Topic 19: Organisms and their Environment -- Challenge Exam 1
1 hour 15 minutes
80
7
75:00
0610
Instructions
Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.
Question Navigation
This paper covers the whole of Topic 19. Like a real Cambridge paper it ranges across every sub-topic — 19.1 energy flow, 19.2 food chains, food webs and trophic levels, 19.3 ecological pyramids and energy efficiency, 19.4 the carbon and nitrogen cycles, and 19.5 populations and the sigmoid curve — and it mixes them inside single questions. All three Topic 19 papers do; they differ in the angle they come at it from, not in what they cover.
Question 1 — A Web With Nothing Named
Total: 12 marks
Fig. 1.1 shows a food web from an area of grassland and hedgerow. The organisms have been given letters only. Work from the arrows, not from what you think each organism is.
Key:A grass · B dandelion · C grasshopper · D rabbit · E field mouse · F snail · G frog · H shrew · J thrush · K grass snake · L fox · M kestrel
(a)[3]
State the letters of the two producers in Fig. 1.1, and explain how the diagram alone allows you to identify them.
Model Answer — 1(a)
A and B [1]
they have arrows leaving them but no arrow arriving, so no organism transfers energy into them [1]
they must therefore make their own organic nutrients, which is the definition of a producer [1]
⚠ If you missed marks here: The mark is for the reasoning, not for recognising grass and dandelion. A producer is identified on any web by the absence of an incoming feeding arrow — that test works even when every organism is unfamiliar, which is exactly why Cambridge letters the diagram. Do not write “because they are at the bottom”: position on the page carries no meaning.
(b)[4]
Construct the longest food chain shown in Fig. 1.1, using letters. State the number of trophic levels in it, and name the trophic level occupied by the third organism in your chain.
Model Answer — 1(b)
a five-organism chain, for example A → C → G → K → L [1]
arrows drawn in the correct direction, from the organism eaten to the organism eating it [1]
five trophic levels [1]
the third organism (G) is a secondary consumer [1]
⚠ If you missed marks here: Count organisms, not arrows — a five-organism chain has four arrows, and counting arrows is what turns a secondary consumer into a tertiary one. Check every link exists in the web before you commit to a chain; a long chain with one invented link scores nothing. And remember that consumer numbering always runs one behind the trophic level, because the producer takes level 1.
(c)[3]
A disease removes every individual of organism D from the habitat. Suggest, with a reason in each case, the effect on organism A, on organism L and on organism E.
Model Answer — 1(c)
A increases, because D fed on it and it is now grazed less [1]
L decreases (at least at first), because it has lost one of its food sources [1]
E may decrease, because L and the other predators of D will now take more E instead — or E may increase, because there is more A and B for it to eat; the two effects oppose one another [1]
⚠ If you missed marks here: Three directions, three reasons. The mark for E is the one most students never reach: it is not connected to D by a feeding arrow at all, and the effect reaches it sideways, through the predators that have switched prey. Where two effects oppose, say so and say why — “the outcome is difficult to predict because…” is a full answer, not a hedge. Never write “the web will collapse”.
(d)[2]
Explain what the arrows in Fig. 1.1 represent, and state why an arrow drawn in the opposite direction would be wrong.
Model Answer — 1(d)
an arrow means is eaten by, and shows the direction in which energy is transferred [1]
reversing it would state that the predator is eaten by its prey, and would show energy flowing the wrong way through the chain [1]
⚠ If you missed marks here: This is the commonest lost mark in the whole topic, and it is lost silently because a reversed diagram still looks tidy. Before handing in any chain, read it in your head with the words “is eaten by” on every arrow. “The arrow shows what the animal eats” is the wrong answer that most students give.
Question 2 — Where the Energy Goes
Total: 11 marks
Table 2.1 shows the energy present at each trophic level of a grassland, measured over one year.
Trophic level
Energy / kJ m⁻² y⁻¹
producers
20 000
primary consumers
1600
secondary consumers
160
tertiary consumers
24
(a)[3]
Calculate the percentage of energy transferred from the producers to the primary consumers, and from the secondary consumers to the tertiary consumers. Show your working, and comment on what the two figures show.
Model Answer — 2(a)
1600 ÷ 20 000 × 100 = 8 % [1]
24 ÷ 160 × 100 = 15 % [1]
the efficiency of the transfer is not constant; it varies from level to level, so “10 %” is a rough generalisation and not a rule [1]
⚠ If you missed marks here: Write the division out even when you can do it in your head — the working carries a mark if the arithmetic slips. The comment mark is the one people miss: examiners set two transfers precisely so that you notice they differ. Never quote 10 % from memory when data are in front of you.
(b)[4]
Explain why so little of the energy at one trophic level reaches the next. Give four different reasons.
Model Answer — 2(b)
respiration releases energy which is transferred to the surroundings as heat [1]
some material is not digested and its energy leaves in the faeces [1]
energy is lost in excretion, as urea in the urine following deamination [1]
not all of the organism is eaten — bark, roots, bone and teeth are usually left [1]
⚠ If you missed marks here: Four distinct losses are wanted, so do not give the same one twice in different words. “Energy is used for movement” and “energy is used to keep warm” are both respiration, and will be credited once between them. Never write “energy is destroyed” or “used up”: energy is transferred into a form nothing can feed on.
(c)[2]
Use the figures in Table 2.1 to explain why this grassland has no fifth trophic level.
Model Answer — 2(c)
a fifth level would receive roughly a tenth of 24 kJ, that is only about 2–3 kJ m⁻² y⁻¹ [1]
that is too little energy to support a viable population of another predator, which would use more energy hunting than it gained [1]
⚠ If you missed marks here: Use the numbers — the question says “use the figures”, so an answer with no arithmetic in it caps itself at one mark. And avoid the absolute version, “there is no energy left”: there is some, and the mark is for saying that there is not enough.
(d)[2]
A country can either grow wheat and sell it for people to eat, or feed the wheat to cattle and sell beef. Explain, in terms of energy, which choice feeds more people from the same area of land.
Model Answer — 2(d)
eating the wheat directly feeds more people, because it involves one energy transfer instead of two [1]
at the extra transfer the cattle lose energy in respiration as heat, in faeces, in urine and in parts not eaten, so only a small fraction of the crop energy reaches the human [1]
⚠ If you missed marks here: Count the transfers first, then name the losses — a vague “energy is lost” scores half. Do not argue that plants contain more energy per kilogram; meat is energy-dense, and that is not the argument. In a “discuss” version, add that some land is too poor, steep or dry for crops but will support grazing.
Question 3 — Three Ways of Drawing the Same Woodland
Total: 12 marks
Fig. 3.1 and Fig. 3.2 describe the same oak woodland chain in the same year.
(a)[3]
Fig. 3.1 is a pyramid of numbers for the chain oak tree → insect → small bird → sparrowhawk. Explain the shape of the pyramid.
Model Answer — 3(a)
a pyramid of numbers takes no account of the size of the organisms [1]
here one very large producer supports very many small primary consumers, so the bottom bar is the narrowest [1]
each organism is counted as one individual, so the single oak counts the same as a single aphid [1]
⚠ If you missed marks here: Two halves are needed: the rule and the instance. “Because the oak tree is very big” is the instance without the rule and scores one. “Because there are more insects than trees” simply describes the diagram again and scores nothing at all — a description is not an explanation.
(b)[3]
Fig. 3.2 is a pyramid of biomass for the same chain: oak 5000 kg, insects 80 kg, small birds 8 kg, sparrowhawks 0.5 kg. State what biomass means, explain why this pyramid has the expected shape, and calculate the percentage of the oak’s biomass present in the insects.
Model Answer — 3(b)
biomass is the dry mass of living material — the water is removed because its content varies and it contains no energy [1]
measuring mass takes account of the size of the organisms, so the one enormous oak forms a broad base [1]
80 ÷ 5000 × 100 = 1.6 % [1]
⚠ If you missed marks here: “Dry” is the word that earns the first mark; “mass of living material” on its own is usually not enough. In the calculation, divide the higher level by the lower one: 5000 ÷ 80 gives 62.5, which is not a percentage of anything meaningful. Check the units match before dividing — a question may give one figure in grams to see whether you notice.
(c)[3]
A pyramid of biomass for an area of open ocean is inverted, with less phytoplankton than zooplankton. Explain how this is possible, and state which type of pyramid would be guaranteed to show the expected shape and why.
Model Answer — 3(c)
a pyramid of biomass is a snapshot at one moment in time [1]
phytoplankton reproduce very rapidly and are grazed almost as fast as they are produced, so little is standing there at any instant, although far more is produced over a year [1]
a pyramid of energy can never be inverted, because energy is lost at every transfer, so a level can never contain more than the level below supplied [1]
⚠ If you missed marks here: The word that unlocks this is snapshot. Do not answer that phytoplankton contain more water: biomass is already measured dry. And do not claim the data must be wrong — a pyramid of biomass genuinely can be inverted, and only the energy pyramid carries a guarantee.
(d)[3]
Describe how an ecological pyramid should be drawn to scale, and state one advantage and one disadvantage of using a pyramid of energy.
Model Answer — 3(d)
producers at the bottom, one horizontal bar per trophic level, with the width of each bar proportional to the quantity and all bars the same height, every bar labelled [1]
advantage: it measures energy over a period of time, so it takes account of the rate of production and can never be inverted; it also allows the efficiency of each transfer to be calculated [1]
disadvantage: the data are difficult and slow to collect, because energy has to be measured over a whole year [1]
⚠ If you missed marks here: Height carries no meaning at all — only width does — and bars go in trophic order even when that produces an odd shape. On the evaluation marks, remember that the energy pyramid is the best of the three biologically and the worst practically; a “discuss” question is asking for exactly that tension.
Question 4 — Following a Carbon Atom
Total: 11 marks
Fig. 4.1 shows the carbon cycle. Each lettered arrow represents a process. Note that the letter Q appears on three arrows.
(a)[4]
Name the processes represented by the letters P, Q, S and U on Fig. 4.1.
Model Answer — 4(a)
P — photosynthesis [1]
Q — respiration [1]
S — decomposition [1]
U — combustion [1]
⚠ If you missed marks here: Find P first: it is the only arrow pointing away from the carbon dioxide box, and only one process removes carbon dioxide from the air. Everything else then falls into place around it. Q appears three times because plants, animals and decomposers all respire, and a question may ask you to explain exactly that.
(b)[3]
Describe how a carbon atom in a molecule of carbon dioxide in the air could become part of a protein molecule in a fox. Name the processes involved.
Model Answer — 4(b)
it is absorbed by a plant and built into glucose by photosynthesis [1]
the plant uses the glucose, together with nitrate ions, to make amino acids and then plant protein [1]
a herbivore eats the plant and a fox eats the herbivore — feeding, followed by digestion of the protein to amino acids and their assembly into fox protein [1]
⚠ If you missed marks here: There is only one way out of the air, so every answer must begin with photosynthesis. Stop where the question stops: it ends at protein in the fox, so writing about deamination or urea afterwards earns nothing and costs time. Name the processes explicitly — the marks are for the named steps, not for a general story.
(c)[2]
Explain why the carbon locked in coal was outside the carbon cycle for millions of years, and name the process that returns it.
Model Answer — 4(c)
the organisms died in conditions where decomposers could not break them down, with little or no oxygen, so the carbon compounds were buried and compressed rather than being returned [1]
combustion returns it to the air as carbon dioxide [1]
⚠ If you missed marks here: Fossil fuels are decomposition that never happened. A common wrong answer reverses the timeline and says the remains were burned before they could decompose — combustion is what ends the storage, millions of years later, not what caused it.
(d)[2]
A student writes: “The carbon cycle shows that energy is recycled in an ecosystem.” Explain why this is wrong.
Model Answer — 4(d)
the cycle recycles atoms; the same carbon atoms are used repeatedly [1]
the energy that travelled with them is transferred to the environment as heat at every respiration arrow and is never returned, so energy flows through in one direction and must be continually supplied by the Sun [1]
⚠ If you missed marks here: This is the most heavily penalised misconception in the topic. Note the shape of a full answer: it does not simply say “energy is not recycled”, it says what is recycled and where the energy goes instead. Decomposers return mineral ions, not energy.
Question 5 — Nitrogen, Bacteria and a Waterlogged Field
Total: 12 marks
Fig. 5.1 shows the nitrogen cycle with lettered arrows.
(a)[4]
Name the processes P, Q, R and V on Fig. 5.1, and for each one state the type of organism responsible, where there is one.
Model Answer — 5(a)
P — nitrogen fixation, by nitrogen-fixing bacteria (and by lightning, which is not an organism) [1]
Q — decomposition, by decomposers (bacteria and fungi) [1]
R — nitrification, by nitrifying bacteria [1]
V — denitrification, by denitrifying bacteria [1]
⚠ If you missed marks here: These are the only four microbial roles the syllabus requires, and the genus names of the bacteria are not required, so do not waste time on them. The error that costs most marks is swapping fixation and denitrification: fixation runs air to soil, denitrification runs soil to air. Check the direction of the arrow before you name the process.
(b)[3]
Describe what happens between the nitrate ions in the soil and the proteins in an animal, naming the processes involved.
Model Answer — 5(b)
nitrate ions are absorbed by the roots, by active transport against the concentration gradient, using energy from respiration [1]
the plant uses them to make amino acids, which it joins together to form plant proteins [1]
the animal eats the plant and digests the protein to amino acids, absorbs them, and builds them into its own proteins [1]
⚠ If you missed marks here: Three distinct steps and the marks are one each. The detail examiners look for is active transport: a root absorbs nitrate against the gradient and therefore needs energy, which is why a waterlogged, oxygen-poor soil damages uptake twice over. Do not write that the animal absorbs plant protein; protein is digested to amino acids first.
(c)[3]
Two fields receive the same fertiliser and the same rainfall. Field A is drained and ploughed; field B is flat and stays waterlogged for weeks. Field A contains 42 mg of nitrate per kg of soil, field B only 9 mg per kg. Calculate how many times greater the nitrate concentration is in field A, and explain the difference.
Model Answer — 5(c)
42 ÷ 9 = 4.7 times greater [1]
field A contains more oxygen because it is drained and ploughed, so nitrifying bacteria are more active and produce more nitrate [1]
in the waterlogged field B, oxygen is short, so denitrifying bacteria convert nitrate back into nitrogen gas, which is lost to the air [1]
⚠ If you missed marks here: Take the calculation mark first: it takes ten seconds and cannot be argued with. Then notice that the question has controlled fertiliser and rainfall deliberately — the examiner is pointing at the one variable left, which is oxygen. Both bacterial effects push the same way, and a full answer gives both.
(d)[2]
A farmer harvests and removes his whole crop each year and the yield falls steadily. Explain why growing clover and ploughing it into the soil raises the yield of the following crop.
Model Answer — 5(d)
harvesting removes the nitrogen in the crop protein from the field, and nothing is returned to be decomposed, so nitrate is not replaced [1]
clover has root nodules containing nitrogen-fixing bacteria, so ploughing it in returns nitrogen-rich material which decomposers convert to ammonium and nitrifying bacteria convert to nitrate [1]
⚠ If you missed marks here: Say that it is the bacteria in the nodules that fix nitrogen, not the clover itself — examiners look for that distinction. The other half of the answer is easy to forget: explain why the yield was falling in the first place, because the question asks about a change over years, not just about clover.
Question 6 — A Population in a Sealed Flask
Total: 11 marks
Fig. 6.1 shows the growth of a population of yeast in a sealed flask of nutrient broth. Nothing was added and nothing was removed.
(a)[3]
Define the terms population, community and ecosystem.
Model Answer — 6(a)
population: a group of organisms of one species, living in the same area, at the same time [1]
community: all of the populations of different species in an ecosystem [1]
ecosystem: a unit containing the community of organisms and their environment, interacting together [1]
⚠ If you missed marks here: Definitions are marked phrase by phrase, so keep every qualifier. If your ecosystem definition does not mention the environment or the physical surroundings, you have written a definition of a community and it will not score. Size is not the difference between the two — a rock pool and a rainforest are both ecosystems.
(b)[2]
Name the phases labelled W, X, Y and Z on Fig. 6.1, and state which phase contains the largest number of cells.
Model Answer — 6(b)
W lag, X exponential (or log), Y stationary, Z death [1 for all four]
the largest number of cells is in phase Y [1]
⚠ If you missed marks here: Both names for X are accepted, so write “exponential (log)” and you cannot be wrong. Read the second part carefully: the population is largest in Y but growing fastest in X, and questions swap those two words deliberately.
(c)[4]
Explain, in terms of the factors acting on the culture, the shape of the curve in phase X and in phase Y.
Model Answer — 6(c)
in X the population doubles in a fixed time, because food, space and oxygen are plentiful and nothing is limiting growth [1]
so the birth rate greatly exceeds the death rate, and each organism produced can itself divide [1]
in Y the rate of reproduction equals the death rate, so the number stays constant [1]
because the food supply is running short, toxic waste products have accumulated and competition for the remaining resources is intense — whichever runs short first is the limiting factor [1]
⚠ If you missed marks here: The mark that decides this question is “the rate of reproduction equals the death rate”. A flat line means two large rates cancelling, not two rates of zero, and “they have stopped reproducing” is what most students write. Note the wording difference too: food is running short in the stationary phase and exhausted in the death phase.
(d)[2]
The count at 12 hours is 1400 cells per cm³ and at 18 hours it is 4700 cells per cm³. Calculate the mean rate of increase over this period, and state one factor named by Cambridge that would have limited the population if the flask had been open to the air.
Model Answer — 6(d)
(4700 − 1400) ÷ 6 = 3300 ÷ 6 = 550 cells per cm³ per hour, with the unit given [1]
any one of food supply, competition, predation or disease [1]
⚠ If you missed marks here: Rate means change divided by time; dividing 4700 by 6 is the standard slip and gives 783. Give the unit, because a naked number regularly loses the mark on a rate question. For the second part, stick to the four factors Cambridge names rather than inventing a fifth.
Question 7 — People, Webs and a Sealed Sphere
Total: 11 marks
(a)[3]
A predatory fish is introduced into a lake where it has never lived. Within ten years it is the most numerous fish present and two native species have almost disappeared. Suggest three reasons for the rapid increase of the introduced fish.
Model Answer — 7(a)
it has no natural predators in that lake, so nothing controls its numbers [1]
there was plenty of food and space at first, so there was little competition [1]
the native species had no defence or resistance against a predator they had never encountered [1]
⚠ If you missed marks here: Three separate mechanisms are wanted, so do not give the same idea twice. “It was stronger”, “it was fitter” and “it was better adapted” all name no mechanism and earn nothing. Nor does an individual adapt itself to a new habitat within its lifetime — populations become adapted over many generations, which is far slower than an invasion.
(b)[3]
Define overharvesting, and use a food chain to explain two effects that overharvesting one fish species could have on the rest of a marine community.
Model Answer — 7(b)
overharvesting is removing individuals faster than the population can replace them by reproduction [1]
organisms that fed on that species lose a food source, so their numbers may fall or they switch to other prey, reducing those [1]
the organisms the fish preyed on are eaten less, so their numbers rise, which in turn reduces whatever they feed on [1]
⚠ If you missed marks here: The definition turns on rate: a well-managed fishery is harvested indefinitely without harm, and “too many were taken” is not a definition. For the effects, trace the chain in both directions — up to the predators and down to the prey — because one direction alone caps the answer at one mark.
(c)[5]
A sealed glass sphere contains sea water, algae, a few small shrimps, gravel and a bubble of air. It is kept on a windowsill in the light. Nothing is added and nothing is removed, and it stays alive for years. Explain how this is possible, and state what must keep entering the sphere and why.
Model Answer — 7(c)
the algae take carbon dioxide from the water and fix it by photosynthesis; the shrimps obtain it by feeding [1]
all the organisms respire, returning carbon dioxide, and decomposers in the gravel break down dead material and waste, so the same carbon atoms are used repeatedly [1]
nitrogen is recycled too: shrimp waste and dead material are decomposed to ammonium ions, which nitrifying bacteria convert to nitrate ions [1]
the algae absorb the nitrate and use it to make amino acids and proteins, which the shrimps then eat [1]
light energy must keep entering, because energy is not recycled: it is transferred to the surroundings as heat at every respiration and passes out through the glass, so it has to be continually resupplied [1]
⚠ If you missed marks here: This question is the whole topic in one place, and the final mark is the one that matters most: nutrients cycle, energy flows. A common half-answer says the sphere works because “everything is recycled” — but if that were true the sphere would survive in a cupboard, and it would not. Name the processes rather than describing them vaguely; each named process is a mark.
Self-Assessment
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80
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A* : 56+
A : 48-55
B : 40-47
C : 32-39
D : 24-31
E : 16-23
U : <16
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