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General Certificate of Education (A-level)
January 2011
Biology
BIOL4
(Specification 2410)
Unit 4: Populations and Environment
Report on the Examination
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Report on the Examination – General Certificate of Education (A-level) Biology – Unit 4: Populations
and Environment – January 2011
General Comments
The paper produced a relatively good spread of marks and overall candidates seemed to find
the paper accessible. In the longer recall questions there were some very impressive
answers with candidates displaying an excellent understanding of the unit content covered
on the examination paper. As expected, questions involving interpretation of data presented
graphically or in tables proved more challenging but were generally well answered.
However, apart from applying the Hardy-Weinberg equation, questions involving calculations
caused problems for many candidates. One area of concern was the poor responses in
parts of question 2 relating to the understanding of genetically-based family trees. There
was also considerable variation in the ability of candidates to express their ideas clearly and
logically. Weaker candidates often failed to gain credit because of the use of imprecise or
inappropriate scientific terminology.
Centres should encourage candidates to take into account the number of marks allocated to
each question and write relevant answers of an appropriate length. Routinely extending
answers to extra pages is unnecessary and can result in candidates falling short of time later
in the unit test.
Question 1
(a)
Most candidates had little difficulty correctly explaining the term population. The main
reason for candidates not obtaining the mark was the failure to refer to organisms
living in a habitat or the same place.
(b)(i) Almost two thirds of candidates were able to write a correct formula showing a
population that stays the same size. A common incorrect response was
‘B + E = D +I’.
(b)(ii) Candidates who provided the answer B+I = D+E for (b)(i) often correctly substituted =
with > to show a population increasing in size. However, there were fewer correct
responses compared to (b)(i) with a number of candidates demonstrating limited
mathematical ability.
(c)(i)
Over ninety five percent of candidates obtained this mark usually by referring to
improved medical care or improved sanitation.
(c)(ii) Approximately a third of candidates were able to calculate successfully the population
of Mexico in 2008. The most frequent incorrect response was 122 million. It was
evident that a significant number of candidates had no idea how to approach this
question.
Question 2
(a)(i)
The majority of candidates gained the mark for explaining what is meant by a
recessive allele. Unfortunately, some candidates simply stated that it is ‘not
expressed in the phenotype’.
(a)(ii) Again, this was well answered with most candidates expressing themselves clearly
with appropriate scientific terminology. Incorrect responses suggested that
codominance indicates different genes.
(b)(i)
Surprisingly, only one in five candidates gained any marks for this question. Most
candidates suggested that parents 1 and 2 having produced a rhesus positive child
was evidence that the allele for Rhesus positive is dominant. It should also be noted
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Report on the Examination – General Certificate of Education (A-level) Biology – Unit 4: Populations
and Environment – January 2011
that many candidates suggested that 4 was the child of 1 and 2, indicating a lack of
understanding of family trees.
(b)(ii) Again, candidates struggled to gain any marks. A significant number simply stated
that as males and females have the condition it can’t be on the X chromosome. A
similar number of candidates suggested that the gene was carried on the Y
chromosome. Candidates who did gain credit often referred to 3 being Rhesus
positive as evidence that the gene is not on the X chromosome. Fewer candidates
cited 9 being Rhesus negative as evidence. Very few candidates were able to
provide a suitable explanation to gain both marks.
(c)
Almost half the candidates gained all three marks. The most common error was to
assume that q=0.16/16% rather than q2. However, most of these candidates still
gained a mark for indicating that 2pq represented heterozygotes. A significant
number of candidates gained two marks for the answer 0.48.
Question 3
(a)
Most candidates obtained at least one mark often for referring to NBPT attaching to
the active site. Better candidates referred to competitive inhibition or provided a
suitable description to gain the second mark. Weaker candidates often referred to an
active site on NBPT or described non-competitive inhibition.
(b)(i)
Very few candidates stated that the inhibitor reduced the loss of ammonia up to day 8
or day 9. Most candidates gave very general descriptions of the effect of NBPT on
the loss of ammonia from urea fertiliser or suggested it was lower for only 4-6 days.
(b)(ii) This also proved problematic for candidates. Most candidates suggested that the
increase in urea in the soil would lead to an increase in the loss of ammonia.
However, this would not fully explain the increase from day 2 to day 4. Very few
candidates gained both marks by suggesting that an increase in bacteria would lead
to more urease being present. However, a number of candidates did gain one mark
for referring to more enzyme-substrate complexes being formed.
(c)
Most candidates gained at least one mark for indicating that less ammonia would be
lost from the soil. Approximately two thirds of candidates gained a second mark by
explaining this would be converted to nitrite/nitrate in the soil. Better candidates then
described how this nitrogen source could be used to form proteins or nucleic acids.
Question 4
(a)(i)
The vast majority of candidates had little difficulty obtaining at least one mark by
referring to the decrease in spadefoot toads as the number of newts increased. The
more observant candidates, approximately forty percent, also noted that although
southern toads were similarly affected, this trend was only evident up to 4 newts per
pond after which the number of southern toads increased.
(a)(ii) Most candidates appreciated that an increase in newts resulted in less competition for
the spring peeper frogs leading to an increase in their percentage survival. However,
many candidates did not link this to the newts feeding on the toads or they did not
always clearly distinguish between toads and frogs in terms of prey.
(b)
This proved more demanding with approximately half the candidates gaining one
mark for explaining that an increase in newts/predators caused a decrease in the
number of toads. However, fewer candidates linked the increase in available food to
an increase in the growth or mass of the remaining tadpoles. There was a significant
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Report on the Examination – General Certificate of Education (A-level) Biology – Unit 4: Populations
and Environment – January 2011
number of candidates who answered this question in terms of an increase in the rates
of reproduction of frogs and toads.
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Report on the Examination – General Certificate of Education (A-level) Biology – Unit 4: Populations
and Environment – January 2011
Question 5
(a)(i)
Approximately half the candidates correctly identified light and temperature as the
limiting factors between X and Y on the graph. Most incorrect responses referred to
only one of these factors or included carbon dioxide.
(a)(ii) Many candidates gained at least one mark by using information from the graph to
explain how one of the factors limits the rate of photosynthesis. Better candidates
provided evidence to explain how both light and temperature limited the rate of
photosynthesis. However, a number of candidates did not refer to evidence from the
graph or used evidence which did not relate to rate of photosynthesis between X and
Y.
(b)
There were very few correct responses. Many candidates failed to notice the
different axes for photosynthesis and respiration. It was also evident that many
candidates do not fully understand gross/net photosynthesis when presented in an
unfamiliar context.
(c)
Most candidates gained at least one mark for indicating that a reduction in light
intensity would decrease photosynthesis. Although many candidates did appreciate
that overall growth would be reduced, they did not all obtain a second mark as their
explanations often contained contradictory statements. The effect of temperature on
respiration was overlooked by many candidates. A significant number of candidates
who did mention respiration stated that increased respiration leads to increased
growth. Consequently, very few candidates gained maximum marks.
Question 6
(a)(i)
Most candidates gained one mark for use of oxygen by the woodlouse. Candidates
failing to gain any marks for this question often referred to ‘air being absorbed’. Many
also gained a second mark by indicating that the carbon dioxide released by the
woodlouse would be absorbed by the potassium hydroxide. Very few candidates
obtained the final marking point by indicating that the pressure inside the tube would
decrease. Answers such as ‘a vacuum is created’ were not credited.
(a)(ii) Over a third of candidates failed to gain any marks on this question. Reference to
how far the bubble travelled and time was the only mark point for most candidates.
Approximately one in five candidates obtained a second mark usually for referring to
the radius or diameter of the capillary tubing. Very few candidates referred to the
mass of the woodlouse. Candidates who realised that mass was one of the required
measurements often referred to measuring the mass of oxygen, carbon dioxide or the
apparatus.
(b)
As expected this proved to be a challenging question. Nevertheless, it was surprising
that forty percent of candidates failed to obtain any marks. Considering the
information provided it was disappointing to see some candidates describing details
of photosynthesis. Although there were some excellent answers gaining maximum
credit, these were a rarity with most candidates gaining a single mark. Usually, this
mark was awarded for linking less ATP production to less or no proton movement.
However, a number of candidates incorrectly referred to ATP being used to transport
protons across membranes. Many candidates also mistakenly interpreted the fact
that oxygen uptake remained constant as an indication that no oxygen was being
used. Very few candidates linked the increase in heat production to the electron
transport chain. Many interpreted this increase as an indication that respiration was
occurring at a faster rate.
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Report on the Examination – General Certificate of Education (A-level) Biology – Unit 4: Populations
and Environment – January 2011
Question 7
(a)(i)
Generally well answered with most candidates realising that this would reduce cost.
Almost half the candidates also referred to less food being required.
(a)(ii) Almost half the candidates obtained this mark. Although some candidates showed no
understanding of a negative value, many who failed to gain credit did so because of
imprecise language. For example, a common incorrect response was ‘they eat less
than they need to’ rather than the credited response ‘they eat less than expected’.
(b)
This was generally well answered although very few candidates gained all four
marks. Most candidates gained credit for explaining that different types of food could
have different energy values or could affect the growth rate. However, few
candidates related this to the content of the food in terms of fat, carbohydrate, etc.
The effect of environmental temperature on heat loss/gain/respiration was generally
appreciated and many candidates referred to the need to maintain body temperature.
(c)(i)
Most candidates suggested a correct null hypothesis. Candidates who failed to gain
the mark often provided a hypothesis or their null hypothesis did not refer specifically
to RFI and methane production.
(c)(ii) The vast majority of candidates gained one mark for linking low RFI to less methane
being produced. Most candidates gained the second mark by referring to methane as
a greenhouse gas. There were a few responses which referred to the ozone layer.
(d)(i)
Most candidates correctly referred to sulfate without straw. A common incorrect
response was sulfate.
(d)(ii) Over three quarters of candidates gained at least one mark usually for referring to the
effect of the treatment on crop yield. A second mark was obtained by many for
referring to the cost of the treatment or its effect on the environment.
(d)(iii) Most candidates were able to describe how the oxygen content would be affected by
the rice field being flooded or not flooded. Better candidates then described how
methane production was reduced in fields that were not flooded due to fewer
anaerobic microorganisms being present.
Question 8
(a)
It was clearly evident that the majority of candidates had a good understanding of the
process of succession. Forty percent of candidates gained maximum marks and over
ninety percent of candidates scored at least two marks. Candidates provided either a
general description of the key features of succession or described a specific example
of succession in sufficient detail to include all the relevant mark points. There were
some rather simplistic accounts but generally the standard of the responses in terms
of clarity of expression and use of scientific terminology was high. However, some
candidates provided a time frame of thousands or millions of years for succession to
occur. A few candidates misinterpreted succession and described evolution or
eutrophication.
(b)
This was also well answered although not to the same extent as part (a).
Nevertheless, a third of candidates obtained four or five marks. Marks related to the
advantages of using biological agents were regularly awarded for statements related
to their specificity, only one application being required and pests not developing
resistance. Statements such as ‘pests do not become immune’ were not credited.
The main disadvantages outlined referred to the possibility of a biological agent
becoming a pest and the time lag for pest reduction following introduction of a
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Report on the Examination – General Certificate of Education (A-level) Biology – Unit 4: Populations
and Environment – January 2011
biological agent. Unfortunately, although the vast majority of candidates appreciated
that this question related to biological control, a minority of candidates wrote about
the advantages of using chemical pesticides.
(c)
There were some excellent answers with good candidates using the information to
provide a detailed account of the processes involved in speciation. At the other end
of the range ability, there were some very confused responses which often included
statements such as ‘genes dying out’ and ‘species selecting the most advantageous
alleles’. Poor powers of expression often prevented weaker candidates gaining credit
although it was also evident that many answers were based on a poor understanding
of natural selection. There was considerable confusion between genes and alleles
and the idea of separate gene pools was often included as a consequence, rather
than as a cause, of speciation. Despite the reference to 10 000 years in the stem of
the question, only better candidates conveyed the idea that speciation had occurred
over a long time period.
Mark Ranges and Award of Grades
Grade boundaries and cumulative percentage grades are available on the Results statistics
page of the AQA Website.
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