IGCSE Biology Paper 4 (Theory / Extended)

Topic 19: Organisms and their Environment -- Challenge Exam 3

Duration1 hour 15 minutes

Total Marks80

Questions7

Time Remaining75:00

Syllabus0610

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 — An East African Grassland

Total: 12 marks

Fig. 1.1 shows a food web from an area of East African grassland with scattered trees. The organisms are shown as letters only.

Key: A grass · B acacia tree · C grasshopper · D zebra · E impala · F giraffe · G lizard · H mongoose · J cheetah · K eagle · L lion · M hyena

(a) [3]

Define the term trophic level. State the letters of the producers in Fig. 1.1 and explain how the diagram alone identifies them.

Model Answer — 1(a)

a trophic level is the position of an organism in a food chain, food web or ecological pyramid [1]

the producers are A and B [1]

they are the only organisms with no arrow arriving at them, so nothing transfers energy into them and they must make their own organic nutrients [1]

⚠ If you missed marks here: The definition must mention position, and it covers webs and pyramids as well as chains — “the level of a food chain an animal is on” is too narrow. On the diagram, use the arrows: nothing about being drawn low on the page makes an organism a producer.

1 mark — trophic level defined as position in a chain, web or pyramid 1 mark — A and B identified as producers 1 mark — reason: no incoming arrows

(b) [4]

Construct two different food chains from Fig. 1.1 that both end at organism L. One must contain three organisms and one must contain four. State the trophic level of L in each.

Model Answer — 1(b)

a valid three-organism chain, for example A → D → L [1]

in that chain L is a secondary consumer [1]

a valid four-organism chain, for example A → C → G → ... or B → E → J → ...; any chain in which L eats an organism that itself ate a consumer [1]

in that chain L is a tertiary consumer [1]

⚠ If you missed marks here: Check every link exists in the web before you commit: a beautiful long chain with one invented arrow scores nothing. Count organisms rather than arrows, and give the trophic level for each chain separately — the whole point of the question is that one organism can sit at more than one level.

1 mark — a valid three-organism chain ending at L 1 mark — L identified as a secondary consumer in it 1 mark — a valid four-organism chain ending at L 1 mark — L identified as a tertiary consumer in it

A long drought removes almost all of organism D. Suggest, with reasons, the effect on organism A, on organism L and on organism E.

Model Answer — 1(c)

A increases, because D grazed on it and is no longer doing so [1]

L decreases, at least at first, because it has lost one of its food sources [1]

E decreases, because L and M will now feed more heavily on E instead — although E also has more A available, so the two effects oppose one another [1]

⚠ If you missed marks here: Up, down and sideways, with a reason on each. The sideways effect on E is the mark that separates a top answer, because E is not joined to D by any arrow — the effect travels through the shared predators. Do not write that the web will collapse; a web with alternative food sources very rarely does.

1 mark — A increases because it is grazed less 1 mark — L decreases because it has lost a food source 1 mark — E affected by predator switching, or opposing effects identified

(d) [2]

Explain what is meant by an arrow in a food web, and state why a food web can never begin with a consumer.

Model Answer — 1(d)

an arrow means is eaten by and shows the direction in which energy is transferred [1]

the energy entering the web comes from the Sun and is fixed by producers, so every chain must begin with a producer; a consumer has no way of obtaining energy other than by feeding on another organism [1]

⚠ If you missed marks here: The reversed arrow is the commonest lost mark in this topic and it is lost silently. Read every chain in your head with “is eaten by” on each arrow before you hand it in. And note that the Sun itself never appears in a chain: it is not an organism.

1 mark — arrow means is eaten by and shows the direction of energy transfer 1 mark — chains begin with a producer because that is where energy enters

Question 2 — Counting and Weighing the Same Grassland

Total: 11 marks

Table 2.1 shows data collected from one hectare of grassland.

Trophic level	Number of organisms	Dry mass / kg per hectare
producers	1 200 000	4800
primary consumers	9000	380
secondary consumers	60	26

(a) [3]

Calculate the percentage of the producers’ dry mass that is present in the primary consumers, and in the secondary consumers. Comment on what the two figures show.

Model Answer — 2(a)

380 ÷ 4800 × 100 = 7.9 % (accept 7.9–8 %) [1]

26 ÷ 380 × 100 = 6.8 % (accept 6.8–7 %) [1]

only a small proportion of the biomass at one level is present at the next, and the proportion is not the same at each step, so no fixed percentage rule should be quoted [1]

⚠ If you missed marks here: Show both divisions. Divide the higher level by the lower one — the other way round gives a number greater than 100, which should tell you at once that something is wrong. The comment mark is the one people skip, and it is deliberately set up by giving you two transfers rather than one.

1 mark — 7.9 % calculated with working 1 mark — 6.8 % calculated with working 1 mark — comment that the proportion is small and not constant

(b) [4]

Using Table 2.1, describe and explain the shape of a pyramid of numbers and the shape of a pyramid of biomass for this grassland. State one advantage of the pyramid of biomass.

Model Answer — 2(b)

the pyramid of numbers would be the expected pyramid shape, narrowing at each level, because each producer here is a small individual grass plant and there are very many of them [1]

the pyramid of biomass would also narrow at each level, from 4800 to 380 to 26 kg per hectare [1]

the advantage of the biomass pyramid is that it takes account of the size of the organisms [1]

so it is not distorted when a single very large producer, such as one oak tree, supports many small consumers — which is when a pyramid of numbers becomes inverted [1]

⚠ If you missed marks here: The question asks you to describe and explain, so quote figures from the table as well as naming the shapes. The advantage mark needs the reason, not just the statement: say what a pyramid of numbers ignores, and give the case where that matters.

1 mark — describes the pyramid of numbers as the expected shape, with a reason 1 mark — describes the pyramid of biomass as the expected shape, quoting figures 1 mark — advantage: biomass takes organism size into account 1 mark — gives the case where a pyramid of numbers becomes inverted

Describe how you would draw an ecological pyramid to scale.

Model Answer — 2(c)

producers at the bottom, one horizontal bar for each trophic level in order, and the bars centred on a vertical line [1]

the width of each bar proportional to the quantity, using one chosen scale throughout, with all bars the same height, and every bar labelled [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 inverted shape — the odd shape is the information. If the largest value makes a true scale impossible, write “not to scale” on the drawing and give the figures.

1 mark — producers at the bottom, one bar per level, in trophic order 1 mark — width proportional to quantity, equal heights, labelled

(d) [2]

The secondary consumers in this grassland contain 26 kg of dry mass per hectare. Suggest two reasons why this figure is so much lower than the 380 kg in the primary consumers.

Model Answer — 2(d)

energy is released in respiration and transferred to the surroundings as heat at each level, and heat cannot be used to build tissue [1]

any one of: not all of the prey is eaten (bone, hair, horn); not all of what is eaten is digested and absorbed, so energy leaves in the faeces; energy is lost in excretion as urea in the urine [1]

⚠ If you missed marks here: Two different reasons are needed; saying “energy is needed for movement” and “energy is needed to keep warm” is respiration twice and will be credited once. Never write that energy is destroyed or used up: it is transferred into a form nothing can feed on.

1 mark — heat lost through respiration 1 mark — a second distinct loss: faeces, excretion or parts not eaten

Question 3 — Two Populations That Rise and Fall Together

Total: 11 marks

Fig. 3.1 shows the numbers of two species of mammal living in the same habitat, counted every year for forty years. One species feeds on the other.

(a) [3]

State which curve, P or Q, shows the predator. Give two pieces of evidence from Fig. 3.1 to support your answer.

Model Answer — 3(a)

the predator is Q [1]

Q is present in far smaller numbers than P at every point — a maximum of about 13 500 compared with about 95 000 [1]

each peak in Q comes after the corresponding peak in P, by about 2–3 years, which is what you expect if Q depends on P for food [1]

⚠ If you missed marks here: Two independent pieces of evidence are wanted, and the second one — the lag — is the stronger. Quote figures or times from the graph rather than describing the shapes in words; a data question that contains no data caps itself. There is far too little energy at the top of a chain for a predator to outnumber its prey, which is why the first piece of evidence works at all.

1 mark — Q identified as the predator 1 mark — evidence from the numbers, with figures quoted 1 mark — evidence from the lag between the peaks, with a time quoted

(b) [3]

Explain why the two populations rise and fall in this repeating pattern.

Model Answer — 3(b)

when P is plentiful there is more food for Q, so more of Q survive and reproduce and the number of Q rises [1]

the increased number of Q then eat more of P, so the number of P falls [1]

with less food, fewer Q survive and reproduce, so Q falls in turn, which allows P to recover and the cycle repeats [1]

⚠ If you missed marks here: Write the cycle as a sequence of causes, one arrow at a time, rather than as a description of the graph. The order matters: prey first, then predator, then prey again. Do not say that the predator “controls” the prey and stop there — the mark is for the feedback going round.

1 mark — more prey means more food, so predator numbers rise 1 mark — more predators eat more prey, so prey numbers fall 1 mark — fewer prey means fewer predators, allowing prey to recover

Predation is one factor affecting the rate of population growth. Name the other three factors given in the syllabus, and explain what all four have in common.

Model Answer — 3(c)

food supply and competition [1]

disease [1]

all four act more strongly as the population becomes larger and more crowded — less food per individual, fiercer competition, more predators attracted, and faster spread of a pathogen between hosts that are close together [1]

⚠ If you missed marks here: Stick to the four named factors; adding temperature or oxygen wastes time and earns nothing. The third mark rewards understanding rather than listing, and it is the link to the sigmoid curve: because every factor intensifies as numbers rise, a population cannot grow indefinitely.

1 mark — food supply and competition named 1 mark — disease named 1 mark — explains that all four act more strongly at higher population density

(d) [2]

Between year 27 and year 32 the number of P rises from about 17 000 to about 95 000. Calculate the mean rate of increase per year, and suggest one reason why the real population would not go on rising at that rate.

Model Answer — 3(d)

(95 000 − 17 000) ÷ 5 = 78 000 ÷ 5 = 15 600 animals per year (accept a close value read from the graph) [1]

any one of: the food supply would run short; competition would intensify; the number of predators would rise; disease would spread more easily in a dense population [1]

⚠ If you missed marks here: Rate is change divided by time. Dividing the final value by the time is the standard slip and here would give 19 000. Give the unit. For the second mark, name one of the four factors rather than saying vaguely that the population “reaches its limit”.

1 mark — rate calculated correctly with working and unit 1 mark — names one of the four factors as a reason growth stops

Question 4 — Nitrogen From Air to Protein and Back

Total: 12 marks

Fig. 4.1 shows the nitrogen cycle. Each lettered arrow represents a process.

(a) [4]

Name the processes P, R, S and V on Fig. 4.1.

Model Answer — 4(a)

P — nitrogen fixation [1]

R — nitrification [1]

S — absorption of nitrate ions by the roots [1]

V — denitrification [1]

⚠ If you missed marks here: Check the direction of each arrow before naming it. P runs from the air into the soil and V runs from the soil back into the air, and swapping those two is the error that costs most marks in this sub-topic. The genus names of the bacteria are not required at IGCSE, so do not spend time on them.

1 mark — P nitrogen fixation 1 mark — R nitrification 1 mark — S absorption of nitrate by roots 1 mark — V denitrification

(b) [3]

State the four roles of microorganisms in the nitrogen cycle named by the syllabus, and describe what each one does to nitrogen.

Model Answer — 4(b)

nitrogen fixation — nitrogen gas from the air is converted into compounds plants can use, by bacteria free in the soil and in root nodules [1]

decomposition — protein in dead organisms, faeces and urine is broken down to ammonium ions; and nitrification — ammonium ions are converted to nitrite and then to nitrate ions [1]

denitrification — nitrate ions in the soil are converted back into nitrogen gas, which is lost to the air; this happens where oxygen is short, as in a waterlogged soil [1]

⚠ If you missed marks here: Four names and four transformations. Learn each one as “what goes in and what comes out” rather than as a word, because that is what makes the four impossible to confuse. Note that lightning also fixes nitrogen but is not a microorganism, so it does not belong in this answer.

1 mark — nitrogen fixation described correctly 1 mark — decomposition and nitrification described correctly 1 mark — denitrification described correctly, including the conditions

Describe how a nitrogen atom in a nitrate ion in the soil becomes part of a protein molecule in a lion, naming the processes involved.

Model Answer — 4(c)

the nitrate ion is absorbed by the roots of a plant, by active transport against the concentration gradient [1]

the plant uses it to make amino acids and joins those into plant proteins [1]

a herbivore eats the plant and digests the protein to amino acids, which it absorbs and builds into its own protein; a lion then eats the herbivore and repeats the process [1]

⚠ If you missed marks here: Naming active transport is the detail that separates a full answer from a general one, and it explains why a waterlogged, oxygen-poor soil is doubly bad for a crop. Do not write that an animal absorbs protein directly — protein is always digested to amino acids first.

1 mark — nitrate absorbed by roots, by active transport 1 mark — plant makes amino acids and then proteins 1 mark — animal digests protein to amino acids and builds its own protein

(d) [2]

Explain how deamination in an animal returns nitrogen to the soil.

Model Answer — 4(d)

excess amino acids cannot be stored, so they are broken down in the liver; the nitrogen-containing part is removed and converted to urea [1]

the urea is excreted in the urine and reaches the soil, where decomposers break it down to release ammonium ions, which are then nitrified to nitrate [1]

⚠ If you missed marks here: Two organs and two processes: liver for deamination, kidney for excretion. Saying that deamination happens in the kidney is the classic near-miss, and it costs the mark. Cambridge names deamination in this sub-topic explicitly, so it is fair game in an ecology question.

1 mark — deamination in the liver produces urea from excess amino acids 1 mark — urea is excreted and decomposed in the soil to ammonium

Question 5 — Carbon Goes Round, Energy Does Not

Total: 11 marks

Fig. 5.1 shows the carbon cycle with lettered arrows.

(a) [3]

Name the six processes that Cambridge includes in the carbon cycle, and state which of them is the only one that removes carbon dioxide from the air.

Model Answer — 5(a)

photosynthesis, respiration, feeding [1]

decomposition, formation of fossil fuels, combustion [1]

only photosynthesis removes carbon dioxide from the air [1]

⚠ If you missed marks here: Knowing that the list has exactly six items tells you how many points a “describe the carbon cycle” question expects. Do not add nitrification, denitrification or transpiration: they belong to other parts of the syllabus and will not be credited here.

1 mark — three of the six processes named correctly 1 mark — the other three named correctly 1 mark — photosynthesis identified as the only process removing carbon dioxide

(b) [3]

A field of wheat receives 1 000 000 kJ m⁻² of light energy in a year and stores 20 000 kJ m⁻² as new plant material. Calculate the percentage stored, and give two reasons why the figure is so low.

Model Answer — 5(b)

20 000 ÷ 1 000 000 × 100 = 2 % [1]

much of the light misses the chloroplasts — it is reflected, passes through the leaf or falls on bare soil between the plants [1]

some of the light is of a wavelength chlorophyll absorbs poorly (such as green), and some of the glucose the plant makes is used in its own respiration [1]

⚠ If you missed marks here: Two different reasons are needed. Reject anything that happens after the material has been made — for example “some is eaten by herbivores” — because the question runs from light arriving to material stored, and that lies outside it.

1 mark — 2 % calculated with working 1 mark — light missing the chloroplasts, reflected or transmitted 1 mark — unusable wavelengths, or the plant respiring some of the glucose

The letter Q on Fig. 5.1 appears on three arrows. Name the process, explain why it appears three times, and state what happens to the energy released by it.

Model Answer — 5(c)

Q is respiration [1]

plants, animals and decomposers all respire, so all three return carbon dioxide to the air [1]

almost all of the energy released is transferred to the environment as heat, and it cannot be passed on to another organism or recovered [1]

⚠ If you missed marks here: Students routinely remember that animals respire and forget that plants respire 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 — decomposition on its own does not release a gas.

1 mark — Q identified as respiration 1 mark — plants, animals and decomposers all respire 1 mark — energy released is transferred to the environment as heat

(d) [2]

Using Fig. 5.1, explain why an ecosystem needs a continuous supply of energy but does not need a continuous supply of carbon.

Model Answer — 5(d)

the carbon atoms are recycled: every arrow on the diagram returns carbon to somewhere it has been before, so the same atoms are used repeatedly [1]

energy is not recycled: it enters once as light, and at every respiration arrow some is transferred to the environment as heat, which no organism can use, so it must be resupplied by the Sun [1]

⚠ If you missed marks here: Say what is recycled as well as what is not; an answer that only says “energy is not recycled” leaves half the question untouched. This is the most heavily penalised misconception in the topic, and the phrase “decomposers return energy to the soil” should never appear in your writing.

1 mark — carbon atoms are recycled and used repeatedly 1 mark — energy leaves as heat and cannot be reused, so it must be resupplied

Question 6 — From a Flask to a Field

Total: 12 marks

Fig. 6.1 shows the growth of a yeast population in a sealed flask of nutrient broth.

(a) [3]

Define the term population. Name the four phases W, X, Y and Z, and state in which phase the population is growing most rapidly.

Model Answer — 6(a)

a population is a group of organisms of one species, living in the same area, at the same time [1]

W lag, X exponential (or log), Y stationary, Z death [1]

growth is fastest in phase X [1]

⚠ If you missed marks here: All three conditions are needed in the definition. And read the last part carefully: the population is largest in Y but growing fastest in X, and examiners swap those two words on purpose.

1 mark — population defined with all three conditions 1 mark — all four phases named 1 mark — phase X identified as the fastest growth

(b) [4]

Explain, in terms of limiting factors, why the curve rises steeply in phase X and then levels off in phase Y.

Model Answer — 6(b)

in X nothing is limiting: food, space and oxygen are plentiful and no toxic waste has built up [1]

so the population doubles in a fixed time, because every organism produced can itself divide, which is why the line steepens rather than running straight [1]

in Y the rate of reproduction equals the death rate, so the number stays constant [1]

because food is running short, waste products have accumulated and competition is intense; whichever runs short first is the limiting factor and sets the maximum population the flask can support [1]

⚠ If you missed marks here: The sentence about the two equal rates is the first marking point on any stationary-phase question, so write it before you write anything about food. A flat line means two large rates cancelling, not two rates of zero, and “the yeast have stopped reproducing” is the answer most students give.

1 mark — nothing is limiting in phase X 1 mark — population doubles in a fixed time / birth rate far exceeds death rate 1 mark — in phase Y reproduction rate equals death rate 1 mark — names limiting factors: food short, waste accumulating, competition

Calculate the mean rate of increase between 8 hours and 14 hours, giving the unit. Then state, with a reason, whether the rate between 20 and 24 hours is higher or lower.

Model Answer — 6(c)

(2600 − 240) ÷ 6 = 2360 ÷ 6 = 393 cells per cm³ per hour (accept 390–400) [1]

the unit given correctly as cells per cm³ per hour [1]

between 20 and 24 hours the rate is much lower, approximately zero, because the culture is in the stationary phase and the number is not changing [1]

⚠ If you missed marks here: Rate is change divided by time; dividing 2600 by 6 is the standard slip. A naked number regularly loses a mark on a rate question, so always give the unit. For the comparison, name the phase as well as describing the graph — the name is what the examiner is listening for.

1 mark — correct rate calculated with working 1 mark — unit given correctly 1 mark — identifies the much lower, near-zero rate and names the stationary phase

(d) [2]

Rabbits are released on an island that has plenty of grass and no predators. Suggest, with reasons, what shape the population curve would have over the following twenty years.

Model Answer — 6(d)

it would be a sigmoid curve: rising slowly at first, then steeply, then levelling off [1]

removing predators removes only one of the four factors; food supply, competition and disease all still act and all intensify as numbers rise, so growth must eventually level off [1]

⚠ If you missed marks here: The trap is to answer that the population will rise for ever because nothing eats the rabbits. Name the factors that still operate. A real island population often overshoots and then falls, because heavy grazing damages the food supply itself, and saying so is credited.

1 mark — predicts a sigmoid curve levelling off 1 mark — explains that the other three factors still act as numbers rise

Question 7 — Feeding People From the Same Land

Total: 11 marks

(a) [3]

A farmer has 10 000 kJ of energy in a crop of maize. He can sell the maize for people to eat, or feed it to chickens and sell the meat. Assume that 10 % of the energy is transferred at each step. Calculate the energy that reaches people by each route, and state which feeds more people.

Model Answer — 7(a)

eating the maize directly: 10 000 × 10 % = 1000 kJ reaches human tissue [1]

via the chickens: 10 000 × 10 % = 1000 kJ in the chickens, then × 10 % = 100 kJ in the people [1]

eating the maize directly feeds more people, by a factor of ten [1]

⚠ If you missed marks here: Note that “10 % is transferred” means 90 % is lost. Subtracting 10 % instead of keeping it is the commonest error on this calculation and it turns the whole argument upside down. Show each step separately so a slip costs one mark rather than all three.

1 mark — 1000 kJ by the direct route 1 mark — 100 kJ by the livestock route 1 mark — states that the direct route feeds more people, with the comparison

(b) [4]

Explain, naming the losses, why so much energy is lost at the extra step in the livestock route, and give two reasons why livestock are nevertheless farmed.

Model Answer — 7(b)

energy is released in respiration by the chickens and transferred to the surroundings as heat; birds and mammals lose a great deal this way because they maintain a constant body temperature [1]

energy is lost in undigested material in the faeces and in excretion [1]

parts of the animal are not eaten — bone, feathers, gut contents [1]

any two of: some land is too poor, steep or dry to grow crops but will support animals; meat, milk and eggs are concentrated sources of protein and of some vitamins and minerals; animals can be fed on crop by-products people cannot eat [1]

⚠ If you missed marks here: Naming the losses is what turns a one-mark answer into a three-mark one, so list them separately rather than writing “energy is lost” three times. The final mark is the “discuss” mark: whenever an argument is put to you, give the other side of it as well.

1 mark — respiration and heat loss named 1 mark — faeces and excretion named 1 mark — parts not eaten named 1 mark — two valid reasons for farming livestock

Explain why food chains rarely contain more than five trophic levels, and why a pyramid of energy for such a chain can never be inverted.

Model Answer — 7(c)

so much energy is lost at each transfer — typically around nine tenths — that very little remains at the top [1]

there would be too little energy to support a viable population of a further predator, which would use more energy hunting than it gained [1]

a pyramid of energy cannot be inverted because energy is lost at every transfer [1]

so a trophic level can never contain more energy than the level below it supplied; and because energy is measured over a period of time, the rate of production is included, unlike a pyramid of biomass, which is a snapshot and can be inverted [1]

⚠ If you missed marks here: Avoid the absolute version, “there is no energy left”: there is some, and the mark is for saying that there is not enough. Do not extend the guarantee to a pyramid of biomass — biomass is a snapshot at one moment and genuinely can be inverted, most famously for ocean plankton.

1 mark — large energy loss at each transfer 1 mark — too little energy to support a viable further population 1 mark — energy lost at every transfer, so a level cannot exceed the one below 1 mark — contrast with the biomass pyramid as a snapshot that can be inverted

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