B io Factsheet

www.curriculumpress.co.uk Number 127

Why Students Lose Marks I: Ecology

This Factsheet analyses students’ real answers to exam questions. Some of the students received an A grade, some received an E. We’ll look at how these
two groups tackled the ecology question. By the end of this Factsheet, you should be more confident about:
• What the examiners want
• The kinds of things you are likely to be asked
• Common mistakes and misunderstandings

Table 1 lists the most common AS/A2 exam questions on ecology. A ! indicates the topic is on a particular specification.
This Factsheet will look at students’ answers on food chains, nitrogen cycle and eutrophication, since they are on every specification.

Table 1. Common AS/A2 exam questions Fig 2. Ecological pyramids

AQA A AQA B CCEA Edexcel OCR WJEC

Food
chains/webs ! ! ! ! ! !
Carbon
cycle ! ! ! ! !
Nitrogen
cycle ! ! ! ! ! ! Numbers Biomass
Eutrophication ! ! ! ! ! !
Predator-prey Common mistakes
graphs ! ! ! ! As you read the students’ answers to the questions, imagine that you are
the examiner and mark the answers. Do this first without looking at the
Ecosystem etc
markscheme. Then read the markscheme and mark it again.
definitions ! ! ! ! ! !
Acid rain !* ! ! !* Explain the shape of the pyramid of numbers

* = option module only The biomass decreases at each level. Biomass is lost at each
..................................................................................................................
level because of respiration and heat.
..................................................................................................................
Food chains and webs
The examiners want you to understand the fundamental principle that
energy from the sun, captured by green plants in photosynthesis, is then ............................................................................................................. [3]
available – in chemical form – to herbivores and thus to carnivores.
Markscheme
Fig 1. Food chain There is less energy available at each (trophic) level;
Respiration;
Not all preceding organism eaten;
Not all food digested/ref to faeces/egestion;

Sun • The student gained one of the three marks.

• Their first sentence describes but doesn’t explain.
peppered moth • The first part of their second sentence almost repeats the first.
redstart • “Biomass is lost at each level” doesn’t gain credit because the
birch tree examiners want a reference to energy.
• The student gained one mark by mentioning respiration.
• Reference to heat doesn’t get a mark – energy is lost as heat during
Typical exam questions want you to know that: respiration, so this is the same marking point.
• The arrows show the transfer of energy
• Energy is lost at each stage because: organisms respire and therefore
lose energy as heat; the peppered moth doesn’t eat all of the birch
tree; the peppered moth can’t digest all of the birch leaves it does
eat – energy is lost in the egested material
• As a consequence of these preceding points, both the numbers and
biomass of organisms usually decrease as you move along a food
chain. Fig 2 illustrates pyramids of numbers and biomass.

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127 Why Students Lose Marks I: Ecology Bio Factsheet
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Nitrogen cycle
Different specifications require different levels of knowledge (Table 2)
Typical exam questions expect you to be able to label a diagram of the
nitrogen cycle and explain each of the stages. The fundamental principle
Table 2. Specification content is that all living organisms need nitrogen (proteins, DNA etc) but only
certain species of bacteria can use the nitrogen gas in the atmosphere.
AQA A AQA B CCEA Edexcel OCR WJEC Gaseous N2 makes up 78% of the atmosphere. Every day, huge amounts
Know names of N2 gas are added to the atmosphere because of denitrification
of bacteria ! ! ! (conversion of NO3− or NH4+ into N2) .To balance this, nitrogen-fixing
bacteria such as Rhizobium take gaseous N2 and convert it to NH3
understand : (ammonia gas). The ammonia gas then dissolves in water to form
fixation ! ! ! ! ! ! ammonium ions which can form ammonium compounds. A third process,
nitrification, converts ammonium ions (NH4+) into nitrite ions (NO2− )
nitrification ! ! ! ! ! ! and then nitrate ions (NO3−).
denitrification ! ! ! ! ! !

Common mistakes
The diagram shows the nitrogen cycle.
Fig 3. The Nitrogen cycle

nitrogen (N2) nitrogen oxides

deposition
A denitrifying ammonia (NH3)
bacteria fertilisers
urine and Transport &
litter faeces Animals feeding plants Industry
bacterial volcano
absorbed
breakdown death
bacteria
in soil ammonia (NH3) nitrates (NO3−)
and
dissolves nitrifying
root nodules
in soil water bacteria
ammonium (NH4+) leaching
decomposition nitrifying bacteria nitrites
(NO 2−)

Is this an A, C or E candidate?

Comment on the significance of: Too vague!

(i) Stage A

No. Bacteria fix nitrogen Legumes fix nitrogen. Plants need nitrogen for growth
......................................................................................................................................
Get the spelling right!
Legumes contain bacteria – Rhizonium – which fixes nitrogen.
......................................................................................................................................
Without these bacteria the plant could not get nitrogen and would die.
......................................................................................................................................

.................................................................................................................................. [4]
Most nitrogen fixing bacteria live freely in the soil, not in Correct, but the question asked about the significance
legumes. Legumes attract bacteria to them by secreting of the process. It’s significant for more than just plants –
a substance into the soil. When the bacteria enter the animals rely on nitrogen-fixing bacteria too.
root a nodule forms around them.

Here’s the markscheme: The examiner gave the student marking point 3 (MP3) for their reference
to “bacteria…fixes nitrogen” and MP5 for their reference to “without
Nitrogen in atmosphere is unavailable to higher organisms; these… the plant could not get nitrogen”. So 2 out of 4 – this is the C
Nitrogen fixers/nitrogen-fixing bacteria convert N2 into NH3; candidate.
NH3 then converted into ammonium compounds/nitrates;
Makes nitrogen available to plants;
Nitrogen then available to herbivores and then carnivores; From this, you can see that markschemes are good things to revise from.
You can get a catalogue to buy markschemes for your board online, or by
post. The web addresses are given at the end of the Factsheet.

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127 Why Students Lose Marks I: Ecology Bio Factsheet
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Eutrophication
The examiners expect you to know that nitrates are very soluble and will therefore readily leach/ be washed into rivers, ponds and reservoirs.
Phosphates are much less soluble and are more likely to be blown (i.e. erode) into water courses. Once there, NO3− and PO43− will act as fertilisers,
promoting rapid growth of algae/phytoplankton. These phytoplankton have a short life, many die and are then broken down by bacteria.

In doing this breakdown, the bacteria use up a lot of oxygen from the water. In other words, there is a high biological oxygen demand (BOD). Other
aerobes – such as fish – may then die as a result of a lack of oxygen. In addition, the phytoplankton may cover the water surface (algal bloom)
stopping light getting to the plants that grow below the water surface. These plants die and they too are broken down by aerobic bacteria.

If plants that are growing on the sides of the river/pond/reservoir die, their roots will no longer hold soil in place. The soil will crumble into the water,
increasing its turbidity (murkiness), reducing light levels further. The addition of excess NO3− and PO43− ions to water is eutrophication. The algal
blooms, increasing turbidity and decreasing O2 are the consequences of eutrophication.

Common mistakes
The diagrams show the consequences of excess fertiliser application near a natural pond (Fig 4)

Fig 4. Excess fertiliser application

excess
input of
nutrients

1 2 3 4

Is this an A, C or E student?

Outline the process that has occurred in this pond MP2

MP1
The fertiliser has got into the pond and caused an algal bloom.
.......................................................................................................................................
This bloom has used all the oxygen in the pond.
.......................................................................................................................................
No. Phytoplankton are plants. They
The fish and plants that need oxygen have died. photosynthesise. They produce O2!
.......................................................................................................................................

.......................................................................................................................................
why?

................................................................................................................................... [6]

Here’s the markscheme The student has 2 out of 6 – this is the E candidate.
Fertiliser/nitrates/phosphates have entered pond/reference to eutrophication; Apart from the weak Biology, this is a good example
Stimulated algal bloom/growth of phytoplankton; of poor exam technique. The student has produced
Phytoplankton has high turnover rate/reference to many phytoplankton/algae dying; three sentences containing four points. The question
Broken down/decomposed by aerobic/oxygen-consuming bacteria; is out of six, so to be sure of getting all the marks, the
Created high BOD/oxygen levels have fallen; student should be looking to make seven or eight clear,
Aerobes/fish/plants/macrophytes die because they cannot respire; different points.
Dead/sinking phytoplankton/soil increase water turbidity/reduce light penetration;
Max 6

Web References:
www.aqa.org.uk
www.edexcel.org.uk
www.ocr.org.uk
www.wjec.co.uk
www.ccea.org.uk
Acknowledgements:
This Factsheet was researched and written by Kevin Byrne.
Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.
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