⚡ Challenge Paper Preparation

Challenge Prep: Organisms and their Environment

IGCSE Biology 0610 — Topic 19 — Extended

This topic looks like the easy one and it is not. Most of the marks are for reading and drawing, not recall: constructing a chain from a paragraph of prose, tracing a knock-on effect through a web in both directions, drawing a pyramid to scale, reading a growth curve. Two sentences carry more marks than anything else. An arrow means is eaten by and shows the direction of energy transfer — reversing one is the commonest lost mark in the topic. And nutrients are recycled; energy is not. Twelve traps, six data-led walkthroughs, six lookalike pairs, a concept map and ten full practice questions below, every one aimed at a place where a sensible-sounding sentence earns nothing at all.

⚠️ Common Traps & Misconceptions

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Twelve traps that cost marks on Topic 19 challenge papers, spread across all five sub-topics. Every one is an answer that sounds right and that mark schemes refuse.

⚠️ TRAP

Trap 1: Drawing the arrow from the predator to its prey

The TrapYou think “the fox eats the rabbit” and your hand draws fox → rabbit. The chain reads sensibly in your head and is wrong on the page. Because the drawing looks tidy, nothing warns you, and the mark is gone silently.

The TruthThe arrow means is eaten by, and it points in the direction in which energy is transferred. grass → rabbit → fox. Every chain begins with a producer, because that is where the energy enters, and every arrow runs left to right away from it.

Why It MattersThis is the single most frequently lost mark in Topic 19, and it can cost several marks at once, because a reversed chain makes every trophic level in the following parts wrong too. Before you hand in any chain, read it aloud in your head with the words “is eaten by” on each arrow. If any part is ridiculous, an arrow is round the wrong way.

Example Question“Construct a food chain using four of the organisms in Fig. 2.1 and state the trophic level of the third organism in your chain. [3]”

⚠️ TRAP

Trap 2: Saying that energy is recycled

The Trap“Decomposers break down the dead organisms and return the energy to the soil, where plants take it up again.” It is a neat sentence, it feels like a cycle, and it is the error this whole topic is designed to expose.

The TruthNutrients cycle; energy flows. Carbon and nitrogen atoms are used over and over for ever. Energy enters once as light from the Sun and leaves as heat transferred to the environment, mostly from respiration. Decomposers return mineral ions, not energy — and decomposers respire too, so the energy they obtain also leaves as heat.

Why It MattersExaminers ask for the difference between the carbon cycle and energy flow specifically to see whether you have this straight. It also explains why an ecosystem needs sunlight every day but needs no delivery of new carbon — a favourite two-mark question.

Example Question“Explain why an ecosystem requires a continuous input of energy but not a continuous input of carbon. [3]”

⚠️ TRAP

Trap 3: Writing that producers “make energy”

The Trap“Producers make energy using sunlight”, or the closely related “plants get their food from the soil”. Both sound like things a textbook might say. Neither will score.

The TruthA producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. It transfers light energy into chemical energy stored in organic nutrients. Roots absorb water and mineral ions; mineral ions are needed to make proteins and chlorophyll and they contain no usable energy at all.

Why It MattersThe definition of a producer is a one-mark recall question that appears constantly, and it is marked phrase by phrase. Getting “organic nutrients” instead of “energy” or “food” is worth doing precisely.

Example Question“Define the term producer. [2]”

⚠️ TRAP

Trap 4: Answering a knock-on question in one direction only

The Trap“If the rabbits are removed, there will be fewer foxes.” True, and worth one mark out of the four on offer. The student has looked up the web, found the predator, and stopped.

The TruthTrace three directions. UP: whatever ate it has lost a food source, so its numbers fall or it switches prey. DOWN: whatever it ate is no longer being eaten as much, so those numbers rise. SIDEWAYS: its predators now eat their other prey more heavily, so those numbers fall; and anything else that shared its food now has more, so those rise.

Why It MattersFour-mark web questions are built with one mark per direction plus one for a reason. The sideways step is the one almost everybody misses, and it is often the mark that separates an A from an A*.

Example Question“All of the shrews are removed from the habitat shown in Fig. 3.1. Suggest, with reasons, the effect on three other organisms in the web. [4]”

⚠️ TRAP

Trap 5: Believing that 10 % is a rule

The TrapA question gives you 20 000 kJ and 1600 kJ and you write “10 %, because 10 % is transferred between trophic levels”. The correct answer was 8 %, and it was sitting in front of you.

The TruthEnergy transfer is often around a tenth, but it varies enormously with the organisms involved and it is not a rule you may quote instead of doing the arithmetic. Always divide the numbers you are given. Efficiency = energy in the higher level ÷ energy in the lower level × 100.

Why It MattersData questions are the AO2 marks, and this is the fastest way to lose one. There is also a related trap: “10 % is transferred” means 90 % is lost, so 10 000 kJ becomes 1000 kJ, not 9000 kJ.

Example Question“Use the data in Table 4.1 to calculate the percentage of energy transferred from the primary to the secondary consumers. Show your working. [2]”

⚠️ TRAP

Trap 6: “A pyramid of biomass can never be inverted”

The TrapYou are told that biomass fixes the problem with numbers, and you over-generalise it into a rule. Then a question shows you an inverted biomass pyramid for open ocean plankton and you conclude the data must be wrong.

The TruthOnly a pyramid of energy can never be inverted, because energy is lost at every transfer so a level can never hold more than the level below supplied. A pyramid of biomass is a snapshot at one moment: microscopic algae reproduce and are grazed so fast that very little is standing there at any instant, even though they produce far more material over a year.

Why It Matters“Explain why this pyramid of biomass is inverted” is a standard Supplement question, and the word that earns the mark is snapshot or at one point in time. It is also the reason a pyramid of energy is worth the trouble of measuring.

Example Question“The pyramid of biomass for this marine habitat is inverted. Explain why, and suggest what measurement would give a pyramid of the expected shape. [3]”

⚠️ TRAP

Trap 7: Explaining an inverted pyramid of numbers with only half the answer

The Trap“Because the oak tree is very big.” That is the instance without the rule, and it is half of a two-mark answer. The mirror error, “because there are more insects than trees”, is a description of the diagram dressed up as an explanation.

The TruthBoth halves: a pyramid of numbers takes no account of the size of the organisms, and here one very large producer supports very many small primary consumers. The same shape appears from the other end when small organisms feed on one large individual — aphids on a rose bush, for instance.

Why It MattersThe oak tree is the standard example and it appears every year in some form. Having the two-part sentence ready means the question takes twenty seconds.

Example Question“Explain why the pyramid of numbers in Fig. 5.2 is not the expected shape, and state one advantage of drawing a pyramid of biomass instead. [3]”

⚠️ TRAP

Trap 8: “There is no energy left” to explain short food chains

The Trap“Food chains have fewer than five levels because there is no energy left at the top.” It is too absolute, and mark schemes reject the absolute version.

The TruthThere is energy left — there is simply too little to support another trophic level. Follow the arithmetic: 20 000 → 1600 → 160 → 24 kJ m⁻² y⁻¹. A fifth-level carnivore would have to hunt over an enormous area to stay alive and would spend more energy hunting than it gained.

Why It MattersThe mark is for the consequence: not enough energy to support a viable population. And beware the alternative wrong answer, “the animals would be too big”, which is neither the reason nor reliably true.

Example Question“Explain, in terms of energy, why food chains usually contain fewer than five trophic levels. [3]”

⚠️ TRAP

Trap 9: Confusing nitrogen fixation with denitrification

The Trap“Nitrogen-fixing bacteria convert nitrates in the soil into nitrogen gas.” The words are all correct and the process has been run backwards, which turns a right-sounding answer into a wrong one.

The TruthNitrogen fixation is the way in: nitrogen gas from the air becomes a compound plants can use, done by nitrogen-fixing bacteria (in root nodules and in the soil) and by lightning. Denitrification is the way out: nitrate ions in the soil become nitrogen gas, done by denitrifying bacteria in waterlogged soil where oxygen is short.

Why It MattersCambridge names four microbial jobs — decomposition, nitrification, nitrogen fixation and denitrification — and expects you to identify each from a description. Two of the four run in opposite directions, so getting them the wrong way round loses both.

Example Question“Name processes X and Y on Fig. 6.1 and state the type of bacteria responsible for each. [4]”

⚠️ TRAP

Trap 10: Adding processes to the carbon cycle that are not on the list

The TrapYou describe the carbon cycle and reach for extra ideas to fill the marks. Some are from another topic entirely; some are not carbon at all.

The TruthThe carbon cycle is examinable limited to six processes: photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion. Learn the six and where each puts carbon. Only one removes carbon dioxide from the air — photosynthesis. Three put it back — respiration, combustion and decomposition (through the decomposers respiring).

Why It Matters“Describe the carbon cycle [6]” is marked against exactly those six. Knowing the list tells you how many points to make and stops you writing about anything else.

Example Question“Describe how a carbon atom in a molecule of glucose in a leaf could return to the air as carbon dioxide by two different routes. [4]”

⚠️ TRAP

Trap 11: “In the stationary phase nothing is reproducing”

The TrapA flat line looks like nothing happening, so the answer becomes “the organisms have stopped reproducing because there is no food left”. Both halves are wrong.

The TruthIn the stationary phase the rate of reproduction equals the death rate. Large numbers are still being born and large numbers are still dying; the two rates cancel. The causes are that food is running short, waste products have built up, competition is intense and space has run out — not that the food is gone, which is the death phase.

Why It Matters“Explain why the population remains constant” is worth two or three marks and the sentence about the two equal rates is the first of them. Without it, the rest of the answer cannot be credited.

Example Question“Explain why the number of yeast cells remains constant between 18 and 26 hours. [3]”

⚠️ TRAP

Trap 12: Explaining an invasion with the word “stronger”

The Trap“The introduced species increased because it was stronger than the native species.” It sounds like an explanation and contains no biology at all.

The TruthThree real reasons, and you usually need two: it has no natural predators in the new habitat, so nothing controls its numbers; there is plenty of food and space at first, so little competition; and the native species have no defence or resistance against a predator or pathogen they have never met. Then say what it does: it competes with native species for food, space or light, or eats them.

Why It Matters“Stronger”, “fitter” and “better” are on every examiner’s list of words that earn nothing. The same rule applies to overharvesting questions: say removed faster than the population can reproduce, not “too many were taken”.

Example Question“Suggest why the introduced species increased rapidly, and describe two effects it would have on the native food web. [4]”

🔍 Step-by-Step Walkthroughs

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Six challenge-level questions with real data, worked through in the order you should actually think about them. Try each part before revealing the next step.

Walkthrough 1 — A Food Web With Nothing Named

Key: A algae · B pondweed · C water flea · D mayfly nymph · E pond snail · F dragonfly nymph · G stickleback · H perch · J heron

(a) State the letters of the two producers, and explain how you can tell. [2] (b) Construct the longest food chain in the web and state the trophic level of G in it. [3] (c) J occupies two different trophic levels in this web. Explain, using two chains. [2] (d) A disease removes all of organism E. Suggest, with reasons, the effect on B, on G and on J. [4]

A producer has no arrow coming into it

You do not need to know what the organisms are. Look at the arrows: A and B have arrows leaving them and none arriving. Nothing eats energy into them, which means they must be making their own organic nutrients — they are the producers. (The key confirms it: algae and pondweed.) Everything else in the web has at least one incoming arrow, so everything else is a consumer. This test works on any web, in any habitat, even if every organism is unfamiliar.

Start at a producer and take the longest route to a top predator

Start at A. A is eaten by C; C is eaten by G; G is eaten by H; H is eaten by J. That is A → C → G → H → J — five organisms, so five trophic levels, which is as long as this web gets. Count organisms, not arrows: A is level 1, C level 2, G level 3. So in this chain G is a secondary consumer at the third trophic level. The commonest slip here is counting the four arrows and calling G tertiary.

An organism can sit at more than one trophic level

Find two chains ending at J that have different lengths. In A → C → G → J, J eats G, and G eats a primary consumer — so G is secondary and J is a tertiary consumer. In A → C → G → H → J, J eats H instead, and H eats G — so H is tertiary and J is a quaternary consumer. Both answers are correct, and a full-mark answer names the chain each one belongs to.

Note what this does not apply to. G eats C, D and E, and all three of those eat producers directly, so G is a secondary consumer in every chain in this web. Before you claim that an organism sits at two levels, check the direction of every arrow into it — the claim is only true if its prey sit at different levels themselves.

Up, down and sideways — one sentence each, each with a reason

B (down): E is a primary consumer that eats B, so removing E means B is grazed less and increases. G (up): G eats E, so G has lost a food source and its numbers may fall — but G also eats C and D, so it will switch rather than starve. J (sideways and above): J does not eat E directly, so the effect reaches it through G and H. If G falls, J has less food and may fall too; if instead G switches to C and D, then C and D fall and the effect passes down rather than up. A full-mark answer says which chain the effect travels along, and says that the direction of change in J is uncertain because two opposing effects act on it.

Walkthrough 2 — An Energy Budget That Has to Balance

A young bullock is kept in a pen for 100 days and everything is measured. Over that period it eats grass containing 1000 MJ of energy.

Measurement	Energy / MJ
energy in the food 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) Calculate the energy stored as new body tissue. [1] (b) Calculate the percentage of the energy eaten that is available to an animal that eats the bullock. [2] (c) Explain why the figure for faeces is so large in a grass-eating animal. [2] (d) A pig fed on grain shows a much smaller faeces figure and a much larger figure for new tissue. Suggest why. [2]

Everything eaten has to appear somewhere

1000 − 600 − 280 − 40 = 80 MJ. Energy is never destroyed, so a budget question always balances, and if yours does not you have missed a row. Write the subtraction out; the working carries a mark if you slip.

A predator eats a body, not a meal

The tempting wrong answer is 400 MJ — the energy the bullock absorbed. But absorbed energy that has been respired is gone as heat, and energy in urine has left the body. What a predator eats is the new tissue. So 80 ÷ 1000 × 100 = 8 %. Notice that this is not 10 %, which is exactly why you must divide rather than recite.

Grass is mostly cellulose

Plant cell walls are made of cellulose, and a mammal produces no enzyme that digests it. A large proportion of what the bullock swallows therefore cannot be digested or absorbed, passes through the alimentary canal and leaves in the faeces, taking its energy with it. That energy is not wasted from the ecosystem’s point of view — decomposers use it — but it never enters the bullock’s body, so it is not available to the next trophic level.

The change in diet changes one term in the budget, and the rest follows

Grain is mostly starch, which mammals digest easily, and it contains far less cellulose than grass. So a much higher proportion is digested and absorbed, the faeces figure falls, and more energy is available inside the body. With more absorbed and respiration roughly similar, more is left to be stored as new tissue, so the transfer to the next level is more efficient. A second acceptable point: a pig is smaller and often less active than a bullock in a pen, so it may lose less energy as heat per unit of food.

Walkthrough 3 — Two Pyramids of the Same Community

Both diagrams describe the same oak woodland chain in the same year. (a) Explain why the pyramid of numbers has this shape. [2] (b) Explain why the pyramid of biomass does not. [2] (c) Calculate the percentage of the oak tree’s biomass present in the insects. [1] (d) State one advantage and one disadvantage of drawing a pyramid of energy for this woodland instead. [2]

Two halves, one mark each

The rule: a pyramid of numbers takes no account of the size of the organisms — one oak tree is counted as one, exactly like one aphid. The instance: one very large producer supports very many small primary consumers, so the base bar is the narrowest in the diagram. Give both halves. “The tree is big” is the instance without the rule and scores one.

Biomass counts size, so the tree becomes the base

A pyramid of biomass measures the dry mass of living material at each level. The single oak has an enormous dry mass — 5000 kg here — while the three thousand insects together weigh 80 kg. Measuring mass rather than counting individuals therefore puts the producer where you expect it, and the pyramid narrows all the way up. That is the standard answer to “state an advantage of a pyramid of biomass over a pyramid of numbers”.

Read the two figures off the diagram and divide

80 ÷ 5000 × 100 = 1.6 %. Two things to watch. Both figures must be in the same unit — here both are kilograms, but a question may give one in grams to see whether you notice. And divide the higher level by the lower: 5000 ÷ 80 gives 62.5, which is not a percentage of anything sensible.

Best biologically, worst practically

Advantage: a pyramid of energy measures the energy passing through each level over a period of time, so it takes account of the rate at which material is produced and it can never be inverted. It also lets you calculate the efficiency of each transfer. Disadvantage: the data are extremely difficult and slow to collect, because energy has to be measured over a whole year rather than sampled on one day.

Walkthrough 4 — A Carbon Cycle With The Labels Removed

(a) Name the process shown by the arrow labelled photosynthesis on the diagram, and state why only one arrow can point away from the carbon dioxide box. [2] (b) A student says decomposition releases carbon dioxide into the air. Correct and complete this statement. [2] (c) Explain why the carbon in coal was removed from the cycle for millions of years. [2] (d) Describe the fastest possible route by which a carbon atom fixed in a leaf this morning could be back in the air by tonight. [2]

Only photosynthesis removes carbon dioxide from the air

Of the six named processes, photosynthesis alone takes carbon dioxide out of the air and fixes it into organic compounds. Three put it back: respiration, combustion and decomposition (indirectly). So on any blank carbon-cycle diagram, find the single arrow leaving the air box — it must be photosynthesis — and everything else falls into place around it.

It is the decomposers respiring, not the decomposition itself

Decomposition transfers the carbon compounds from the dead material into the decomposers. The carbon dioxide reaches the air when those decomposers respire. That is why a full answer names two processes: decomposition and respiration. Writing “decomposition releases carbon dioxide” alone is worth one mark of two.

No decomposition, no return

The organisms died in conditions where decomposers could not break them down — deep water or waterlogged ground with little or no oxygen. Their carbon compounds were buried, compressed and heated over millions of years to form coal, oil and gas, and the only process that can return that carbon to the air is combustion. Until something burns it, it sits outside the cycle.

The plant respires the glucose itself

Photosynthesis fixes the carbon into glucose; the plant respires that glucose and releases the carbon dioxide back to the air within hours. Plants respire day and night, so no other organism needs to be involved at all. Every other route — being eaten first, or dying and decomposing — takes longer, and the fossil-fuel route takes millions of years.

Walkthrough 5 — A Growth Curve With Lettered Phases

(a) Name the phases labelled W, X, Y and Z. [2] (b) Calculate the mean rate of increase in phase X between 10 and 16 hours. [2] (c) Explain, in terms of limiting factors, why the curve levels off in phase Y. [3] (d) The experiment is repeated but fresh broth is added continuously and waste removed. Predict how the curve would differ after 18 hours, and explain. [2]

Flat, steep, flat, falling

W = lag phase (flat at the start), X = exponential or log phase (rising steeply and getting steeper), Y = stationary phase (level at the maximum), Z = death phase (falling). Both names for X are accepted; write “exponential (log)” and you cannot be wrong.

Read both points carefully before dividing

At 10 hours the count is 600 cells per cm³; at 16 hours it is 3900. Rate = (3900 − 600) ÷ (16 − 10) = 3300 ÷ 6 = 550 cells per cm³ per hour. Two marks: one for the correct method, one for the answer with its unit. The classic error is dividing 3900 by 6 — you need the change, not the final value.

Two equal rates, and the factor that is now limiting

Start with the balance: the rate of reproduction now equals the death rate, so the number stays constant. Then give the causes: the food supply is running short, toxic waste products have accumulated, and competition for the remaining nutrients and space is intense. Whichever of those runs short first is the limiting factor — the same idea you met with limiting factors in photosynthesis, where the factor in shortest supply sets the rate and increasing anything else changes nothing.

Remove the limiting factors and the growth continues

Adding fresh broth continuously keeps the food supply high; removing waste prevents toxic products accumulating. Those were the two things that ended the exponential phase, so the population would continue to rise for much longer and there would be no death phase while the supply lasted. It would still level off eventually, because space in the flask is finite and competition for it would become the new limiting factor. Saying that last part is what turns a good answer into a full one.

Walkthrough 6 — Two Fields and a Nitrogen Problem

A farmer compares two fields of the same size, given the same fertiliser and the same rainfall. Field A is ploughed and has drains beneath it. Field B is flat and holds standing water for weeks after rain. After three years the soil is tested.

	Field A	Field B
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) Calculate how many times greater the nitrate concentration is in field A. [1] (b) Explain the difference in nitrate, naming the bacteria involved. [4] (c) Explain why field B has more ammonium than field A even though it has less nitrate. [2] (d) Suggest why the yield in field B is only about half that of field A. [2]

Do the arithmetic before you start writing prose

42 ÷ 9 = 4.7 times greater (2 significant figures). Ten seconds, one mark, and it cannot be argued with. Calculation parts are the cheapest marks on any data question — never leave them until the end.

Everything else is controlled, so the answer is oxygen

Same size, same fertiliser, same rainfall — the question has controlled those deliberately, which is the examiner pointing at the one thing left. Drainage and ploughing put air, and therefore oxygen, into the soil. In field A there is plenty of oxygen, so nitrifying bacteria are active and convert ammonium ions through nitrite to nitrate ions. In waterlogged field B the water fills the air spaces, oxygen is short, nitrification slows, and denitrifying bacteria, which thrive where oxygen is scarce, convert nitrate back to nitrogen gas, which is lost to the air.

Ammonium piles up because the next step has stalled

Ammonium ions are the input to nitrification. In field B nitrification is slow because of the shortage of oxygen, so ammonium is produced by decomposition but is not converted onwards into nitrate, and it accumulates. This row is the strongest single piece of evidence that the problem is nitrification and not, say, a shortage of decomposers — if decomposition had failed, the ammonium would be low too.

Nitrate is the only form the roots can use

Plant roots absorb nitrate ions, by active transport; they cannot absorb ammonium as their main nitrogen source in this syllabus, and they certainly cannot use nitrogen gas. With only 9 mg per kg available, the crop in field B can make fewer amino acids and less protein, so it grows less and yields less. A second acceptable point: waterlogged soil is also short of oxygen for root respiration, and without energy from respiration the roots cannot carry out active transport at all — which makes the nitrate shortage worse still.

🔍 Spot the Difference

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Six pairs that look almost identical and have different answers. In this topic the distinction is nearly always where the marks live.

Question A

What happens to the nutrients in an ecosystem?

They are recycled. Carbon and nitrogen atoms pass from the air into producers, through consumers, into dead material, and back again. The same atoms are used indefinitely, which is why no new supply is needed.

Question B

What happens to the energy in an ecosystem?

It flows through in one direction. It enters as light from the Sun, is transferred as chemical energy along the chain, and leaves as heat transferred to the environment, mainly from respiration. It is never reused.

Key DifferenceCycle versus flow. This is the most heavily examined distinction in Topic 19 and the source of the commonest wrong sentence in the topic (“decomposers recycle energy”). It also answers the classic question of why an ecosystem needs sunlight every day but not a delivery of carbon.

Question A

Define herbivore.

An animal that gets its energy by eating plants. It describes the diet, and it is true of the animal wherever it appears.

Question B

Define primary consumer.

The organism at the second trophic level — the one that eats the producer. It describes a position in a particular chain, not a diet.

Key DifferenceDiet versus position. The same animal is usually both, which is why they feel interchangeable — but a bear eating berries is a primary consumer and eating a fish is a tertiary consumer, and it is an omnivore throughout. If the question says “state the trophic level”, the word “herbivore” scores nothing.

Question A

Why can a pyramid of numbers be the wrong shape?

Because it takes no account of the size of the organisms. One oak tree counts as one, exactly like one aphid, so a single large producer gives a very narrow base.

Question B

Why can a pyramid of biomass be the wrong shape?

Because it is a snapshot at one moment in time. Fast-reproducing producers such as phytoplankton are grazed almost as fast as they grow, so little is standing there at any instant.

Key DifferenceNumbers ignores size; biomass ignores time. A pyramid of energy ignores neither, which is why it can never be inverted — and why it is the hardest of the three to measure. If a question shows you two pyramids that disagree, the disagreement itself is the information.

Question A

What do nitrogen-fixing bacteria do?

Convert nitrogen gas from the air into compounds plants can use — ammonium ions and amino acids. They live free in the soil and in the root nodules of plants such as peas, beans and clover. This is the way in.

Question B

What do denitrifying bacteria do?

Convert nitrate ions in the soil back into nitrogen gas, which is lost to the air. They thrive in waterlogged soil where oxygen is short. This is the way out.

Key DifferenceDirection. Fixation runs air → soil; denitrification runs soil → air. Swapping them is the single most common nitrogen-cycle error, and because the two answers use identical vocabulary it is invisible unless you check the direction deliberately. Keep the other two straight too: decomposition makes ammonium from dead material; nitrification turns ammonium into nitrate.

Question A

Why is the population constant in the stationary phase?

Because the rate of reproduction equals the death rate. Both are large; they cancel. Food is short, waste has built up and competition is intense, so the environment cannot support any more individuals.

Question B

Why is the population falling in the death phase?

Because the death rate is now greater than the rate of reproduction. The food supply is exhausted and the accumulated waste products are toxic.

Key DifferenceEqual rates versus unequal rates. In both phases reproduction is still happening — that is the point most students miss. Note also the difference in the food: “running short” in the stationary phase, “exhausted” in the death phase. Using the death-phase reason for the stationary phase is a common half-answer.

Question A

Define community.

All of the populations of different species in an ecosystem. Every living thing, and nothing that is not living.

Question B

Define ecosystem.

A unit containing the community of organisms and their environment, interacting together. It includes the soil, the water, the air and the temperature.

Key DifferenceThe non-living part. If your definition of an ecosystem does not mention the environment or the physical surroundings, you have written a definition of a community. Size is not the difference — a rock pool and a rainforest are both ecosystems.

🔗 Organisms and their Environment Concept Map

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Click each node. The whole topic is three stories: energy going one way, atoms going round, and numbers rising until something stops them.

⭐ CORE FRAMEWORK 1

Light in → chemical energy along the chain → heat out. One direction, no return.

Where Energy Enters and What a Producer Really Does ▶

The Sun is the principal source of energy input to biological systems — “principal”, not “only”. A producer makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. It transfers light energy into chemical energy stored in organic molecules; it does not make energy and it does not take food from the soil.

Very little of the light that falls on a field is stored: typically about 2 %. Light is reflected, passes through the leaf or falls on bare soil; some wavelengths cannot be used by chlorophyll; and the plant respires part of what it makes. Any “why is the figure so low” question is answered from that list.

From photosynthesis onwards the energy stays chemical, because eating is simply moving molecules from one body into another. Nothing in a food chain converts it back into light.

The Four Places Energy Leaves ▶

Respiration — released and transferred to the surroundings as heat. The largest loss for a bird or a mammal, which respires heavily just to stay warm.

Undigested material — leaves in the faeces. The largest loss for a grass-eating herbivore, because grass is mostly cellulose and no mammal makes an enzyme for it.

Excretion — excess amino acids are deaminated in the liver and the nitrogen leaves as urea in the urine.

Not eaten at all — bark, roots, bone, teeth, horn. That energy goes to decomposers, which respire it away as heat like everything else.

What survives all four is new tissue, and only new tissue is available to the next trophic level. Everything in 19.3 — inefficient transfers, short food chains, eating crops rather than livestock — is this one list applied to a different question.

The Three Pyramids, and What Each One Cannot See ▶

Numbers counts individuals and ignores size, so one oak tree gives a base narrower than the insects above it. Biomass weighs dry material and fixes that, but it ignores time — a snapshot of fast-turnover plankton can still be inverted.

Energy measures kJ per m² per year and ignores nothing, so it can never be inverted: energy is lost at every transfer, and a level can never contain more than the level below supplied it. Its one weakness is practical — the data are difficult and slow to collect.

Drawing marks are mechanical: producers at the bottom, bars centred, width proportional to the quantity, all bars the same height, every bar labelled. If the scale is impossible, write “not to scale” and give the figures.

⭐ CORE FRAMEWORK 2

Atoms go round: six carbon processes, eight nitrogen steps, four bacterial jobs.

Carbon — One Way Out of the Air, Three Ways Back ▶

The six named processes are photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion. Nothing else is examinable in the carbon cycle at IGCSE.

Only photosynthesis removes carbon dioxide from the air. Respiration (of plants, animals and decomposers) and combustion return it. On a blank diagram, find the single arrow pointing away from the air box first.

Decomposition needs two steps to reach the air: decomposition transfers the carbon into the decomposers, and their respiration releases the gas. Name both. Fossil fuels exist because some organisms died where decomposers could not work, so their carbon sat outside the cycle until something burned it.

Nitrogen — Getting an Unreactive Gas into a Protein ▶

Almost 78 % of the air is nitrogen and no plant or animal can use any of it directly. The whole cycle exists to solve that. Nitrogen fixation is the way in, by nitrogen-fixing bacteria (free in the soil and in root nodules) and by lightning.

The soil route: decomposition of dead material, faeces and urine releases ammonium ions; nitrification oxidises those to nitrite and then nitrate ions; roots absorb nitrate by active transport; the plant makes amino acids and proteins.

The animal route: feeding and digestion of proteins gives amino acids, which the animal builds into its own proteins. Excess amino acids are deaminated in the liver to urea, excreted, and decomposed back to ammonium.

The way out: denitrification, in waterlogged soil short of oxygen, turns nitrate back into nitrogen gas. Which is why draining and ploughing a field raises nitrate twice over — more oxygen speeds nitrification and slows denitrification.

Why the Two Cycles Do Not Contradict Framework 1 ▶

Both cycles move atoms, and atoms genuinely go round for ever. The energy that travelled with those atoms leaves the system at every respiration arrow on either diagram, as heat, and never returns.

That is the whole answer to “why does an ecosystem need a continuous input of energy but not of carbon”, and it is why a sealed jar of pond water on a windowsill can live for years but the same jar in a cupboard dies.

⭐ CORE FRAMEWORK 3

Numbers rise until something limits them — and everything that limits them gets worse as they rise.

Population, Community, Ecosystem ▶

Population = a group of organisms of one species, in the same area, at the same time. All three conditions are needed.

Community = all of the populations of different species in an ecosystem — everything living, nothing else.

Ecosystem = the community and its environment, interacting together. The non-living part is the entire difference, and leaving it out turns your ecosystem definition into a community definition.

The Four Factors and the Four Phases ▶

Cambridge names exactly four factors: food supply, competition, predation and disease. Every one of them bites harder as the population grows — less food each, more competition, more predators attracted, faster spread of a pathogen between crowded hosts. That is why growth must eventually stop.

Lag: few individuals, adjusting to the conditions, growing and making the enzymes they need; little division yet. Exponential (log): food, space and oxygen plentiful, no waste, nothing limiting, so the number doubles in a fixed time.

Stationary: the rate of reproduction equals the death rate — food short, waste accumulating, competition intense. Death: the food is exhausted and the waste is toxic, so the death rate now exceeds the rate of reproduction.

Predator and prey cycles are the same logic across two species: plenty of prey feeds more predators, so the predator peak always follows the prey peak, and the predators then reduce the prey and fall in turn.

What Humans Do to a Web — and the Two Named Cases ▶

Overharvesting = removing individuals faster than the population can replace them by reproduction. Trace the consequences both ways: whatever ate the species loses food; whatever the species ate is released.

Introducing a foreign species: it has no natural predators in the new habitat, there is plenty of food and space at first, and the native species have no defence against it. It then competes with native species for food, space or light, or preys on them.

Any removal or addition is answered the same way: up, down, sideways, one reasoned sentence each. Where two effects oppose one another, say so — “the effect is difficult to predict because…” is a full-mark answer, not a hedge.

❌ “Why Is This Wrong?” Exercises

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Six answers of the kind that read fluently and score badly. Find the fault before you reveal it.

Exercise 1: “Describe how energy moves through an ecosystem. [4]”

Student’s Answer“Producers make energy from the Sun and store it. Animals eat the plants and use the energy up. When they die, decomposers break them down and return the energy to the soil, so the plants can use it again. This is why the energy is recycled in an ecosystem.”

The FlawThree separate errors, and they compound. “Make energy” — nothing makes energy; producers transfer it. “Use the energy up” — energy is transferred to the environment as heat, not consumed. And the whole final claim is the topic’s biggest misconception: decomposers return mineral ions, not energy, and they respire away everything they obtain. As written this answer scores at most one mark.

Correct Answer“Light energy from the Sun is absorbed by producers and transferred into chemical energy stored in organic nutrients by photosynthesis [1]. It is passed along the food chain when organisms are eaten [1]. At every level, energy is released in respiration and transferred to the environment as heat, and more is lost in faeces and in excretion [1]. Energy therefore flows through the ecosystem in one direction and is not recycled — only the nutrients are [1].”

Key RuleFour words to carry into the exam: nutrients cycle, energy flows. If your answer contains “energy is recycled”, “makes energy” or “energy is destroyed”, cross it out and rewrite it before you move on.

Exercise 2: “Fig. 2.1 shows a food web. All of the voles are killed by disease. Suggest the effects on the rest of the web. [4]”

Student’s Answer“There will be fewer owls because owls eat voles. The whole food web will collapse because every organism depends on every other one.”

The FlawThe first sentence is worth one mark and the second is worth none. Only one direction has been traced — upwards. Nothing is said about what the voles ate, nothing about the owls’ other prey, and no reason is given beyond “owls eat voles”. “The web will collapse” is what people write when they have not actually looked at the diagram; a web with alternative food sources almost never collapses, which is the point of it being a web.

Correct Answer“Up: the owls have lost a source of food, so their numbers may fall [1]. Down: the grass and seeds the voles ate are no longer being eaten as heavily, so they increase [1]. Sideways: the owls will eat more of their other prey, such as shrews, so shrew numbers fall [1]; and other animals that ate the same plants as the voles now have more food available, so they may increase [1].”

Key RuleUp, down, sideways — one sentence for each, each containing a reason. Where two effects pull an organism in opposite directions, say so explicitly; “the effect is hard to predict because…” earns marks, while a confident guess with no reasoning does not.

Exercise 3: “A pyramid of numbers for an oak woodland is inverted at the base. Explain why, and suggest a better way of presenting the data. [4]”

Student’s Answer“It is inverted because there are more insects than oak trees. A pyramid of biomass would be better because a pyramid of biomass can never be inverted, unlike a pyramid of numbers.”

The FlawThe first sentence describes the diagram instead of explaining it — “there are more insects than trees” is exactly what the picture already shows, and it never says why that produces an odd shape. The second sentence states something untrue: a pyramid of biomass can be inverted, because it is a snapshot in time. It is the pyramid of energy that never can.

Correct Answer“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, so the base bar is narrow [1]. A pyramid of biomass would be better, because it measures the dry mass of living material and therefore takes size into account, giving the expected shape [1]. Better still would be a pyramid of energy, which measures energy over a period of time and can never be inverted [1].”

Key RuleExplanation questions need the rule as well as the instance. And keep the three pyramids’ weaknesses separate: numbers ignores size, biomass ignores time, energy ignores nothing but is hard to measure.

Exercise 4: “Explain why it is more energy-efficient for humans to eat crop plants than to eat livestock fed on those crops. [4]”

Student’s Answer“Plants have more energy than animals do, so eating plants gives you more energy. Also 10 % of the energy is lost at each stage, so eating meat wastes 10 % of the energy from the crop.”

The FlawThe first claim is not the argument at all — per kilogram, meat generally contains more energy than most crops. The argument is about the number of transfers, not the food itself. The second claim inverts the figure: if 10 % is transferred, then 90 % is lost, so the extra step costs nine tenths of the energy, not a tenth. And no loss is ever named.

Correct Answer“Eating the crop directly involves one energy transfer; feeding it to livestock and eating them involves two [1]. At the extra transfer, energy is lost from the livestock in respiration as heat, in undigested material in the faeces, in urine, and in parts that are not eaten such as bone [1]. Only around a tenth of the energy in the crop therefore reaches the human by the livestock route, compared with the crop route [1]. So a given area of land feeds far more people when the crop is eaten directly [1].”

Key RuleCount the transfers, then name the losses. And if a “discuss” version is asked, add the other side: some land is too poor, steep or dry to grow crops but will support grazing animals.

Exercise 5: “Describe the role of bacteria in the nitrogen cycle. [4]”

Student’s Answer“Nitrogen-fixing bacteria in the soil turn nitrates into nitrogen gas so it can go back into the air. Nitrifying bacteria then break down dead plants and animals to release ammonia. Denitrifying bacteria are found in root nodules and help the plant absorb nitrogen from the air.”

The FlawEvery one of the three has been given somebody else’s job. What is described as nitrogen fixation is actually denitrification; what is described as nitrification is actually decomposition; and the bacteria in root nodules are the nitrogen-fixing ones, not the denitrifying ones. The vocabulary is all present and correct, which is exactly why this answer feels right while scoring nothing.

Correct Answer“Nitrogen-fixing bacteria, free in the soil and in root nodules, convert nitrogen gas from the air into compounds the plant can use [1]. Decomposers break down protein in dead organisms, faeces and urine to release ammonium ions [1]. Nitrifying bacteria oxidise ammonium ions to nitrite and then to nitrate ions, which plants absorb [1]. Denitrifying bacteria, in waterlogged soil where oxygen is short, convert nitrate ions back into nitrogen gas, which is lost to the air [1].”

Key RuleFor each of the four bacterial jobs, learn what goes in and what comes out, not just the name. Fixation runs air → soil; denitrification runs soil → air; decomposition makes ammonium; nitrification turns ammonium into nitrate.

Exercise 6: “Explain the shape of the curve between 18 and 30 hours. [4]”

Student’s Answer“Between 18 and 26 hours the population is constant because the yeast have stopped reproducing and there is no food left. After 26 hours the number goes down because the yeast are dying of old age.”

The Flaw“Stopped reproducing” is the classic misreading of a flat line: the line is flat because two large rates are cancelling, not because both are zero. “No food left” belongs to the death phase — if the food were truly gone, the line would already be falling. And “dying of old age” explains nothing: the causes are the exhausted food supply and the accumulation of toxic waste.

Correct Answer“Between 18 and 26 hours the culture is in the stationary phase: the rate of reproduction equals the death rate [1], because the food supply is running short, waste products have accumulated and competition is intense [1]. After 26 hours it enters the death phase: the food supply is now exhausted and the waste products have reached toxic levels [1], so the death rate exceeds the rate of reproduction and the population falls [1].”

Key RuleWhenever a population graph is flat, say what the two rates are doing. “Rate of reproduction equals death rate” and “death rate exceeds rate of reproduction” are the two sentences those questions are marked against.

✍️ Ultra-Detailed Practice Questions

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Ten Cambridge-style challenge questions, each drawing on more than one sub-topic. Write your answer first, then reveal the model answer and the examiner’s notes.

Question 1

[8 marks]

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) State the letters of the producers and explain how the diagram alone tells you. [2] (b) Construct a food chain containing four organisms and ending in M, and state the trophic level of the organism you have placed second. [3] (c) A disease removes all of E. Suggest, with reasons, the effect on B, on J and on A. [3]

Model Answer(a) A and B [1]. They are the only organisms with arrows leaving them and no arrow arriving, so nothing feeds energy into them — they must make their own organic nutrients [1].
(b) A → D → L is only three; a four-organism chain ending in M is A → E → J → M [1] (grass → impala → cheetah → hyena). The organism placed second is E [1], which eats a producer, so it is a primary consumer at the second trophic level [1].
(c) B (down): E eats B, so removing E means B is browsed less and increases [1]. J (up): J eats E and nothing else in this web, so J loses its only food source and its numbers fall sharply [1]. A (down, and then sideways): E also eats A, so grazing on A falls and A increases — but with E gone, L and M will take more D, and if D falls then A is grazed less still, so A rises for two reasons [1].

Examiner’s NotesPart (a) is the test that works on any unfamiliar web: a producer has no incoming arrow. In (c), J is the interesting case — a specialist with one food source is far more vulnerable than a generalist, and saying so explicitly is what lifts the answer. Note that the marks are for the reasons, not the directions: “J decreases” alone earns nothing.

Question 2

[7 marks]

A grassland ecosystem is studied for one year. The energy figures, in kJ per m² per year, are: producers 20 000; primary consumers 1600; secondary consumers 160; tertiary consumers 24.
(a) Calculate the percentage of energy transferred from producers to primary consumers, and from secondary to tertiary consumers. Show your working. [3] (b) A student says the results prove that 10 % of energy is always transferred. Comment on this. [2] (c) Explain why there is no fifth trophic level in this grassland. [2]

Model Answer(a) 1600 ÷ 20 000 × 100 = 8 % [1 working, 1 answer]. 24 ÷ 160 × 100 = 15 % [1].
(b) The student is wrong. The two transfers here are 8 % and 15 %, so the value clearly varies between levels [1]. Around a tenth is a useful rough generalisation, but it is not a rule, and the figures given in a question must always be used in preference to it [1].
(c) A fifth level would receive roughly a tenth of 24 kJ — only about 2–3 kJ m⁻² y⁻¹ [1]. That is too little energy to support a viable population of another predator: it would have to hunt over an enormous area and would use more energy hunting than it gained [1].

Examiner’s NotesPart (b) is an evaluation question and those are always worth reading twice — the examiner has planted a plausible generalisation to see whether you will check it against the data. Quote both percentages; a comment without figures cannot score. In (c) avoid “there is no energy left”: there is, and the mark is for saying it is not enough.

Question 3

[8 marks]

(a) Name the four phases W, X, Y and Z. [2] (b) Explain the shape of the curve in phase W. [2] (c) Explain, referring to limiting factors, why the curve behaves as it does in phase Y. [3] (d) Sketch in words how the curve would differ if fresh nutrient broth were supplied continuously and waste removed. [1]

Model Answer(a) W lag, X exponential (log), Y stationary, Z death [2 for all four, 1 for two or three].
(b) The organisms are adjusting to their new conditions — taking up water, growing and producing the enzymes needed to use the nutrients available [1] — so few are dividing yet, and with so few individuals present even a doubling is barely visible on this scale [1].
(c) The rate of reproduction now equals the death rate, so the number stays constant [1]. This is because the food supply is running short and toxic waste products have accumulated, and competition for the remaining nutrients and space is intense [1]. Whichever of these is in shortest supply is the limiting factor and it sets the maximum population the flask can support [1].
(d) The population would continue to rise well beyond 18 hours and there would be no death phase, because the two factors that ended growth — shortage of food and accumulation of waste — have been removed; it would eventually level off when space became limiting [1].

Examiner’s NotesNotice how often the phrase “rate of reproduction equals the death rate” is the first marking point on a stationary-phase question. Write it before you write anything about food. In (d) the extra credit is for recognising that removing two limiting factors does not remove all of them — space is still finite.

Question 4

[8 marks]

(a) Name the six processes shown in Fig. 4.1 that Cambridge includes in the carbon cycle. [3] (b) Explain why only one process can remove carbon dioxide from the air. [2] (c) Describe, naming two processes, how carbon in a dead animal reaches the air. [2] (d) State one reason why carbon locked in coal was out of the cycle for millions of years. [1]

Model Answer(a) Photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion [3 for all six, 2 for four or five, 1 for two or three].
(b) Photosynthesis is the only process that takes carbon dioxide from the air and fixes it into organic compounds [1]. Respiration, combustion and decomposition all move carbon in the opposite direction, back into the air [1].
(c) Decomposition: decomposers feed on the dead animal and take its carbon compounds into their own bodies [1]. Respiration: the decomposers then respire those compounds and release carbon dioxide into the air [1].
(d) The organisms died in conditions where decomposers could not break them down (little or no oxygen), so their carbon was buried and compressed instead of being returned, and only combustion can release it [1].

Examiner’s NotesPart (c) is the one people half-answer. “Decomposition” on its own does not put a gas into the air — the decomposers have to respire. Naming both processes is the difference between one mark and two, and the same pairing is worth using whenever a question mentions decomposers and carbon dioxide in the same sentence.

Question 5

[7 marks]

A farmer has grown wheat in the same field for eight years and the yield has fallen each year. He grows clover for one season, ploughs it into the soil, and the following wheat crop is his best for a decade.
(a) Explain why the yield had been falling. [2] (b) Explain how growing clover and ploughing it in raised the nitrate concentration in the soil, naming the organisms responsible at each stage. [4] (c) The neighbouring field is waterlogged and gives a poor yield despite the same treatment. Suggest why. [1]

Model Answer(a) Each harvest removes the whole crop, and the nitrogen in its protein, from the field [1]. Nothing is returned to the soil to be decomposed, so nitrate ions are steadily used up and not replaced, and the wheat can make less protein [1].
(b) Clover has root nodules containing nitrogen-fixing bacteria, which convert nitrogen gas from the air into compounds the plant can use, so the clover grows well and its tissues are rich in protein [1]. Ploughing it in returns that material to the soil rather than removing it [1]. Decomposers (bacteria and fungi) break the protein down to ammonium ions [1]. Nitrifying bacteria then oxidise ammonium to nitrite and then to nitrate ions, which the wheat roots absorb by active transport [1].
(c) In waterlogged soil there is little oxygen, so nitrification is slow and denitrifying bacteria convert nitrate back to nitrogen gas, which is lost to the air [1].

Examiner’s NotesFour marks in (b) means four named steps, so plan them before you write: fixation, return of material, decomposition, nitrification. The word “naming” in the question is instructing you to identify the organisms, and an answer that describes the chemistry without naming the bacteria will lose half the marks.

Question 6

[7 marks]

Fig. 6.1 shows what happens to 1000 kJ of energy eaten by a bullock. (a) Calculate the energy stored as new tissue and the percentage efficiency of the transfer. [2] (b) Explain why the faeces figure is the largest single loss for this animal. [2] (c) A student says this proves that carnivores transfer energy more efficiently than herbivores. Evaluate that claim. [3]

Model Answer(a) 1000 − 600 − 280 − 40 = 80 kJ [1]; 80 ÷ 1000 × 100 = 8 % [1].
(b) Grass is largely cellulose, and a mammal produces no enzyme that digests cellulose [1], so a large proportion of what is swallowed cannot be digested or absorbed and passes out in the faeces, taking its energy with it [1].
(c) The claim is partly supported. A carnivore eats meat, which is far more easily digested than grass, so less energy is lost in the faeces and a higher proportion is absorbed [1]. But that is only one of the losses: carnivores are usually far more active because they must hunt, so more energy is lost as heat in respiration [1]. So the claim cannot be settled from one animal’s figures — you would need comparable data for a carnivore before concluding anything [1].

Examiner’s NotesPart (c) is an “evaluate” question and the third mark is almost always for recognising the limits of the evidence. “You cannot conclude this from one set of data” is a real biological point, not a cop-out, and examiners reward it — provided you have also engaged with the biology on both sides first.

Question 7

[7 marks]

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. Within five years the native fish are almost gone and the water has turned green.
(a) Construct the food chain described, before the introduction. [1] (b) Suggest three reasons why the introduced fish increased so rapidly. [3] (c) Explain, using the food chain, how the loss of the native fish could turn the water green. [3]

Model Answer(a) algae → water flea → native fish [1] — producer first, arrows meaning “is eaten by”.
(b) 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 [1]. The native fish had no defence against a predator they had never encountered [1].
(c) With the native fish removed, the water fleas would at first be eaten less and increase, which would reduce the algae [1]. The water turned green instead, so the introduced fish must be eating the water fleas as well, reducing them [1]. Fewer water fleas means less grazing on the algae, so the algae multiply and the water turns green [1].

Examiner’s NotesPart (c) is deliberately awkward: the obvious one-step answer predicts 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 so explicitly is worth a mark, and it is exactly the kind of thinking a challenge paper is testing.

Question 8

[6 marks]

(a) Define the terms population, community and ecosystem. [3] (b) State the four factors that Cambridge names as affecting the rate of population growth, and explain what all four have in common. [3]

Model Answer(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].
(b) Food supply, competition, predation and disease [1, all four needed]. All four act more strongly as the population becomes larger and more crowded [1]: there is less food per individual, competition intensifies, a large prey population supports more predators, and a pathogen spreads more easily between hosts that are close together. That is why a population cannot grow indefinitely and why the growth curve levels off [1].

Examiner’s NotesDefinitions are marked phrase by phrase, so include every qualifier: “one species”, “same area”, “same time”, “and their environment”. Part (b) is the kind of question that rewards understanding rather than listing — the connection between the four factors and the shape of the sigmoid curve is what the third mark is for.

Question 9

[7 marks]

In an area of open ocean, the pyramid of numbers is a normal pyramid shape but the pyramid of biomass is inverted at the base, with 4 g m⁻² of phytoplankton and 21 g m⁻² of zooplankton.
(a) Explain how a pyramid of biomass can be inverted. [3] (b) Explain why the pyramid of numbers looks normal in this habitat but is inverted for an oak woodland. [2] (c) State which pyramid would be guaranteed to show the expected shape and give one advantage and one disadvantage of using it. [2]

Model Answer(a) A pyramid of biomass measures the dry mass present at one moment in time — it is a snapshot [1]. Phytoplankton are microscopic, reproduce extremely rapidly and are grazed almost as fast as they are produced [1], so very little algal material is standing there at any instant, even though over a whole year they produce far more material than the zooplankton do [1].
(b) In the ocean the producers are individually tiny but astronomically numerous, so counting them gives a very wide base [1]. In an oak woodland one enormous producer is counted as a single individual, so the base is the narrowest bar — in both cases the pyramid of numbers is misleading because it takes no account of size [1].
(c) A pyramid of energy [1 — with either]. Advantage: it measures energy over a period of time and can never be inverted. Disadvantage: the data are difficult and slow to collect [1].

Examiner’s NotesThis question is the whole of 19.3 in one place: numbers ignores size, biomass ignores time, energy ignores neither. Part (b) is the neat one — the same weakness produces opposite-looking errors in two habitats, and noticing that is what a top answer does.

Question 10

[9 marks]

A sealed glass sphere contains sea water, a few small shrimps, algae, gravel and a bubble of air. It is kept on a windowsill in the light. Nothing is ever added and nothing removed, and it stays alive for years.
(a) Explain how the carbon in the sphere is recycled, naming the processes. [3] (b) Explain how the nitrogen in the sphere is recycled, naming the processes and the organisms responsible. [4] (c) Identify the one thing that must keep entering the sphere, and explain why it cannot be recycled like the carbon and the nitrogen. [2]

Model Answer(a) The algae take carbon dioxide from the water and fix it into organic compounds by photosynthesis [1]. The shrimps obtain those compounds by feeding on the algae [1]. All the organisms — algae, shrimps and the decomposers in the gravel — release carbon dioxide by respiration, and decomposition of dead material and waste returns the rest, so the same carbon atoms are used again [1].
(b) The shrimps deaminate excess amino acids and excrete the nitrogen as waste [1]. Decomposers in the gravel break down that waste and any dead material to release ammonium ions [1]. Nitrifying bacteria convert ammonium to nitrite and then to nitrate ions [1]. The algae absorb the nitrate and use it to make amino acids and proteins, which the shrimps then eat [1].
(c) Light [1]. Energy is not recycled: every organism in the sphere respires and transfers energy to the surroundings as heat, which passes out through the glass and cannot be recovered, so a continuous input of light energy is required [1].

Examiner’s NotesThis is the question that rewards seeing the topic as one idea. The sphere is a working model of “nutrients cycle, energy flows”, and part (c) is the whole of 19.1 in two sentences. If you can explain this sealed sphere you can answer almost anything in 19.1, 19.3 and 19.4 — and it explains why the same jar in a cupboard would be dead within weeks.