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4C Cycles in the Ecosystem
Time: 41 minutes
Total marks available: 41
Total marks achieved: ______
AndyTodd
Q1.
(a) Describe how the process of nitrification affects the availability of nitrates to plants.
(2)
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(b) Some farmers grow legume plants such as clover in their fields. They then plough
them into the soil before growing their cereal crops.
Suggest why they do this.
(4)
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(c) Explain why only about 10% of the energy in the cereal crop is transferred to organisms
that eat the crop.
(2)
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(Total for question = 8 marks)
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Q2.
Here is a food chain.
(a) (i) Use the information in this food chain to complete the diagram.
(3)
(ii) Name one type of organism that is a decomposer.
(1)
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(b) Decomposition is a stage in the carbon cycle. The other stages are respiration,
photosynthesis and combustion.
How many of these four stages add carbon dioxide to the air?
(1)
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(Total for question = 5 marks)
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Q3.
The diagram shows the nitrogen cycle. Different stages have been numbered 1 to 8.
(a) The table lists the stages involved in the nitrogen cycle.
Complete the table by giving the correct number, or numbers, corresponding to
each stage.
The first one has been done for you.
(4)
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(b) Explain how nitrates are absorbed into plants at stage 8.
(3)
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(Total for question = 7 marks)
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Q4.
(a) The diagram shows the cell wall of a bacterium.
Complete the diagram by drawing and labelling the parts found inside the cell wall.
(3)
(b) Decomposition by bacteria helps to release mineral ions, such as nitrates, into the soil.
(i) Explain why the rate of decomposition is affected by the pH of the soil.
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(ii) Explain how nitrate ions help plants to grow.
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(iii) Explain how nitrate ions get into the root cells of plants.
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(c) The graph shows the change in plant growth when different masses of nitrate fertiliser
are added to fields.
(i) On the graph, mark with a cross (×) the point at which the concentration of
nitrate ions ceases to be a limiting factor in the growth of the plant.
(1)
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(ii) Calculate the percentage increase in plant growth when the mass of nitrate fertiliser
is increased from 50 to 100 kilograms per hectare.
(2)
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(iii) Suggest why crops still grow when no nitrate fertiliser is added.
(1)
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(Total for question = 14 marks)
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Q5.
(a) The table describes stages in the water cycle.
Complete the table by naming or describing each stage.
The first one has been done for you.
(3)
(b) Describe the biological consequences of pollution of water by sewage.
(4)
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(Total for question = 7 marks)
AndyTodd
Q1.
This question examined understanding of the nitrogen cycle and energy transfer.
In part (a), the better candidates understood that nitrification involves nitrifying
bacteria converting ammonium into nitrite or nitrite into nitrate. However, many
candidates did not understand what the term nitrification means and believed it to
be a catch all term for all the processes in the nitrogen cycle. These candidates
wrote about nitrifying bacteria, nitrogen fixing bacteria and denitrifying bacteria
and duly lost credit. Many candidates failed to read part (c) carefully and wrote
about movement and heat loss in their answer. The examiners were looking for
answers that showed an understanding of the reasons energy is not transferred
from the producer trophic level to the primary consumer trophic level.
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Q2.
This question presented a food chain and in part a) candidates had to use the
information to complete a diagram, almost all gained full marks. Although
most candidates could name one type of decomposer some suggested rose
or another organism from the food chain. Part b) was correctly answered by
most candidates. Candidates had no difficulty with parts c) and d).
Q3.
This question gave a diagram about the nitrogen cycle and in part (a) students had to identify the
correct stages. Many scored 3 or 4 marks out of 4. In part (b) students had to explain how
nitrates are absorbed into plants. The best responses described how ions are absorbed into the
root hair cells by active transport against a concentration gradient using energy from respiration.
Q4.
(a) This proved to a challenging question and many candidates struggled to
gain full marks. Common errors were to label the nucleiod as a nucleus, and
to label the nucleiod or the plasmids as being made from RNA. Many of the
components of eukaryotic cells were drawn and labelled but gained no
credit.
(b) (i) This question required candidates to make a link between pH and the
process of decomposition. Many appreciated that pH can affect the activity
of enzymes and that these molecules are likely to be denatured if they are
not at the optimum pH. The better answers were able to make this link, but
the weaker answers described the process of decomposition, or discussed
the effect of temperature, or stated that low pH would affect the rate and
that high pH would affect the rate, offering no explanation. The examiners
sensed that many candidates did not understand what pH is.
(ii) The better candidates fully appreciated that nitrate ions are essential for
making amino acids and that amino acids are essential for making proteins.
Weaker candidates made bland statements such as, 'nitrates provide plants
with nutrients which help plants to grow'.
(iii) Weaker answers described events in the nitrogen cycle that add nitrates
to the soil and also about what happens to nitrates after they get into the
root cells of plants. A surprising number of answers stated that nitrates
'contain' amino acids and proteins. Candidates need to read questions
carefully so that they answer what is actually asked. In this case, the
examiners rewarded candidates who appreciated that active transport, an
energy requiring process, is used to move nitrate ions against a
concentration gradient into the root hair cells that have a large surface area
to facilitate the uptake.
(c) (i) The correct answer, a cross on the coordinates of 100 kg per hectare
and 8000 kg per hectare, was frequently seen, but there were many who
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put their cross elsewhere and therefore gained no credit. Those who drew a
line to the correct coordinate were given a mark. About 20% of the
candidates did not attempt to put a cross on the graph suggesting that they
either did not read the question at all or that they did not understand the
phrase 'limiting factor'.
(ii) The correct answer of 11.1% was seldom seen. One mark was available
for noting the number 8000, 7200 or 800 in the working and this is where
most candidates gained some credit. Clearly, candidates are able to select
the right values but do not understand how to use them to calculate a
percentage. Candidates who answer incorrectly and do not show their
working fail to give themselves an opportunity to gain a mark.
(iii) The best answers noted that there is a source of nitrates already in soil
that helps plants to grow. Weaker answers suggested that candidates had
failed to read the question carefully and made general comments about
what plants need to be able to grow or carry out photosynthesis, such as
light and water. Many ignored commenting about nitrate ions and made
reference to other ions being in the soil.
Q5.
This question examined student knowledge of the water cycle and the consequences of sewage
pollution. In part (a), most students recalled condensation and precipitation as the correct
responses to naming the stages. However, only the better students were able to offer an
acceptable description of transpiration. A surprising number believe that transpiration is the
runoff into rivers that flow to the sea, or the absorption of water by plant roots. Answers to part
(b) were poor in their construction and tested the leniency of markers in awarding credit. Whilst it
is clear that students know that sewage pollution kills fish because of oxygen depletion created
by bacterial decomposition, the way in which they express this simple idea is often so convoluted
that the marking points start to lose credibility. Marks were available to those students who also
wrote about eutrophication as a possible consequence of sewage pollution.
Q1.
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Q2.
Q3.
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Q4.
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Q5.
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