulation.
(c) (i) About a half correctly referred to the concentration at which 50% were killed.
(ii) There were some good answers giving details of enzyme inhibition. A few
mentioned specific effects, such as damage to the nervous system. Weaker
candidates simply referred vaguely to poisoning and killing.

28. (a) (i) Some were aware of the significance of the carboxyl group, but many invoked some very
fanciful chemistry, e. g. the presence of nitrogen.
(ii) Very few linked the similarity in structure to binding with proteins in the cell wall.
(b) Some sensible ideas were expressed relating to over-rapid stretching of cell walls and to
uncontrolled growth. None of the candidates pointed out that the herbicide would add to
the effect of the IAA naturally present.

29. (a) (i) Most candidates achieved this mark.

(ii) The presence of nitrogen-fixing bacteria in leguminous plants is well known, but
the process by which the fixed nitrogen is made available to a future crop is not.
Very few candidates referred to the role of decomposition in recycling nitrogen and
many implied that the fixed nitrogen was automatically made available. Nitrifying
bacteria were incorrectly given as having the ability to fix nitrogen and a surprising
number of candidates believe clover is able to fix it.

(b) (i) The effect of increasing fertiliser application on the protein content of grain was
usually described adequately, but only the most able candidates identified the small
effect that fertiliser applications had on grain yield. Many candidates ignored the
small increase in yield at low doses of fertiliser by stating that fertiliser had no
effect on yield, without any indication that the yield increase might not be
statistically significant.

(ii) This was well answered by the majority of candidates.


30. Very few candidates achieved all five marks for this question, but the more able candidates were
able to demonstrate a sound understanding of genetics in relation to the conservation of
breeding in plants. Many candidates confused the concept of species conservation with breeding
programmes, giving answers about the process of evolution rather than concentrating on aspects
of selective breeding.
(a) Many candidates gained a mark for explaining that old varieties of a plant could be used
for producing new varieties, but few made any reference to the useful alleles they may
contain. Often answers suggested the old and rare varieties could be used if the species
became extinct, which suggested a poor understanding about the concept of extinction.
(b) This was often the only mark achieved on this question by candidates.
(c) Very few candidates seemed to appreciate that the question was asking about the
advantage of crossing plants to obtain seeds for the seed collection and many took it as a
reference to maintaining the viability of populations in the wild. As a result, many
candidates discussed the advantage of crossing plants to improve evolutionary successes
rather than to consider selective breeding. A few candidates did provide good descriptions
about using crosses to avoid inbreeding, the deleterious effects of homozygous loci and to
maintain genetic variation. A common misconception was observed in answers which
referred to using selective breeding to increase the size of a gene pool or produce new
alleles. This represented a poor understanding about the concept of mutation and the
inappropriate use of some terminology used commonly in explaining the process of
evolution.

31. (i) Most candidates calculated the diversity index correctly and obtained both marks. A
significant few understood how to use the equation but failed to apply the right
mathematical approach; multiplying a number by the value zero was often performed
incorrectly.

(ii) Only rarely was a valid reason given for the difference in diversity index. Many
candidates incorrectly believe that the quantity of available food, rather than the type of
food, determines species diversity.

32. (i) This proved surprisingly difficult. Many wrongly expressed the idea that insects had to
become resistant when they encountered insecticide.

(ii) There were some good answers to this question on evolution of resistance. This seemed to
be a topic that candidates had prepared well.

33. (a) (i) Almost all of the candidates cited the long roots of the plant.

(ii) Almost every candidate correctly identified inter-specific competition.

(iii) Most candidates thought that the nitrogen-fixing bacteria in the root nodules of the
mesquite would remain active somehow after the plant was dead. There were few
attempts to relate the increase in fertility to the decomposition of the plants.


(b) Many had prepared answers for this topic. Unfortunately, one or two used examples they
had learned, rather than the one in the question. The two marks commonly awarded were
for noting that the control organism would be a parasite or predator and that it should be
specific to the pest. Other ideas from the mark scheme were less commonly seen.

34. (a) Surprisingly few candidates scored two marks here. Those who did obtain one mark
usually did so either for the idea that chemical control is rapid, or the idea that biological
control is longer term. Few candidates wrote about the combined advantage of using the
two approaches together. Many attempted rather confused explanations based upon cost
or reduced damage to the environment.

(b) (i) There were very few correct answers. Most candidates worked out the percentage
differences between the control and the normal virus and between the control and
the genetically engineered virus. They then subtracted one from the other. This
approach was awarded one mark. Others ignored the control data completely and
worked out the percentage difference between the effects of the two viruses. This
approach was not credited. A small number of candidates did not attempt the
calculation at all.

(ii) Many candidates obtained one mark for the idea that the genetically engineered
virus reduced the (total) time that caterpillars could eat the leaves. Fewer linked
this specifically to the effect of the toxin from the scorpion.

35. (a) Candidates were required to refer to both the accumulation of pesticide within an
individual and the increased concentration from one trophic level to the next. Many used
the term ‘bioaccumulation’ without any further explanation, so limiting the marks that
could be awarded.

(b) Few candidates mentioned the effect on non-target species. Most tried an environmental
approach, but often their answers were too vague or incomplete.

36. This question was well answered with many candidates scoring six or seven marks.
(a) Definitions of monoculture were often imprecise, with many candidates referring to
‘types’ of crop being the same or failing to mention the large area of cultivation.
(b) This part was very well answered with the majority of candidates scoring both marks.


(c) Although most candidates stated that there would be more animal species in an older
hedge, they did not give a reason relating to available food sources, habitats or niches.
Poor expression was common, with many candidates stating there would be more animals
or more food available.
(d) This was generally well answered, with the majority of candidates describing
bioaccumulation or the consequences of death of a food source. A minority of candidates
incorrectly described eutrophication.

37. (a) This was not well answered by many candidates. Most knew that legumes fixed nitrogen
but references to roots, nodules or bacteria were often missing or incorrect. Very few
discussed the advantage in a crop rotation, and just discussed the need for less fertiliser.
(b) Most gained one mark for stating that applying more than 600 kg of fertiliser did not
increase yield, but many failed to use the data or state that lower rates of application led
to an increase in yield.

(c) This was either well answered or very poorly answered. If candidates made the link
between fertiliser and water potential, they usually scored both marks but many
candidates were unable to make this link.

38. (a) This discriminated well, better candidates gained both marks but weaker ones simply
stated it was bioaccumulation, showing little understanding of why it occurs.
(b) This was well answered and most candidates were able to suggest a reason for leaving a
strip of bare ground between the hedgerow and the conservation headland.

(c) This question was generally poorly answered with vague responses and poor terminology.
Only the best candidates discussed ideas like habitats for pest predators. Weaker
candidates refered to ‘homes for insects’, or stated increased diversity with no
explanation.
(d) This was well answered by many, with candidates gaining three marks for a discussion of
the nitrogen cycle. Few candidates discussed the carbon cycle, and those that did were
rather vague.

39. The carbon cycle and the effect of global warming on the life-cycle and numbers of insect pests
provided the theme for this question.
(a) Despite the number of marking points available, relatively few candidates were able to
gain credit for recalling the principles underpinning the cycling of carbon. There was a
widespread failure to appreciate that carbon is taken up by plants as carbon dioxide in the
process of photosynthesis. This was apparent in the many answers which stated that
carbon or specific carbon-containing compounds were absorbed by roots.


(b) In order to demonstrate the understanding and skills required by the specification,
candidates need to have acquired the mathematical skills defined on page 45. A
significant number of candidates were unable to calculate a rate from a graph, and it was
only the best who identified the part of the curve from which they should have worked.
The most frequent incorrect response was the value of 16 obtained by dividing 400 by 25
– a figure which represented the rate of growth over the full time period.

(c) Although most candidates successfully explained the link between nitrogen and protein,
fewer understood the concept of a ratio. It was not uncommon to read that an increase in
carbon dioxide concentration would lead to a lower carbon:nitrogen ratio, or that a high
carbon:nitrogen ratio would result in the synthesis of more protein.

(d) Those candidates who gained credit here had obviously read the question carefully and
established that it was referring to crop loss due to caterpillars. They often responded in
terms of relevant limitations of the experimental procedure or offered other valid answers
relating to how an increase in carbon dioxide concentration might increase caterpillar
damage. Those who failed to provide appropriate responses tended to concentrate on the
effect of carbon dioxide concentration on the rate of photosynthesis

40. This question allowed for continuous prose and accounted for a considerable number of the
marks available for knowledge and understanding.
(a) This part of the question was generally answered well with most candidates able to
comment sensibly on the raised energy level of electrons and their subsequent passage
down an electron transfer chain. There were also frequent references to the release of
energy allowing the generation of ATP from ADP and phosphate. Better candidates often
made an appropriate reference to oxidation and reduction or to the association of the
electron transfer chain with the chloroplast membranes. There was, perhaps, the
inevitable confusion between photosynthesis and respiration but most problems arose
where candidates had gone far beyond the requirements of the specification. In such cases
detail was often confused and led to a range of contradictory and inaccurate statements.

(b) The specification requires candidates to consider the efficiency of energy transfer
between trophic levels from a quantitative viewpoint. Answers to this part of the question
were frequently extremely superficial and involved little more than a catalogue of feeding
habits with an often repeated statement about energy losses. Most candidates were able to
point out that not all of the energy in sunlight was used in photosynthesis. The
introduction of a quantitative statement about the efficiency of this initial stage was
relatively rare. Where such statements were introduced, they frequently revealed
confusion between the proportion of energy reflected or failing to strike chloroplasts and
the efficiency of transfer to chemical potential energy in the plant. There was much
discussion of energy loss with progression through a food chain. Respiration and heat loss
were frequently mentioned but responses were often marred by references to energy loss
through growth and reproduction. Another common misconception was that the efficiency
of energy transfer decreased with increasing trophic level.


(c) Most candidates produced answers relevant to the question, although there was some
misinterpretation of the specification term domestic livestock as referring to plants, and
some lengthy and irrelevant discussion of economic and ethical issues. Most candidates,
however, appreciated the need to reduce respiratory loss and considered heating and
restriction of movement. References to controlling feed intake were not always phrased
with sufficient precision to gain credit, and there was much inaccurate reference to
animals turning energy into fat or meat.


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