1. 1. This question required the use of extended prose throughout. Full marks were only available to
those able to express the appropriate concepts clearly and unambiguously in scientific terms.
Many candidates produced good answers, using both information from the passage and their
own biological knowledge. Lack of detail, misconceptions and careless expression all
contributed to the loss of marks for other candidates.
(a) The main points given by candidates here were related to deforestation causing loss of
habitat, loss of food, and potential extinction or the need to migrate elsewhere. Some
gave details relating to exposure of the soil and hence erosion and leaching of ions. Very
few mentioned changes in climate or in light levels as a result of removal of the trees.
(b) Many understood that trees could act as carbon sinks by absorbing carbon dioxide (but
not ‘carbon’ as a high proportion of weak candidates stated) for use in photosynthesis and
conversion of organic molecules within the tree. Careless omission of any of these details
resulted in the loss of marks – the third point being the one most commonly left out. Far
too many candidates believed that carbon dioxide was converted into oxygen or that it
was used in ‘respiration’ rather than photosynthesis.
(c) Some candidates did not make it clear whether the scenario they were describing related
to the situation where ploughing took place or, as the question stated, to where ploughing
was reduced. Such ambiguity in the answer cost marks. Once more, ‘carbon’ was often
the substance used rather than dead organic matter and this carbon was often thought to
combine directly with oxygen to form carbon dioxide. Ploughing sometimes introduced
‘air’ into the soil, with oxygen not being specified. Similarly, the process of respiration, if
mentioned in the context of saprotrophic organisms in the soil, was rarely qualified as
being aerobic. Some candidates even imagined a peculiar situation whereby carbon
dioxide was still formed in the soil but it was trapped due to a lack of ploughing.
(d) This section was specifically about the role of nitrifying bacteria. These were frequently
confused with saprotrophic and with nitrogen fixing bacteria. For those thinking the latter,
ploughing introduced nitrogen gas into the soil ready for fixation. Only correct, relevant
details were rewarded, such as the conversion of ammonium ions to nitrite and then to
nitrate, using the extra oxygen introduced by ploughing. Many candidates knew that
nitrate ions were taken up by crop plants and some explained how these could be
converted into named organic compounds, such as DNA or protein, which could be used
in turn by the plant to increase its growth or yield (hence being of benefit to the crop
plants, as required by the question). Such completeness of detail was the domain of the
better candidates who frequently scored full marks.
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2. 2. (a) Some accounts of the activities of decomposers and nitrifying bacteria in the recycling of
carbon and nitrogen from fallen leaves were truly excellent. Aspects which tended to be
omitted were mainly those associated with the early stages of saprotrophic nutrition, i.e.
the secretion of hydrolytic enzymes for external digestion followed by absorption of the
products by diffusion or active transport. Details of respiration releasing carbon dioxide
to be used by the trees in photosynthesis, of the release of ammonium compounds its
conversion to nitrite and nitrate and the subsequent use of the latter by the trees to
produce organic nitrogen compounds such as amino acids were frequently given.
However, weaker candidates typically knew few of these details and often described how
‘carbon’ was released into the soil, in which state it was apparently taken up by the trees’
roots. Some candidates were determined to display all they knew about the nitrogen cycle
and included irrelevant details of denitrification and nitrogen fixation.
(b) For candidates who kept to the point, this was a very straightforward question: felled
trees no longer removed carbon dioxide from the atmosphere by photosynthesis and
burning them released more carbon dioxide back into the atmosphere – hence a predicted
rise in the carbon dioxide concentration. Some became too involved in telling a story and
often forgot about one or other of the key processes.
(c) Answers to this section were very disappointing. Many candidates merely repeated
information given in the stem of the question but did not use this, nor did they attempt to
apply biological principles to the situation with which they were presented. Others treated
examiners to the answer they had prepared earlier, with sound biological facts irrelevant
to this question. One area which was hardly ever explored was how the protective canopy
of the softwood trees might actually have modified the environment to make it suitable for
the growth of hardwoods beneath it – such aspects as protection from wind and
rainstorms, provision of a humid environment, shedding their leaves making ions
available to the hardwoods, reducing soil erosion and leaching of ions due to their root
systems being already established in the soil. Usually answers contained a reference to
the light penetrating following tree felling so that seeds could germinate and the idea that
succession to a climax community would occur, and little else of any relevance. Hardly
any candidates considered that being exposed to light might possibly be advantageous for
photosynthesis in the seedling trees. Better candidates realised that more seeds could be
transported into the cleared area from the adjacent forest, and that animals might return
because new habitats and food would be available for them as the new trees began to
grow.
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3. 3. Part (a)(i) appeared to indicate a problem over the term ‘organic’, as the most common incorrect
answers were ammonia and nitrate. In part (ii) the process of death or decay was often given
rather than a source of the material. Nitrifying, nitrogen fixing and denitrifying bacteria caused
confusion in answers to part (b); a substantial number of candidates did not know which type of
bacteria did what. Although this flow diagram was not the usual form of the nitrogen cycle,
enough clues were given to allow candidates to relate it to their own knowledge, and many were
able to do so. 183 was a common incorrect answer to part (c), obtained by deducting the value
for ‘cattle’ from that for inorganic nitrogen’. Many gave 339 which suggested that they had the
principle correct, but missed out the effect of fixation in providing some of the ‘nitrogen’
requirements of clover. While a lot of the candidates knew that clover had nitrogen fixing
bacteria in root nodules, a significant number thought that the clover did its own nitrogen, fixing
in part (d). A common circular argument was that cattle eat the clover, add the nitrogen from the
clover to the soil via dung and urine to benefit the grass which is an advantage to the cattle.
Only a few suggested that growing clover might save the farmer having to buy and apply
fertiliser.
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