Topic 19: Organisms and their Environment -- Challenge Exam 2
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 — Reading a Pond
Total: 12 marks
Fig. 1.1 shows a food web from a freshwater pond. The organisms are given as letters only.
Key:A algae · B pondweed · C water flea · D mayfly nymph · E pond snail · F dragonfly nymph · G stickleback · H perch · J heron
(a)[3]
Define the term food chain, and state which letters in Fig. 1.1 could begin a food chain, giving a reason.
Model Answer — 1(a)
a food chain shows the transfer of energy from one organism to the next, and it begins with a producer [1]
the chains must begin with A or B [1]
because those are the only organisms with no arrow arriving at them, so they make their own organic nutrients [1]
⚠ If you missed marks here: The definition is worth learning word for word; “shows what eats what” is not enough because it leaves out both the energy and the producer. On the diagram, use the arrows rather than your knowledge of the organisms — the absence of an incoming arrow identifies a producer on any web at all.
(b)[4]
Organism J occupies two different trophic levels in this web. Explain this, using two named chains, and state the trophic level of J in each.
Model Answer — 1(b)
in A → C → G → J, J eats G, which eats a primary consumer [1]
so in that chain J is a tertiary consumer [1]
in A → C → G → H → J, J eats H, which eats G [1]
so in that chain J is a quaternary consumer [1]
⚠ If you missed marks here: An organism sits at two levels only when its prey sit at different levels themselves — simply having two food sources is not enough, and that is the commonest wrong reasoning here. Count organisms, not arrows, and name the chain each answer belongs to; an unqualified “J is a tertiary consumer” is only half a mark scheme point.
(c)[3]
A pollutant-free change in the pond removes all of organism C. Suggest, with reasons, the effect on organism A, on organism F and on organism D.
Model Answer — 1(c)
A increases, because C grazed on it and is no longer doing so [1]
F decreases, because it has lost one of its two food sources [1]
D may decrease, because F and G will now feed more heavily on D instead — although D also has more A and B available, so the two effects oppose one another [1]
⚠ If you missed marks here: Three directions and a reason for each. The D mark is the sideways step, and it is the one that separates a three-mark answer from a one-mark answer. Where two effects pull an organism in opposite directions, say so explicitly and say why: an examiner credits a properly argued “difficult to predict”, but not a bare guess.
(d)[2]
Define the terms consumer and decomposer, and state one reason why decomposers are not usually drawn on a food web.
Model Answer — 1(d)
a consumer gets its energy by feeding on other organisms; a decomposer gets its energy from dead or waste organic material [1]
decomposers act on organisms at every trophic level, so drawing all the arrows would make the diagram unreadable [1]
⚠ If you missed marks here: The decomposer definition names no organism, which matters: many decomposers are fungi, not bacteria. “Or waste” matters as well, because faeces and urine are a large part of what they live on. Do not answer that decomposers are not part of the web — they very much are; they are simply left off the drawing.
Question 2 — An Energy Budget That Has to Balance
Total: 11 marks
A young bullock is kept in a pen for 100 days and every energy transfer is measured. Table 2.1 shows the results.
Measurement
Energy / MJ over 100 days
energy in the grass eaten
1000
energy in the faeces produced
600
energy released in respiration
280
energy in the urine produced
40
energy stored as new body tissue
to be calculated
(a)[3]
Calculate the energy stored as new body tissue, and the percentage of the energy eaten that is available to an animal which eats the bullock. Show your working.
Model Answer — 2(a)
1000 − 600 − 280 − 40 = 80 MJ [1]
80 ÷ 1000 × 100 = 8 % [1]
the figure used is the new tissue, because a predator eats the animal’s body, not its meals [1]
⚠ If you missed marks here: The tempting wrong answer is 400 MJ, which is the energy absorbed; but energy that has been respired away as heat or excreted has left the body, so a predator cannot obtain it. An energy budget always balances, and if yours does not you have missed a row.
(b)[3]
Explain why the energy lost in the faeces is so large for this animal, and predict how the figure would differ for a pig fed on grain. Give a reason.
Model Answer — 2(b)
grass is largely cellulose, and no mammal produces an enzyme that digests it [1]
so a large proportion of the food cannot be digested or absorbed and passes out in the faeces, taking its energy with it [1]
for a pig fed on grain the faeces figure would be much smaller, because grain is largely starch, which is digested easily, so more is absorbed and more can be stored [1]
⚠ If you missed marks here: Name the molecule. “Grass is hard to digest” is worth about half a mark; “grass is largely cellulose and no mammal has an enzyme for it” is worth the mark. Note also that the energy in the faeces is not wasted from the ecosystem’s point of view — decomposers use it — but it never entered the bullock’s body, so it is not available to the next trophic level.
(c)[3]
Explain, in terms of energy, why a given area of land feeds more people if the crop grown on it is eaten directly rather than fed to cattle. Give one reason why livestock are nevertheless farmed on some land.
Model Answer — 2(c)
eating the crop directly involves one energy transfer; feeding it to cattle and eating the cattle involves two [1]
at the extra transfer energy is lost in respiration as heat, in faeces, in urine and in parts that are not eaten, so only a small fraction of the crop energy reaches the human [1]
some land is too poor, too steep or too dry to grow crops but will support grazing animals, and meat and milk are concentrated sources of protein [1]
⚠ If you missed marks here: Count the transfers, then name the losses — a bare “energy is lost” is worth half. The third mark is the “discuss” mark: whenever a question invites you to evaluate an argument, give the other side as well. Do not claim crops contain more energy per kilogram than meat; that is not the argument and is not reliably true.
(d)[2]
A student writes that the energy lost by the bullock is destroyed. Correct this statement, and state where most of that energy finally goes.
Model Answer — 2(d)
energy is never destroyed; it is transferred into a form that no organism can feed on [1]
most of it is transferred to the environment as heat, released by respiration, and the energy in the faeces and dead material is obtained by decomposers, which respire it away as heat in turn [1]
⚠ If you missed marks here: “Destroyed” and “used up” are words examiners strike out, even in an otherwise correct sentence. The second mark rewards following the energy all the way: decomposers do not recycle it, they release it as heat like every other organism, which is why energy has to keep arriving from the Sun.
Question 3 — Numbers, Mass and Energy
Total: 12 marks
Fig. 3.1 shows a pyramid of energy for the chain grass → grasshopper → shrew → kestrel, measured over one year.
(a)[3]
Calculate the percentage of energy transferred at each of the three steps in Fig. 3.1, and state what the three figures show about the efficiency of energy transfer.
the efficiency varies between transfers, so “10 % is always transferred” is a rough generalisation rather than a rule [1]
⚠ If you missed marks here: Three divisions, three different answers — which is exactly why examiners set questions with more than one transfer. Always divide the figures in front of you rather than quoting a remembered percentage, and show the division so a slip in arithmetic does not cost you the method mark.
(b)[3]
Explain why a pyramid of energy can never be inverted, and give one advantage and one disadvantage of using a pyramid of energy rather than a pyramid of biomass.
Model Answer — 3(b)
energy is lost at every transfer, so a trophic level can never contain more energy than the level below supplied to it [1]
advantage: it measures energy over a period of time, so it takes account of the rate at which material is produced and is never misleading; it also allows efficiency to be calculated [1]
disadvantage: the data are difficult and slow to collect, because energy must be measured over a whole year [1]
⚠ If you missed marks here: Do not extend the guarantee to a pyramid of biomass: because biomass is a snapshot at one moment, it genuinely can be inverted. The disadvantage mark is the one people leave out, and a “discuss” question caps an answer that only gives one side.
(c)[3]
The same community is drawn as a pyramid of numbers, using a single oak tree as the producer instead of grass. Describe the shape you would expect, and explain it.
Model Answer — 3(c)
the base bar would be the narrowest, so the pyramid would be inverted at the bottom [1]
because a pyramid of numbers takes no account of the size of the organisms [1]
and here one very large producer supports very many small primary consumers [1]
⚠ If you missed marks here: Describe the shape and then explain it — a question worth three marks is not asking for a one-line answer. “Because the tree is big” gives the instance without the rule; “because there are more insects than trees” simply restates the diagram and is not an explanation at all.
(d)[3]
A pyramid of biomass for the open ocean is inverted at the base. Explain how this is possible, and state how the data for a pyramid of biomass are collected.
Model Answer — 3(d)
a pyramid of biomass shows the mass present at one moment in time — a snapshot [1]
the microscopic producers reproduce and are grazed extremely rapidly, so little material is standing there at any instant, although a great deal is produced over the year [1]
biomass is measured as dry mass: the organisms are dried to remove the water, because water content varies between organisms and water contains no energy [1]
⚠ If you missed marks here: The word that earns the first mark is snapshot, or “at one point in time”. Do not explain the inversion by water content: the water has already been removed, which is exactly what “dry mass” means, and saying otherwise loses both marks at once.
Question 4 — Carbon, In and Out
Total: 11 marks
Fig. 4.1 shows the carbon cycle with lettered arrows. The letter Q appears three times.
(a)[3]
Name the processes represented by Q, R and T on Fig. 4.1, and explain why Q appears on three different arrows.
Model Answer — 4(a)
Q — respiration; R — feeding [1]
T — formation of fossil fuels [1]
Q appears three times because plants, animals and decomposers all respire, and all three release carbon dioxide into the air [1]
⚠ If you missed marks here: The third mark is the interesting one and it is often missed: students remember that animals respire and forget that plants do so day and night, and that decomposers respire everything they obtain. That last point is what makes decomposition a route back to the air at all.
(b)[3]
Only one process removes carbon dioxide from the air. Name it, describe what it does to the carbon, and explain how this can be used to identify it on an unlabelled diagram.
Model Answer — 4(b)
photosynthesis [1]
it takes carbon dioxide from the air and builds the carbon into organic compounds such as glucose, which the plant converts to starch, cellulose and, with nitrate ions, into protein [1]
it is the only arrow pointing away from the carbon dioxide box; every other arrow points towards it [1]
⚠ If you missed marks here: Say “organic compounds” or name them; “turns carbon dioxide into food” is loose. The diagram test in the last mark is worth carrying into the exam — one arrow out, three arrows in, and once photosynthesis is placed everything else falls around it.
(c)[3]
Describe two different routes by which a carbon atom in a glucose molecule in a leaf could return to the air as carbon dioxide, and state which route is faster.
Model Answer — 4(c)
route one: the plant respires the glucose itself and releases carbon dioxide [1]
route two: the leaf dies and falls, decomposers feed on it and then respire, releasing carbon dioxide; alternatively an animal eats the leaf and respires [1]
the plant respiring it directly is faster — it can happen within hours, while decomposition requires the leaf to die and be broken down first [1]
⚠ If you missed marks here: Notice that the decomposition route needs two named processes: decomposition transfers the carbon into the decomposers, and their respiration puts the gas into the air. Writing “decomposition releases carbon dioxide” without mentioning respiration is the standard half-answer.
(d)[2]
Explain why the carbon in a lump of coal took hundreds of millions of years to re-enter the cycle.
Model Answer — 4(d)
the organisms it came from died in conditions where decomposers could not break them down, with little or no oxygen, so their carbon compounds were buried and compressed instead of being returned [1]
the only process that returns that carbon to the air is combustion, so the carbon remained out of the cycle until the coal was burned [1]
⚠ If you missed marks here: Fossil fuels are simply decomposition that never happened. A frequent wrong answer reverses the sequence and claims the remains were burned before they could decompose — combustion is what ends the storage, hundreds of millions of years later.
Question 5 — Two Fields and Their Bacteria
Total: 11 marks
Two fields are given the same fertiliser and receive the same rainfall. Field A is drained and ploughed each autumn; field B is flat and holds standing water for weeks after heavy rain. Table 5.1 shows the soil after three years.
Field A (drained, ploughed)
Field B (waterlogged)
nitrate in soil / mg per kg
42
9
ammonium in soil / mg per kg
6
21
crop yield / tonnes per hectare
8.4
4.1
(a)[3]
Calculate how many times greater the nitrate concentration is in field A than in field B, and describe two other differences shown in Table 5.1.
field B contains more ammonium — 21 compared with 6 mg per kg [1]
field A gives roughly twice the yield — 8.4 compared with 4.1 tonnes per hectare [1]
⚠ If you missed marks here: Describe means quote figures. “Field B has more ammonium” without the numbers is worth half; the mark scheme wants both values or the difference between them. Take the calculation first — it is the quickest mark on the question and cannot be argued with.
(b)[4]
Explain the difference in nitrate concentration between the two fields, naming the bacteria involved.
Model Answer — 5(b)
draining and ploughing put air, and therefore oxygen, into the soil of field A [1]
nitrifying bacteria need oxygen, so in field A they are more active and convert ammonium ions to nitrite and then to nitrate ions [1]
in the waterlogged field B, water fills the air spaces so oxygen is short and nitrification is slow [1]
denitrifying bacteria, which thrive where oxygen is scarce, convert nitrate ions back into nitrogen gas, which is lost to the air [1]
⚠ If you missed marks here: Everything here follows from oxygen, and the two bacterial effects push in the same direction, which is why field A wins twice over. The error that costs most marks is swapping fixation and denitrification: fixation runs air to soil, denitrification runs soil to air.
(c)[2]
Explain why field B contains more ammonium than field A, even though it contains far less nitrate.
Model Answer — 5(c)
decomposition continues in field B and releases ammonium ions from dead material and waste [1]
but nitrification is slow for lack of oxygen, so the ammonium is not converted onwards into nitrate and accumulates [1]
⚠ If you missed marks here: This row is the real test in the table, because it distinguishes two possible explanations. If decomposition had failed, the ammonium would be low as well — the fact that it is high is direct evidence that the block is at nitrification. Data questions reward that kind of reasoning far more than recall does.
(d)[2]
Suggest why the roots of the crop in field B may absorb even less nitrate than the soil figure alone would suggest.
Model Answer — 5(d)
nitrate ions are absorbed by active transport, against the concentration gradient, which requires energy from respiration [1]
a waterlogged soil is short of oxygen for the roots, so aerobic respiration is reduced and less energy is available for active transport [1]
⚠ If you missed marks here: This is the cross-topic link the question is really testing, back to Topic 3. Do not answer “because there is less nitrate” — the question specifically asks for an effect beyond the concentration, and that answer earns nothing.
Question 6 — Growth, and What Stops It
Total: 12 marks
Fig. 6.1 shows the growth of a yeast population in a sealed flask of nutrient broth.
(a)[2]
Name the four phases W, X, Y and Z, and state the approximate time at which the population reaches its maximum.
Model Answer — 6(a)
W lag, X exponential (or log), Y stationary, Z death [1 for all four]
the maximum is reached at about 22–24 hours [1]
⚠ If you missed marks here: Write “exponential (log)” and both accepted names are covered. For the reading, take the value from the highest plotted point rather than from the middle of the flat region, and give a time that the graph actually supports.
(b)[4]
Explain the shape of the curve in phase W and in phase X.
Model Answer — 6(b)
in W the organisms are adjusting to their new conditions, taking up water, growing and producing the enzymes they need to use the nutrients available [1]
so few are dividing yet, and with so few cells present even a doubling is barely visible on this scale [1]
in X food, space and oxygen are plentiful, waste has not built up and nothing is limiting growth [1]
so the birth rate greatly exceeds the death rate and the population doubles in a fixed time, which is why the line curves upwards rather than running straight [1]
⚠ If you missed marks here: Do not explain the lag phase by a shortage of food: at the start there is more food per organism than at any later time. And in the exponential phase, notice that exponential means multiplying by a constant factor, not adding a constant number — that is precisely why the line steepens.
(c)[4]
Explain the shape of the curve in phase Y and in phase Z, and state one difference between the two explanations.
Model Answer — 6(c)
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 has accumulated and competition is intense [1]
in Z the death rate exceeds the rate of reproduction, so the population falls [1]
the difference is that in Y the food is running short while in Z it is exhausted and the waste has reached toxic levels [1]
⚠ If you missed marks here: The single most valuable sentence in this sub-topic is “the rate of reproduction equals the death rate”. A flat line means two large rates cancelling — it does not mean that reproduction has stopped, which is what most students write. The wording of the food difference is what separates the two phases.
(d)[2]
The experiment is repeated with fresh broth supplied continuously and waste removed. Predict how the curve after 18 hours would differ, and explain your prediction.
Model Answer — 6(d)
the population would continue to rise for much longer and there would be no death phase, because the two factors that ended growth — shortage of food and accumulation of waste — have been removed [1]
it would still level off eventually, because space in the flask is finite and competition for it would become the new limiting factor [1]
⚠ If you missed marks here: The second mark is the one that lifts this answer: removing two limiting factors does not remove all of them. Whichever resource is in shortest supply sets the ceiling — exactly the same logic as limiting factors in photosynthesis.
Question 7 — What People Do to a Web
Total: 11 marks
(a)[3]
A lake contains algae, water fleas that graze on the algae, and small native fish that eat the water fleas. A large predatory fish is introduced. Construct the original food chain, and suggest two reasons why the introduced fish became abundant.
Model Answer — 7(a)
algae → water flea → native fish, with arrows in the correct direction [1]
it had no natural predators in the lake, so nothing controlled its numbers [1]
there was plenty of food and space at first, so little competition — and the native fish had no defence against a predator they had never met [1]
⚠ If you missed marks here: Producer first, arrows meaning “is eaten by”. On the reasons, avoid “it was stronger” and “it was better adapted”: neither names a mechanism and neither scores. An individual does not adapt itself to a new habitat — populations become adapted over many generations.
(b)[4]
Five years later, the native fish have almost disappeared and the water of the lake has turned green. Explain, using the food chain, how this could have happened.
Model Answer — 7(b)
the introduced fish preyed on the native fish, so their numbers fell sharply [1]
with fewer native fish, the water fleas would be eaten less and would be expected to increase, which would make the water clearer — so that alone cannot explain the observation [1]
the introduced fish must therefore also be feeding on the water fleas, reducing their numbers [1]
with fewer water fleas the algae are grazed far less, so they multiply and the water turns green [1]
⚠ If you missed marks here: This is a deliberately awkward question: the obvious one-step prediction is the opposite of what happened. When your reasoning contradicts the stem, the stem is right and your model is incomplete — go back and ask what else the new organism might be eating. Saying that out loud is worth a mark in itself.
(c)[4]
Define overharvesting. A fishery is harvested at the same rate for forty years without decline, and then the catch is doubled and the population crashes within four years. Explain why the earlier rate did no harm, and describe one effect of the crash on the rest of the community.
Model Answer — 7(c)
overharvesting is removing individuals faster than the population can replace them by reproduction [1]
at the earlier rate, the number of young fish reaching adulthood matched or exceeded the number removed, so the population size was maintained [1]
at the doubled rate, removal outpaced reproduction, so the population fell each year and could not recover [1]
one effect: organisms that fed on the fish lose a food source and decline or switch prey — or the organisms the fish preyed on increase, because they are eaten less [1]
⚠ If you missed marks here: The definition turns on rate, which is exactly what the two halves of this story illustrate; an answer that says “too many fish were taken” has not defined anything. For the last mark, trace the effect through the chain in one clear direction with a reason attached, rather than listing organisms.
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A : 48-55
B : 40-47
C : 32-39
D : 24-31
E : 16-23
U : <16
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