IGCSE Biology Topic 19: Organisms and their Environment

19.1 Energy Flow 19.2 Food Chains, Webs & Trophic Levels 19.3 Pyramids & Energy Efficiency 19.4 The Carbon & Nitrogen Cycles 19.5 Populations & the Sigmoid Curve 19.6 Exam Technique

Hi Tara. Almost everybody meets this topic, decides it is the easy one, and then drops marks all over it. The reason is that Topic 19 is not really a recall topic at all. Look at what the objectives actually ask you to do: construct a food chain, interpret a food web, draw a pyramid, identify the phases of a graph, explain why a transfer is inefficient. Five of those six verbs are reading-and-drawing verbs. You will be handed information you have never seen — an unfamiliar lake, a set of organisms with odd names, a curve with no labels — and asked to do something precise with it.

So this guide is built around the doing. 19.1 is one idea and it is the idea the whole topic hangs on: energy arrives as light from the Sun, is passed along, and leaves as heat. 19.2 is chains, webs and trophic levels, with a long drill on constructing chains from a description and on tracing a knock-on effect through a web. 19.3 is the pyramids and the energy arithmetic; it is split off from 19.2 because drawing a pyramid to scale and explaining energy loss are a separate block of work with their own marks and their own traps. 19.4 is the carbon and nitrogen cycles. 19.5 is populations and the sigmoid curve. 19.6 is the vocabulary check and the exam technique.

Two things carry more marks than anything else here, and they are both one-liners. First: an arrow in a food chain means “is eaten by”, and it points in the direction the energy travels. Reversing an arrow is the single commonest lost mark in the topic, and it is lost silently, because the drawing looks fine. Second: nutrients are recycled; energy is not. Carbon atoms and nitrogen atoms go round and round for ever. The energy that came with them leaves the system as heat at every single step and never comes back. If you hold on to those two sentences, half of this topic writes itself.

19.1 Energy Flow ▼

▶ Watch: Energy Flow

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Everything Starts at the Sun

Cambridge states it in one line, and it is worth having word for word: the Sun is the principal source of energy input to biological systems. Note “principal”, not “only” — there are odd communities deep in the ocean that run on chemical energy instead, and the word “principal” is how the syllabus quietly allows for them. You are not asked about those, but you are asked not to write “the only source”.

Now follow one packet of energy the whole way through. Light energy leaves the Sun and falls on a leaf. A tiny fraction of it — most light misses the chlorophyll, or is the wrong wavelength, or passes straight through — is absorbed and used in photosynthesis to build glucose. At that moment the energy stops being light and becomes chemical energy stored in the bonds of organic molecules. That is what a producer does: it does not make energy, it transfers energy from one form to another and stores it in a form that can be eaten.

Words that lose marks here

“Plants make energy.” No. Plants make organic nutrients; the energy is transferred, never created. “The energy is used up.” No — it is transferred to the environment as heat. “Energy is recycled by decomposers.” No: decomposers recycle nutrients, and they release the energy they get as heat, exactly as everything else does.

From there the chemical energy moves by being eaten. When a rabbit eats grass, the chemical energy in the grass’s molecules becomes chemical energy in the rabbit’s molecules — but only some of it. Look carefully at where the rest goes, because those losses are the whole of 19.3 and they start here.

Energy in at the top left, energy out down the right. Nothing goes back to the Sun.

The three places energy leaves

Respiration. Every living organism respires, every minute of its life, to release energy for movement, for active transport, for building molecules and for keeping warm. Almost all of that energy ends up as heat transferred to the environment. It is gone: no organism can eat heat.
Undigested material. A herbivore cannot digest all of what it swallows — cellulose in particular. That energy passes straight through and leaves in the faeces, so it never enters the animal’s body at all.
Excretion. Excess amino acids are deaminated, and the nitrogen-containing part leaves as urea in the urine, carrying some energy with it.

What is left after all three is the energy actually built into new tissue — the growth. Only that is available to the next organism along, because only that is the animal’s body. And notice that the energy in the faeces and the dead bodies is not wasted from the ecosystem’s point of view: decomposers get it. But decomposers respire too, so it still ends up as heat. There is no route by which energy re-enters the food chain.

The sentence to carry into the exam

“Nutrients cycle. Energy flows.” Four words. Nutrients — carbon, nitrogen — go round and round for ever and the same atoms are used again and again. Energy travels in one direction only: in as light, out as heat. That is why an ecosystem needs the Sun every single day, but does not need a delivery of new carbon.

Worked example A student writes: “Energy is recycled in an ecosystem. Producers make energy from sunlight, animals use it up, and decomposers return the energy to the soil so plants can use it again.” Identify three errors and correct each one. [3]

Error 1 — “energy is recycled”

Energy is not recycled. It flows through in one direction: it enters as light energy from the Sun and leaves as heat energy transferred to the environment. It is nutrients that are recycled.

Error 2 — “producers make energy”

Producers do not make energy. They transfer light energy into chemical energy stored in organic nutrients, by photosynthesis.

Error 3 — “decomposers return the energy to the soil”

Decomposers return mineral ions and nutrients to the soil. The energy they obtain is released in their own respiration and transferred to the environment as heat, so no energy goes back to the plants. (“Animals use it up” is also loose — energy is transferred, not destroyed — but the three above are the ones a mark scheme lists.)

Energy flows one way — light in, heat out. Only nutrients go round.

Worked example A field receives 1 000 000 kJ of light energy per square metre in a year. The plants growing in it store 20 000 kJ per square metre in that year. Calculate the percentage of the light energy that is stored, and suggest two reasons why the figure is so low. [3]

Step 1 — the arithmetic

20 000 ÷ 1 000 000 × 100 = 2 %. Always write the division you did; a bare “2 %” can lose the working mark if the answer is wrong.

Step 2 — two reasons, and they must be different from each other

Much of the light misses the chloroplasts — it falls on bare soil between the plants, is reflected off the leaf surface, or passes straight through the leaf. Of the light that is absorbed, some is the wrong wavelength to be used by chlorophyll. And some of the glucose the plant does make is used in the plant’s own respiration, so it is not stored.

2 %. Light is reflected, transmitted or falls between plants; some wavelengths cannot be used; and the plant respires some of what it makes.

Check Yourself: 19.1 Energy Flow

12 multiple choice questions. Click an option to check your answer.

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Question 1

Which statement about energy in an ecosystem is correct?

A Energy is recycled by decomposers and returned to producers.

B Producers make energy from carbon dioxide and water.

C Energy enters as light and is eventually transferred to the environment as heat.

D Energy is destroyed as it passes from one trophic level to the next.

C is the shape of the whole topic: one way in, one way out. A is the misconception the syllabus is built to catch — decomposers recycle nutrients, not energy. B says “make energy”, which no organism ever does; producers make organic nutrients. D breaks a law of physics: energy is never destroyed, only transferred to a form nothing can eat.

Question 2

In which form does energy leave a food chain?

A light energy reflected back into the atmosphere

B heat energy transferred to the environment, mostly from respiration

C chemical energy in the mineral ions returned to the soil

D electrical energy in nerve impulses

Respiration in every organism releases energy, most of which warms the organism and then the surroundings. Option A confuses the light that never entered the chain with the energy that did. Option C confuses nutrients with energy again — mineral ions returning to the soil is the nutrient cycle, not an energy output. Option D is a real energy transfer inside an animal but a trivial one, and it too ends up as heat.

Question 3

A leaf absorbs 100 kJ of light energy. Only 4 kJ is stored as chemical energy in new plant material. Which of the following does not help to explain the difference?

A some of the energy is eaten by primary consumers

B some of the light is of a wavelength chlorophyll absorbs poorly

C some of the glucose made is used in the plant’s own respiration

D photosynthesis may be limited by the supply of carbon dioxide

Being eaten by primary consumers is about what happens after the plant material exists, so it cannot explain why only 4 kJ was stored in the first place. The other three all reduce the amount stored: respiration burns some of it, poorly absorbed wavelengths contribute little, and a limiting factor caps the rate of photosynthesis. Read the boundary of the question — “absorbed” to “stored” — and reject anything outside it.

Question 4

Why does an ecosystem need a continuous supply of energy from the Sun, but not a continuous supply of new carbon atoms?

A because carbon atoms are made by decomposers

B because there is far more carbon in the air than there is energy in sunlight

C because plants can make carbon atoms by photosynthesis when they run short

D because carbon atoms are recycled through the ecosystem, while energy passes through once and leaves as heat

This is the “nutrients cycle, energy flows” sentence asked as a question. The two answers in which decomposers or plants make carbon atoms both have organisms creating atoms, which is chemistry rather than biology and impossible either way; photosynthesis rearranges carbon atoms that already exist. The “far more carbon in the air” answer compares two quantities that are not comparable and answers nothing.

Question 5

Which is the best description of what a producer does to energy?

A it creates chemical energy from nothing using chlorophyll

B it stores heat energy from the Sun in its leaves

C it transfers light energy into chemical energy stored in organic nutrients

D it absorbs energy from mineral ions taken up by its roots

C is the wording to copy: transfer, light to chemical, stored in organic nutrients. A creates energy. B swaps light for heat — a leaf does warm up, but heat is not what photosynthesis uses. D is the “plants get their food from the soil” misconception in disguise; mineral ions are needed in small amounts to build proteins and chlorophyll, but they supply no energy.

Question 6

A cow eats 1000 kJ of grass. 600 kJ leaves in the faeces, 280 kJ is released in respiration and 40 kJ leaves in the urine. How much is available to an animal that eats the cow?

A 120 kJ

B 80 kJ

C 400 kJ

D 720 kJ

1000 − 600 − 280 − 40 = 80 kJ, which is the energy built into new tissue. Option C (400) is what you get if you subtract only the faeces — that is the energy absorbed, not the energy stored. Option D subtracts only the respiration. Option A is a slip. In this kind of question, always ask which quantity the next animal actually eats: it eats the cow’s body, not the cow’s meals.

Question 7

Which organism obtains its energy from dead or waste organic material?

A a producer

B a primary consumer

C a quaternary consumer

D a decomposer

That is the definition of a decomposer, word for word. Learn it as “dead or waste” — the waste half matters, because faeces and urine are a large part of what decomposers live on. Remember also that decomposers are not only bacteria: many fungi are decomposers too.

Question 8

Which sequence correctly describes the forms energy takes as it moves from the Sun into a fox?

A light → chemical (in the plant) → chemical (in the rabbit) → chemical (in the fox)

B light → heat → chemical → heat

C chemical → light → chemical → chemical

D light → chemical → light → chemical

Once photosynthesis has happened, the energy stays as chemical energy in molecules all the way along, because eating is just moving molecules from one body to another. Heat is produced at every stage, but it leaves the chain rather than continuing along it, which is why B is wrong as a description of the path. Nothing in a food chain converts chemical energy back into light.

Question 9

Two fields of the same size receive the same light energy. Field 1 grows a crop of wheat; field 2 is bare soil. Which best explains why more chemical energy is stored in field 1?

A the soil in field 2 reflects more light than the wheat does

B only field 1 contains chloroplasts, so only field 1 can transfer light energy into chemical energy

C field 2 has no decomposers to recycle the energy

D field 1 is warmer, so it can absorb more heat energy

The point of energy input is photosynthesis, and photosynthesis needs chlorophyll in chloroplasts. Option A may even be true but it explains nothing about storage. Option C is the recycling misconception yet again. Option D confuses absorbing heat with storing chemical energy: a warm stone stores no food.

Question 10

Why is the energy in an animal’s faeces not counted as part of the energy transferred to the next trophic level?

A because faeces contain no energy at all

B because the energy in faeces has already been released as heat

C because that material was never built into the animal’s body, so a predator eating the animal does not obtain it

D because decomposers destroy the energy in faeces

A predator eats a body, and undigested material has left the body. That energy is real and decomposers make good use of it, which is why A and D are wrong — it is neither absent nor destroyed. B confuses the two separate losses: material that is never absorbed cannot have been respired.

Question 11

Which statement about decomposers and energy is correct?

A decomposers return energy to producers in a usable form

B decomposers are producers, because they release mineral ions into the soil

C decomposers do not respire, which is why they can recycle energy

D decomposers obtain energy from dead and waste material and release most of it as heat in respiration

Decomposers are ordinary living organisms: they feed, they respire, they lose heat. What they return to the soil is mineral ions, which plants need for making proteins and chlorophyll but which carry no usable energy. “Decomposers do not respire” describes an organism that does not exist. Calling decomposers producers confuses returning nutrients with making organic nutrients from carbon dioxide.

Question 12

A student says: “The Sun is the only source of energy for all living things.” What is the most precise correction?

A the Sun is the principal source of energy input to biological systems

B the statement is completely correct

C the Sun supplies no energy to animals, only to plants

D most organisms get their energy from mineral ions in the soil

“Principal” is the syllabus word and it is doing real work: it is honest about the rare communities that run on chemical energy instead. Saying the Sun supplies no energy to animals is a bad correction because animals do get their energy from the Sun, just indirectly, through what they eat. Energy from mineral ions in the soil is the “food from the soil” misconception, which is the single most persistent wrong idea in this whole topic.

19.2 Food Chains, Food Webs and Trophic Levels ▼

▶ Watch: Food Chains, Food Webs and Ecological Pyramids

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Cambridge puts food chains, food webs and the pyramids in one sub-topic. This guide teaches them as two, because drawing and interpreting pyramids and reasoning about energy loss is a distinct block of work with its own marks and its own traps — that is section 19.3.

The Arrow Is the Whole Thing

Start with the definition, because it contains the instruction. A food chain shows the transfer of energy from one organism to the next, beginning with a producer. Two things are built into that sentence. It begins with a producer — always, no exceptions, because that is where the energy enters. And it shows the transfer of energy — which is what the arrow is for.

A → B means “A is eaten by B”

The arrow points from the eaten to the eater — the same direction the energy travels. It does not mean “eats”, and it does not point at the food. If you draw fox → rabbit you have said that a fox is eaten by a rabbit, and you will lose the mark however good the rest of the answer is.

Producer first, arrows pointing the way the energy goes, trophic levels named underneath.

A two-second check that has saved thousands of marks

Before you hand in any chain you have drawn, read it aloud in your head putting the words “is eaten by” on every arrow. “Grass is eaten by grasshopper is eaten by shrew is eaten by kestrel.” If any part of that sentence is ridiculous — “kestrel is eaten by grass” — an arrow is the wrong way round. Do this every single time. It takes two seconds and it catches the commonest error in the topic.

The Vocabulary Cambridge Actually Marks

These are definitions, not descriptions, and the exact words earn the marks.

Term	Definition to learn	The trap
Producer	an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis	writing “makes its own energy” or “makes its own food from the soil”. It makes organic nutrients.
Consumer	an organism that gets its energy by feeding on other organisms	saying “an animal”. Many consumers are not animals.
Herbivore	an animal that gets its energy by eating plants	confusing it with “primary consumer”, which is a position, not a diet.
Carnivore	an animal that gets its energy by eating other animals	assuming a carnivore is always a top predator.
Decomposer	an organism that gets its energy from dead or waste organic material	writing “bacteria”. Many fungi are decomposers too, and the definition never names an organism.
Trophic level	the position of an organism in a food chain, food web or ecological pyramid	saying “the level of the food chain an animal is on” — the definition covers webs and pyramids as well.

The trophic levels themselves run: producer → primary consumer → secondary consumer → tertiary consumer → quaternary consumer. Count the organisms, not the arrows. A four-organism chain has four trophic levels but only three arrows. There are two different labels, and they are always one apart: the trophic level is a number that counts every organism, starting with the producer at level 1; the consumer name counts only the consumers, so it is always one behind the trophic level.

So in grass → grasshopper → shrew → kestrel, grass is the producer (level 1), the grasshopper is the primary consumer (level 2), the shrew is the secondary consumer (level 3) and the kestrel is the tertiary consumer (level 4). If you count arrows instead of organisms, you will put the kestrel at the 3rd trophic level, one level too low. A quaternary consumer needs a five-organism chain, for example grass → grasshopper → frog → snake → hawk, where the hawk is the quaternary consumer at the 5th trophic level.

Two words that are not interchangeable

Herbivore describes what an animal eats. Primary consumer describes where it sits in a particular chain. Usually the same animal is both — but a bear eating berries is a primary consumer in that chain and eating a fish it is a tertiary consumer, and it is an omnivore throughout. Answer the question that was asked: if the question says “state the trophic level”, “herbivore” scores nothing.

Constructing a Chain From a Description

This is a guaranteed question and it is worth doing methodically rather than by instinct. You will be given a paragraph of prose and asked to build the chain. Work in this order.

Find the producer. Anything green, anything called algae, plankton, a plant, a tree, a crop. That goes on the far left. If you cannot find one, look again — there is always one, and starting a chain with an animal is an automatic lost mark.
Find who eats the producer. That is your primary consumer.
Chain forwards until you run out of eaters.
Draw the arrows last, all pointing right, and then run the “is eaten by” check.
Do not include decomposers in the chain unless the question asks for them, and never include the Sun as an organism.

Worked example “In a rocky shore community, limpets scrape a film of green algae from the rocks. Dog whelks bore into limpets and feed on them. Oystercatchers, which are wading birds, pull dog whelks off the rocks and eat them.” Construct a food chain for this community, and state the trophic level of the dog whelk. [3]

Step 1 — find the producer

The green algae. It is the only thing here that photosynthesises, so it starts the chain, and it is the first trophic level.

Step 2 — chain forwards

Limpets eat algae. Dog whelks eat limpets. Oystercatchers eat dog whelks. Nothing eats the oystercatcher in this passage, so the chain ends there.

Step 3 — write it with arrows and check it aloud

green algae → limpet → dog whelk → oystercatcher. Read it: “algae is eaten by limpet is eaten by dog whelk is eaten by oystercatcher.” That is sensible, so the arrows are right.

Step 4 — count organisms, not arrows

Algae is level 1, limpet level 2, dog whelk level 3. So the dog whelk is a secondary consumer, at the third trophic level. “Carnivore” is true but does not answer the question.

green algae → limpet → dog whelk → oystercatcher. The dog whelk is a secondary consumer (third trophic level).

Food Webs

A food web is a network of interconnected food chains. Nothing more complicated than that — and every single chain inside it still starts with a producer and still has arrows meaning “is eaten by”.

Trace one chain with your finger from a producer to a top carnivore: that is a food chain, and there are dozens of them in here.

Three things to notice in that web, because all three are examined.

One organism can sit at more than one trophic level. The kestrel eats the field mouse, which is a primary consumer, so in that chain the kestrel is a secondary consumer. The kestrel also eats the shrew, which is a secondary consumer, so in that chain the kestrel is a tertiary consumer. Both answers are correct — but only if you name the chain you are talking about. If a question asks “what is the trophic level of the kestrel”, the safe answer states both and says which chain each belongs to.
Most consumers have more than one food source. That is the whole point of a web, and it is what makes the knock-on questions answerable.
Decomposers are usually left off the drawing. They act on every organism in it, so drawing all the arrows would bury the diagram.

The Knock-On Question — and How to Score All the Marks

Here is the question you are certain to meet: “All of the rabbits are removed by a disease. Suggest the effects on the rest of the web.” Most students look up the web, find the fox, write “fewer foxes”, and stop. That is one mark out of three or four.

The mark scheme wants you to trace both directions. Above the missing organism, its predators lose a food source. Below it, whatever it used to eat is now left alone. And then there is a third wave: the predators switch to their other prey, which drives those numbers down too.

Up, down, sideways

UP — who ate it? They have less food, so their numbers fall (or they eat something else).
DOWN — what did it eat? That is no longer being eaten as much, so its numbers rise.
SIDEWAYS — what else do its predators eat? Those things are now eaten more, so their numbers fall. And what else eats its food? Those things now have more food, so they rise.

Write one sentence for each direction, each with a reason. Three directions, three linked sentences, and you have covered every mark point the examiner has on the page.

Worked example Using the grassland web above: a disease removes all the rabbits. Suggest, with reasons, the effect on the fox, the grass and the field mouse. [4]

Up — the fox

The fox has lost one of its four food sources, so at first there is less food for the fox and its numbers may fall. But the fox also eats field mice, shrews and grass snakes, so it will not starve; it will switch.

Down — the grass and the dandelion

Rabbits ate both. With the rabbits gone, less grass and dandelion is eaten, so both increase. That is more food for the grasshoppers, the field mice and the snail, so those numbers may rise too.

Sideways — the field mouse, pulled two ways

This is the mark most people miss, because the mouse is affected twice and in opposite directions. Foxes now eat more field mice, which pushes the mouse numbers down. But there is more grass and dandelion for the mice to eat, which pushes them up. A good answer says both and says which you think wins, or says the effect is hard to predict because the two effects oppose each other.

Fox: less food, numbers may fall, but it switches prey. Grass and dandelion: less grazing, so both increase. Field mouse: eaten more by foxes but has more plant food, so the net effect is uncertain.

The phrase that turns a description into an explanation

Never write “the fox decreases” on its own. Write “the fox decreases because it has lost a source of food”. Marks in this topic are almost always for the because, and the statement without it is worth nothing. If you can add a number from a graph or a table as well, do it — a data mark is the cheapest mark on the paper.

What Humans Do to Food Webs

The syllabus asks you to use food chains and webs to describe the impact humans have in two specific ways. Only these two belong to this topic.

Overharvesting of food species

Overharvesting means taking a species out of an ecosystem faster than it can reproduce and replace itself, so its population falls. The classic case is fishing: catching more cod each year than the number of young cod reaching adulthood. Trace it through the web and the marks come easily. Fewer cod means less food for whatever ate cod, so those numbers fall. Fewer cod also means less predation on whatever the cod ate, so those numbers rise — and if the cod ate the animals that graze on plankton, the plankton may fall as well. One removal, effects in both directions, exactly as before.

Introducing foreign species to a habitat

A species brought into a habitat where it has never lived may arrive with no natural predators in that place, so nothing controls its numbers and its population grows without check. It then competes with the native species for food, for space or for light, and being unchecked it often out-competes them; or, if it is a predator, it eats native species that have never evolved any defence against it. The best-known example is the cane toad in Australia, introduced to eat beetles in sugar-cane fields: it is poisonous to the native predators that try to eat it, so its numbers exploded and the predators declined.

Answering “suggest why the introduced species increased so rapidly”

Three ideas, and you will usually need two of them: no natural predators in the new habitat; plenty of food and space, so little competition at first; and the native species have no defence or resistance against it. Do not answer “because it is stronger” — that explains nothing and mark schemes reject it.

Worked example A lake contains algae, water fleas that eat the algae, and small native fish that eat the water fleas. A large predatory fish is introduced, and it eats both the native fish and the water fleas. Within five years the native fish have almost disappeared, and the water in the lake has turned green. Explain, using the food chain, why the water turned green. [3]

Step 1 — write the chain out before you reason

algae → water flea → native fish → introduced fish. Add a second arrow, water flea → introduced fish, because the stem says the predator eats water fleas too. Now every step of the argument has somewhere to sit.

Step 2 — follow the arrows that touch the algae

The algae are eaten only by the water fleas. The introduced fish eats water fleas, so there are fewer water fleas. With fewer water fleas, fewer algae are eaten. The algae reproduce faster than they are removed, so their population rises and the water turns green.

Step 3 — check the other route as well

The predator also eats the native fish, and fewer native fish would leave more water fleas. The stem tells you which effect won: the water turned green, so the water fleas must have fallen overall. Direct predation on the fleas was stronger than the release from the native fish.

Step 4 — the version where it does NOT eat water fleas

If the introduced fish ate only the native fish, the native fish would fall, so fewer water fleas would be eaten. The water fleas would rise, more algae would be eaten, and the water would become clearer. The same introduction gives the opposite result depending on one arrow. That is why you must draw the chain exactly as the question gives it, and never add an arrow the stem did not give you.

The introduced fish eats the water fleas, so their numbers fall; fewer algae are eaten; the algae increase, so the water turns green. Marks are for: predator reduces the grazer, less grazing on algae, algae increase.

Check Yourself: 19.2 Food Chains, Webs and Trophic Levels

12 multiple choice questions. Click an option to check your answer.

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Question 1

What does the arrow in a food chain represent?

A the direction in which the predator moves to find its prey

B the direction in which nutrients are recycled

C eats — it points from the animal to its food

D is eaten by — the direction in which energy is transferred

“Eats”, pointing from the animal to its food, is the reversed-arrow misconception and it is the commonest single error in this topic; it looks reasonable, which is exactly why it is dangerous. The arrow always runs from the organism that is eaten to the organism that eats it, because that is the way the energy goes. The nutrient-recycling option describes a completely different diagram.

Question 2

In the chain oak leaf → caterpillar → blue tit → sparrowhawk, what is the blue tit?

A a primary consumer at the second trophic level

B a secondary consumer at the third trophic level

C a tertiary consumer at the third trophic level

D a producer

Count organisms from the left: oak leaf 1, caterpillar 2, blue tit 3. The third organism is the secondary consumer — the numbering of consumers always runs one behind the trophic level, because the producer takes level 1. Option C is what you get if you count arrows instead of organisms, which is the standard slip.

Question 3

Which is the correct definition of a producer?

A an organism that produces energy from sunlight

B any green plant living in a habitat

C an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis

D an organism that takes in nutrients from the soil and grows

Learn C word for word; every part of it is doing something. A produces energy, which is never allowed. B is a description of the commonest example rather than a definition, and it leaves out algae and some bacteria. D is the “plants get their food from the soil” misconception — roots take up water and mineral ions, not food.

Question 4

A student writes the chain fox → rabbit → grass. What is wrong with it?

A nothing — it correctly shows that the fox eats the rabbit, which eats the grass

B the chain is too short to be valid

C the arrows are reversed and the chain does not begin with a producer

D grass is a consumer, so it cannot be at the end

Two rules broken at once, and each is worth a mark. Read it with “is eaten by”: “fox is eaten by rabbit is eaten by grass”. A is exactly the reasoning that produces the error — the student is thinking “eats”. Three-organism chains are perfectly valid, so B is irrelevant.

Question 5

What is a food web?

A a network of interconnected food chains

B a diagram showing all the organisms in a habitat and where they live

C a food chain that contains more than four trophic levels

D a chart showing how much energy each organism contains

A is the syllabus definition and it is only five words, so there is no excuse for losing it. B describes a habitat map, which shows position rather than feeding. D describes a pyramid of energy.

Question 6

In a web, a hawk eats voles (which eat grass) and also eats stoats (which eat voles). Which statement is correct?

A the hawk must be a quaternary consumer in both chains

B the hawk can only be counted once, so it belongs to the higher level

C the hawk is a secondary consumer in one chain and a tertiary consumer in the other

D the hawk has no trophic level because it feeds at two

grass → vole → hawk makes it secondary; grass → vole → stoat → hawk makes it tertiary. Both are correct and an examiner expects you to say so, naming the chain each time. B invents a rule that does not exist, and D throws away a straightforward answer.

Question 7

Sea otters eat sea urchins; sea urchins graze on kelp, a large seaweed. Otters are hunted until almost none remain. What is the most likely result?

A urchins decrease and kelp decreases

B urchins increase and kelp decreases

C urchins increase and kelp increases

D neither is affected, because kelp is a producer

Go down one level at a time. Fewer otters means less predation on urchins, so urchins increase; more urchins means more grazing on kelp, so kelp decreases. The trap is C, which comes from thinking “removing a predator is good for everything below it” — the effect alternates as you go down, and the number of steps decides the direction.

Question 8

Which best explains why an introduced species may increase rapidly in a new habitat?

A it has no natural predators in the new habitat, so nothing controls its numbers

B introduced species are naturally stronger than native species

C it reproduces asexually, unlike native species

D it has adapted itself to the new habitat within one generation

“No natural predators” is the mark scheme answer, usually alongside “plenty of food and space, so little competition”. The “naturally stronger” answer rests on the empty word “stronger”, which explains nothing and scores nothing. The asexual-reproduction answer is an invented fact. Adapting within one generation is the misconception that an individual organism adapts itself — populations become adapted over many generations, and that takes far longer than an invasion.

Question 9

What is meant by overharvesting?

A harvesting a crop before it is fully ripe

B feeding livestock more than they need

C introducing too many of one species into a habitat

D removing individuals of a species faster than the population can replace them by reproduction

The definition hinges on rate: harvesting is only over-harvesting when it outruns reproduction, which is why a well-managed fishery can be harvested for ever. Putting too many of one species into a habitat describes introducing a species, the other human impact named in this sub-topic; do not swap the two.

Question 10

Which organism in a food web would be described as a decomposer?

A a fungus growing on a fallen log

B a vulture that eats the flesh of animals killed by lions

C a caterpillar eating a living leaf

D algae in a pond

A fungus on dead wood is feeding on dead organic material, which is the definition. This question also kills the idea that decomposers are only bacteria. The vulture is a fair argument — it does eat dead material — but at IGCSE it is treated as a consumer that eats whole tissues rather than a decomposer breaking material down externally; the fungus is unambiguously right, so choose it.

Question 11

Read this description: “Phytoplankton drift near the surface. Krill filter them from the water. Squid eat krill, and sperm whales dive to feed on the squid.” Which food chain is correct?

A sperm whale → squid → krill → phytoplankton

B sunlight → phytoplankton → krill → squid → sperm whale

C krill → phytoplankton → squid → sperm whale

D phytoplankton → krill → squid → sperm whale

Producer first, arrows meaning “is eaten by”. The chain that begins with the sperm whale is reversed. The one that begins with krill puts a consumer first. The one that begins with sunlight is the tempting one: the Sun really is the energy source, but it is not an organism, so it never appears in a food chain — a food chain begins with a producer.

Question 12

In the grassland web on this page, all the shrews are removed. Which single statement is the best-supported prediction?

A the grasshopper population will fall, because it has lost a predator

B the kestrel will eat more field mice and thrushes, so their numbers may fall

C the grass will decrease, because there are fewer organisms in the web

D nothing will change, because the shrew is not a top predator

B is the sideways step, and it is the one that separates a three-mark answer from a one-mark answer: the shrew’s predators switch to their other prey. A gets the direction backwards — losing a predator lets the grasshoppers rise. C has the grass falling when in fact more grasshoppers means more grazing, so the grass falls for a quite different reason than the one given; the stated reasoning is nonsense. D ignores the whole idea of a web.

19.3 Ecological Pyramids and Energy Efficiency ▼

Three Pyramids, Three Different Questions

A food chain tells you who eats whom. A pyramid tells you how much. All three pyramids are drawn the same way — producers at the bottom, one horizontal bar per trophic level, bar width proportional to the quantity — and they differ only in what is being measured.

Pyramid of…	measures	can it be the wrong shape?
numbers	the number of organisms at each trophic level	Yes, often. Size is ignored, so one oak tree counts the same as one aphid.
biomass	the dry mass of living material at each trophic level	Rarely, but it can — it is a snapshot of one moment in time.
energy Supplement	the energy in each trophic level, over a period of time (kJ per m² per year)	No. Never. This is the whole point of it.

Pyramid of Numbers

Count the organisms at each level and draw a bar for each. Usually it comes out looking like a pyramid, because it takes a lot of grass plants to feed a rabbit and a lot of rabbits to feed a fox.

The shape everyone expects: fewer organisms at every step up.

Now the case Cambridge asks about every single time. Change the producer from grass to a single oak tree.

The standard inverted case. One large producer supports thousands of small consumers, so the bottom bar is the narrowest.

Say “one oak tree” and you have the mark

The question is always some form of “explain why this pyramid of numbers is not pyramid-shaped”, and the answer is always some form of “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”. Both halves are needed: the rule (size is ignored) and the instance (one big producer, many small consumers). A bare “because the tree is big” is half an answer.

There is a second awkward case worth recognising: a small organism living on a large one. Aphids on a rose bush give exactly the same shape.

The same problem from the other end — many small feeders on one large individual.

Pyramid of Biomass — and Why It Is Better

Biomass is the mass of living material, and it is measured as dry mass, with the water removed, because water content varies enormously and carries no energy. Now redraw the oak-tree chain measuring biomass instead of numbers.

Same chain, same organisms, different measurement — and the shape comes right, because the one huge oak now counts as huge.

That is the advantage, and it is the answer to “discuss the advantages of a pyramid of biomass over a pyramid of numbers”: a pyramid of biomass takes account of the size of the organisms, so it is almost always the correct pyramid shape, and it gives a much better idea of how much living material each level actually supports.

Supplement

…but a pyramid of biomass can still be inverted

Do not let anyone tell you it cannot. Picture the open ocean. The producers are microscopic phytoplankton with very short lives; they are eaten almost as fast as they reproduce. If you go out on a Tuesday and measure the biomass present at that moment, you may find less phytoplankton than zooplankton, and the pyramid comes out upside down — even though, over the whole year, the phytoplankton produced far more material than the zooplankton ever did.

The reason is that biomass is a snapshot. It measures what is standing there right now, not what has been produced over time. That is precisely the problem the pyramid of energy solves.

Supplement

Pyramid of Energy

A pyramid of energy shows the energy contained in each trophic level, measured over a period of time — usually kJ per square metre per year. Because it counts everything produced across the whole year rather than what happens to be alive on one afternoon, the fast turnover of the phytoplankton is included, and the shape always comes out right.

Here is the same chain drawn both ways so the difference is visible.

Biomass: dry mass standing in one square metre at one moment.

Energy: kilojoules passing through the same square metre across a whole year.

Read the energy figures across: 20 000, then 1600, then 160, then 24 kJ per m² per year. Work out the transfers and you can see the topic in three numbers. 1600 ÷ 20 000 = 8 %. 160 ÷ 1600 = 10 %. 24 ÷ 160 = 15 %. Small numbers, and different at every step — which is exactly why you should never write “10 % is transferred” as though it were a law. Use the numbers you are given.

The advantages of a pyramid of energy over the other two: it can never be inverted, because energy is always lost between levels and a level can never contain more energy than the one below it supplied; it takes account of the rate at which material is produced rather than what is standing there at one moment; and it lets you calculate the efficiency of each transfer. The drawback, worth a mark if you are asked to evaluate it, is that the data are extremely difficult and slow to collect — you have to measure energy over a whole year.

Where All the Energy Goes

You already met the losses in 19.1. Here they are with numbers, because “explain why the transfer of energy from one trophic level to another is often not efficient” is a Supplement objective and it wants specifics.

One thousand kilojoules eaten; eighty kilojoules available to the next level.

Supplement

The four reasons, in the order a mark scheme lists them

Not all of the organism is eaten. Roots, bark, bones, hooves, teeth and horns are usually left. That energy goes to decomposers instead.
Not all of what is eaten is digested and absorbed. Cellulose in particular passes through, so its energy leaves in the faeces.
Energy is released in respiration and transferred to the surroundings as heat. This is the biggest loss of all, and it is much bigger in birds and mammals, which keep their body temperature constant and therefore respire a great deal simply to stay warm.
Some energy is lost in excretion — urea in the urine, produced by deamination of the amino acids the animal could not use.

Only what is left is built into new tissue, and only new tissue can be eaten. That is why the transfer is inefficient.

Supplement

Why food chains rarely have more than four or five trophic levels

Follow the arithmetic. Start with 20 000 kJ in the producers and lose roughly nine tenths of it at each step: 20 000 → 1600 → 160 → 24. A fifth level would be left with two or three kilojoules per square metre per year.

Now say what that means, because that is the mark: there is not enough energy left at the top to support a viable population of another predator. A fifth-level carnivore would have to eat an enormous area’s worth of prey simply to stay alive, and it would spend more energy hunting than it gained. So the chain stops.

Two ways to lose this mark

“There would be no energy left.” Too absolute — there is some, just not enough. Write “too little energy to support another trophic level”.

“The animals would be too big to feed.” That is not the reason and it is not always true. The reason is the total energy available, not body size.

Supplement

Why it is more energy-efficient for humans to eat crops than livestock

This is the same idea turned into an applied question, and it is asked often.

Every extra trophic level costs you an entire round of losses.

The chain wheat → human has one transfer. The chain wheat → cattle → human has two, and at the extra step the cattle lose energy in respiration, in faeces, in urine, and in the parts of the animal nobody eats. So a given area of farmland feeds far more people when the crop is eaten directly.

Two refinements that pick up the higher marks. First, name the loss: it is not vague inefficiency, it is respiration and heat loss in the cattle, undigested material in their faeces, and inedible parts such as bone. Second, be fair about the other side, because “discuss” questions want it: some land is too poor, too steep or too dry to grow crops but will support grazing animals, and meat and milk supply protein and vitamins in a concentrated form. The energy argument is not the only argument — but it is the one the question is asking about.

Drawing a Pyramid to Scale

“Draw” is a syllabus verb here, which means you can be asked to produce one, and the marks are mechanical. Get these five things right and you cannot lose them.

Producers at the bottom, one horizontal bar per trophic level, stacked upwards in order.
Bars centred on a vertical line, so the pyramid is symmetrical.
Width proportional to the quantity, using a sensible scale you choose and then keep. If 200 rabbits is 4 cm, then 5000 grass plants is 100 cm — which will not fit, so pick a scale that works for the biggest bar first and then draw the others from it.
All bars the same height. The height carries no meaning at all; only the width does.
Label every bar with the organism and, if you are given them, the numbers.

If one value is so much larger than the others that a true scale is impossible, say so on the drawing — write “not to scale” and give the figures. An examiner would far rather see an honest note than a pyramid that quietly lies.

Worked example In a meadow, the producers contain 12 000 kJ m⁻² y⁻¹, the primary consumers 900 kJ m⁻² y⁻¹ and the secondary consumers 63 kJ m⁻² y⁻¹. (a) Calculate the percentage of energy transferred from producers to primary consumers. (b) State, with a reason, whether the transfer to the secondary consumers was more or less efficient. [4]

(a) Show the division

900 ÷ 12 000 × 100 = 7.5 %. Write the fraction down even if you can do it in your head — the working carries a mark if the arithmetic slips.

(b) Do the second one before you judge

63 ÷ 900 × 100 = 7 %. So the second transfer was slightly less efficient.

Give a reason, not just a comparison

Secondary consumers are carnivores and generally more active hunters, so a larger proportion of their energy goes into movement and is lost as heat in respiration. A perfectly acceptable alternative reason: 7 % and 7.5 % are close enough that the difference may not be significant — and saying so, with the figures, is a mature answer that examiners reward.

(a) 7.5 %. (b) 7 %, so marginally less efficient — more energy lost as heat through the activity of hunting, though the two figures are very close.

Check Yourself: 19.3 Pyramids and Energy Efficiency

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Why can a pyramid of numbers for the chain oak tree → insect → small bird be inverted at the base?

A because a pyramid of numbers ignores size, and one very large producer supports very many small consumers

B because the oak tree contains less energy than the insects

C because insects reproduce faster than oak trees

D because energy flows upwards through the pyramid

The “ignores size” answer contains the rule and the instance, which is what a two-mark version of this question wants. Less energy in the oak tree than in the insects is impossible: the tree must contain more energy, or it could not support the insects — that is why a pyramid of energy for this chain is the right shape. Insects reproducing faster is true but is not the reason the bar is narrow; the bar is narrow because there is literally one tree.

Question 2

What is measured in a pyramid of biomass?

A the number of organisms at each trophic level

B the volume occupied by the organisms at each trophic level

C the energy transferred per year at each trophic level

D the dry mass of living material at each trophic level

“Dry” is the word that earns the mark: water is removed because its content varies wildly between organisms and it contains no energy. Energy transferred per year is a pyramid of energy, and confusing the two is the commonest mix-up in this section.

Question 3

Which pyramid can never be inverted, and why?

A numbers, because there are always fewer predators than prey

B biomass, because mass cannot increase up a chain

C energy, because energy is lost at every transfer, so a level can never hold more than the level below supplied

D all three, provided the data are collected accurately

Only the energy pyramid is guaranteed, and the guarantee comes from the losses. B is the trap: biomass usually behaves, but because it is a snapshot at one moment it can be inverted, most famously in open ocean plankton. A is simply false, as the oak-tree pyramid shows.

Question 4

A pyramid of biomass for a stretch of open ocean is inverted, with less phytoplankton than zooplankton. What is the best explanation?

A biomass is measured at one moment, and the phytoplankton reproduce and are eaten so fast that little is standing there at any instant

B the zooplankton must be producers as well as consumers

C phytoplankton have a much higher water content, which reduces their dry mass

D the measurement must be an error, because a pyramid of biomass cannot be inverted

The key word is snapshot: high turnover means low standing biomass even though total production over the year is enormous. The water-content answer sounds plausible but biomass is already measured dry, so water content has been removed. “The measurement must be an error” asserts a rule that does not exist.

Question 5

Producers contain 20 000 kJ m⁻² y⁻¹ and primary consumers 1600 kJ m⁻² y⁻¹. What percentage of the energy was transferred?

A 12.5 %

B 8 %

C 10 %

D 80 %

1600 ÷ 20 000 × 100 = 8 %. Option C is the dangerous one: people answer “10 %” from memory without dividing, because they have been told energy transfer is “about 10 %”. It is a rough generalisation, not a rule, and the data in front of you always win. Option A divides the wrong way round.

Question 6

Which is not a reason why energy transfer between trophic levels is inefficient?

A energy is released in respiration and transferred to the surroundings as heat

B parts of the organism, such as bone and bark, are not eaten

C some of the energy is destroyed as it passes between organisms

D some material is not digested and leaves in the faeces

Energy is never destroyed — it is transferred into a form (heat) that nothing can feed on. Writing “destroyed” or “used up” in an answer will cost you the mark even when the rest of the sentence is right, so train yourself out of the word now.

Question 7

Why do food chains rarely have more than four or five trophic levels?

A because there are not enough species of predator in most habitats

B because decomposers take the remaining energy before it can be passed on

C because animals higher up the chain are too large to catch enough prey

D because so much energy is lost at each transfer that too little remains to support another level

The whole argument is energy. Write “too little energy to support a further trophic level” rather than “no energy left” — there is some, it is simply not enough for a viable population. Decomposers taking the remaining energy describes something real that happens to dead material, but it is not why the chain stops.

Question 8

A hectare of land produces 10 000 kJ of wheat. Route A: humans eat the wheat. Route B: cattle eat the wheat and humans eat the cattle. Assume 10 % is transferred at each step. How much energy reaches humans by each route?

A A: 10 000 kJ, B: 1000 kJ

B A: 1000 kJ, B: 100 kJ

C A: 1000 kJ, B: 900 kJ

D A: 9000 kJ, B: 8100 kJ

Each transfer keeps a tenth: 10 000 → 1000 → 100. Option A forgets that eating the wheat is itself a transfer with losses. Option D subtracts 10 % instead of keeping 10 %, which is the misreading to watch for — “10 % is transferred” means 90 % is lost.

Question 9

Which statement about drawing a pyramid to scale is correct?

A the width of each bar should be proportional to the quantity, and all bars should be the same height

B the height of each bar should be proportional to the quantity

C the area of each bar should be proportional to the quantity, with the width kept constant

D the bars should be drawn in order of size, largest at the bottom

Width carries the meaning; height carries none. Drawing the bars in order of size is subtly but importantly wrong: bars go in trophic order, producers at the bottom, and if that produces an odd shape then the odd shape is the information.

Question 10

A bullock eats 1000 kJ of grass and stores 80 kJ as new tissue. What is the efficiency of this transfer, and where did most of the rest go?

A 8 %; most was lost as heat from respiration

B 12.5 %; most was used for growth

C 80 %; most was lost in urine

D 8 %; the single largest loss is usually the undigested material in the faeces, with respiration next

80 ÷ 1000 = 8 %. In the worked figures on this page the faeces take 600 kJ and respiration 280 kJ, so for a grass-eating herbivore the undigested fraction really is the biggest single loss — grass is mostly cellulose. “Most was lost as heat from respiration” is a reasonable general statement but the data on this page say otherwise, and data always beat generalisations.

Question 11

Which is a genuine disadvantage of using a pyramid of energy?

A it can be inverted, which makes it hard to interpret

B it takes no account of the size of the organisms

C the data are difficult and slow to collect, because energy must be measured over a long period

D it cannot be used for aquatic habitats

This is the mark that separates “discuss” from “state”: the energy pyramid is the best of the three biologically and the worst of the three practically. A and B describe the other two pyramids, and D is invented — pyramids of energy are used for aquatic habitats constantly, since that is where the biomass pyramid fails.

Question 12

Two farms have the same area. Farm A grows soya for people to eat. Farm B grows soya and feeds it to pigs, which people eat. Why can farm A feed more people?

A because soya contains more energy per kilogram than pork does

B because farm B has an extra trophic level, at which energy is lost in respiration, faeces, urine and inedible parts

C because pigs compete with humans for space on the farm

D because plants transfer energy more efficiently than animals do at every stage

Naming the losses is what turns this from a one-mark answer into a three-mark one. A is not the argument and is not reliably true per kilogram. D is a vague half-truth: the point is not that plants are efficient, it is that farm B has one extra transfer with all the usual losses attached to it.

19.4 The Carbon and Nitrogen Cycles ▼

▶ Watch: The Carbon and Nitrogen Cycles

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

The Carbon Cycle — Six Processes, No More

Cambridge is unusually specific here. The carbon cycle is examinable limited to: photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion. Six processes. Learn those six, be able to say where each one puts carbon and where it takes it from, and you have the whole objective.

Every arrow labelled in amber is one of the six named processes.

Process	Takes carbon from	Puts carbon into
Photosynthesis	carbon dioxide in the air	carbon compounds (glucose, starch, cellulose) in plants — the only arrow taking carbon out of the air
Respiration	carbon compounds in any living organism — plant, animal or decomposer	carbon dioxide in the air
Feeding	carbon compounds in plants	carbon compounds in animals
Decomposition	carbon compounds in dead organisms and waste	the decomposers, which then respire and release carbon dioxide
Formation of fossil fuels	dead organisms that did not decompose	coal, oil and natural gas, over millions of years
Combustion	fossil fuels (and wood)	carbon dioxide in the air

The one-in, three-out rule

Only one process takes carbon dioxide out of the air: photosynthesis. Three put it back: respiration, combustion, and decomposition (through the respiration of the decomposers). If you are asked to label a blank carbon-cycle diagram, find the single arrow pointing away from the air first — that one must be photosynthesis, and everything else falls into place around it.

A detail worth understanding rather than memorising: why do fossil fuels exist at all? Because some organisms died in conditions where decomposers could not work — deep water, waterlogged ground, no oxygen. Without decomposition the carbon compounds were not broken down; they were buried, compressed and heated over millions of years, and became coal, oil and gas. Combustion is the only thing that brings that carbon back, which is why fossil fuel carbon sat out of the cycle for so long.

Worked example A carbon atom is in a molecule of carbon dioxide in the air. Describe a route by which the same atom could end up in a molecule of protein in a fox, naming the processes involved. [4]

Step 1 — get it out of the air

There is only one way: the carbon dioxide is absorbed by a plant and the carbon atom is built into glucose by photosynthesis.

Step 2 — get it into the first animal

The plant uses the glucose to make amino acids and then plant proteins. A rabbit eats the plant — feeding — and digests the protein to amino acids, which it absorbs.

Step 3 — get it into the fox

The rabbit builds those amino acids into rabbit protein. A fox eats the rabbit — feeding again — digests the protein, absorbs the amino acids and assembles them into fox protein.

Step 4 — say it in the right order and name every process

Marks here are for the named processes and the correct sequence. Do not stray into deamination or urea unless asked; the question stopped at protein in the fox, so stop there too.

Photosynthesis fixes the carbon into glucose in the plant; the plant makes protein; feeding and digestion transfer it to the rabbit, which builds rabbit protein; feeding and digestion transfer it to the fox, which builds fox protein.

Supplement

The Nitrogen Cycle

Every organism needs nitrogen, because proteins contain nitrogen and nothing lives without proteins. Almost 78 % of the air is nitrogen gas — and no plant or animal can use a single molecule of it. Nitrogen gas is remarkably unreactive, and the whole nitrogen cycle exists to get around that one problem.

Follow the nitrogen atom round: air, soil, plant, animal, soil, air.

The syllabus names eight steps. Here they are in the order that makes them easiest to hold, with the point of each one.

Step	What happens	Done by
Nitrogen fixation	nitrogen gas from the air is converted into a usable compound. Lightning supplies enough energy to make nitrogen react with oxygen, which ends up in the soil as nitrate ions. Nitrogen-fixing bacteria, some free in the soil and some in root nodules of plants such as peas, beans and clover, convert nitrogen gas into ammonium ions and amino acids.	bacteria, and lightning
Decomposition	proteins in dead organisms, in faeces and in urine are broken down, releasing ammonium ions into the soil.	decomposers (bacteria and fungi)
Nitrification	ammonium ions are oxidised to nitrite ions and then to nitrate ions. This needs oxygen, which is why a well-drained soil is a fertile soil.	nitrifying bacteria
Absorption	nitrate ions are absorbed from the soil by plant roots — by active transport, against the concentration gradient, using energy from respiration.	plants
Making proteins	the plant combines nitrate ions with carbohydrate to make amino acids, and joins those amino acids into plant proteins.	plants
Feeding and digestion	an animal eats the plant, digests the protein to amino acids, absorbs them, and builds them into its own proteins.	animals
Deamination	amino acids cannot be stored. Excess ones are broken down in the liver: the nitrogen-containing part is removed and converted to urea, which is excreted in the urine and returns to the soil.	animals (in the liver)
Denitrification	nitrate ions are converted back into nitrogen gas, which is lost to the air. This happens in waterlogged soils, where there is little oxygen.	denitrifying bacteria

Four bacterial jobs, and only four

The syllabus limits the roles of microorganisms to decomposition, nitrification, nitrogen fixation and denitrification. Four words. You are not required to know the genus names of any of them, so do not spend a minute learning any.

And keep the two opposites apart, because swapping them is the classic error. Nitrogen fixation takes nitrogen out of the air and puts it into the soil, which helps plants. Denitrification takes nitrate out of the soil and puts it back into the air, which does not. Fixation is the way in; denitrification is the way out.

Why farmers plough and drain their fields

This is the applied question and it is worth learning as a pair. Draining and ploughing let air into the soil. More oxygen means nitrifying bacteria work faster, producing more nitrate; and it means denitrifying bacteria work more slowly, because they only thrive where oxygen is short. Both effects push in the same direction: more nitrate in the soil, so more protein, so better growth.

The same logic run backwards answers “why is a waterlogged field unproductive”: less oxygen, so less nitrification and more denitrification, so less nitrate available to the roots.

Worked example A farmer grows wheat in a field for several years and the yield falls steadily. In the next year he grows clover instead, ploughs it into the soil at the end of the season, and the wheat yield the following year is higher than it has been for a decade. Explain, using the nitrogen cycle. [4]

Step 1 — why the yield fell in the first place

Each wheat harvest removes the plants, and with them the nitrogen. Nothing is returned to the soil to decompose, so nitrate ions are steadily used up and not replaced.

Step 2 — what clover does

Clover has root nodules containing nitrogen-fixing bacteria. These convert nitrogen gas from the air into compounds the plant can use, so the clover grows well even in a soil low in nitrate, and its tissues become rich in protein.

Step 3 — why ploughing it in matters

The clover is not harvested but returned to the soil, where decomposers break its protein down to ammonium ions. Nitrifying bacteria then convert those to nitrate ions.

Step 4 — close the loop

The soil now contains far more nitrate for the wheat roots to absorb, so the wheat can make more amino acids and proteins, so it grows more and the yield rises.

Harvesting removed nitrogen; clover fixes nitrogen from the air using bacteria in its root nodules; ploughing it in lets decomposers and nitrifying bacteria convert that nitrogen into nitrate; the wheat absorbs the nitrate and makes more protein.

Check Yourself: 19.4 The Carbon and Nitrogen Cycles

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Which process removes carbon dioxide from the air?

A respiration

B combustion

C photosynthesis

D decomposition

Photosynthesis is the only one of the six named processes with an arrow pointing away from the air, which is why it is the first thing to find on a blank diagram. The other three all put carbon dioxide back — decomposition indirectly, through the respiration of the decomposers themselves.

Question 2

Carbon in a dead tree becomes carbon dioxide in the air. Which pair of processes is responsible?

A decomposition and respiration

B photosynthesis and feeding

C nitrification and combustion

D feeding and excretion

Decomposers feed on the dead wood and then respire, and it is their respiration that releases the carbon dioxide. Saying “decomposition” alone is worth a mark but the pair is worth two, because decomposition on its own does not put a gas into the air. Nitrification belongs to a different cycle altogether.

Question 3

Why did fossil fuels form from some dead organisms and not others?

A because those organisms contained more carbon than others

B because they were burned before they could decompose

C because they died in conditions where decomposers could not break them down, and were then buried and compressed over millions of years

D because plants form fossil fuels and animals decompose

The key condition is the absence of decomposition, usually because there was little or no oxygen — deep water or waterlogged ground. “Burned before they could decompose” reverses the timeline: combustion is what releases the carbon later. “Plants form fossil fuels and animals decompose” invents a rule; both plant and animal remains contributed.

Question 4

Which of these is not one of the six processes Cambridge names in the carbon cycle?

A nitrogen fixation

B feeding

C combustion

D formation of fossil fuels

The six are photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion. Nitrogen fixation belongs to the nitrogen cycle. Knowing the list is worth doing precisely, because “describe the carbon cycle” is marked against exactly these six.

Question 5

What do nitrogen-fixing bacteria do?

A convert nitrate ions in the soil into nitrogen gas

B convert nitrogen gas from the air into compounds that plants can use

C convert ammonium ions into nitrite and then nitrate ions

D break down protein in dead organisms into ammonium ions

Option A is denitrification and it is the classic swap — the two processes run in opposite directions, so getting them the wrong way round turns a correct-sounding answer into a wrong one. C is nitrification and D is decomposition. All four of the bacterial jobs are in this question; be able to name each from its description.

Question 6

Which form of nitrogen do plant roots absorb?

A nitrogen gas

B protein

C nitrate ions

D urea

Nitrate ions, taken up by active transport against the concentration gradient. Option A is the misconception the entire cycle exists to correct: nitrogen gas is all around a plant and completely useless to it. Urea and protein must be decomposed to ammonium and then nitrified before a root can do anything with them.

Question 7

Why does a waterlogged soil usually contain less nitrate than a well-drained one?

A because there is little oxygen, so nitrifying bacteria are less active and denitrifying bacteria are more active

B because nitrate ions dissolve in water and float to the surface

C because plants absorb nitrate faster when the soil is wet

D because decomposers cannot survive in wet conditions at all

Both halves of the answer about nitrifying and denitrifying bacteria matter and both are about oxygen: nitrification needs it, denitrification thrives without it, so a waterlogged soil loses on both counts. “Cannot survive in wet conditions at all” overstates the case — decomposition is slowed, not abolished. This is also the reason farmers plough and drain.

Question 8

Where does deamination take place, and what is produced?

A in the kidneys; nitrate ions are produced

B in root nodules; amino acids are produced

C in the soil; ammonium ions are produced

D in the liver; urea is produced and later excreted in the urine

Liver, urea — and the kidneys then remove that urea from the blood, which is why the kidneys answer is such a tempting near-miss. Deamination is the point where Topic 13 joins the nitrogen cycle, and Cambridge names it explicitly in this sub-topic, so it is fair game in a 19.4 question.

Question 9

Clover is grown in a field and ploughed back into the soil instead of being harvested. Why does this improve the soil for the next crop?

A the clover releases nitrogen gas into the soil as it rots

B nitrogen fixed by bacteria in its root nodules is returned to the soil, decomposed to ammonium and nitrified to nitrate

C the clover roots absorb nitrogen gas directly from the air in the soil

D ploughing kills the denitrifying bacteria completely

Three named steps in B — fixation, decomposition, nitrification — which is exactly how the marks are allocated. C credits the plant itself with fixing nitrogen; it is the bacteria in the nodules that do it, and the relationship benefits both. A produces the wrong gas in the wrong direction.

Question 10

Which sequence correctly shows nitrification?

A nitrogen gas → ammonium ions → nitrate ions

B protein → amino acids → urea

C nitrate ions → nitrite ions → nitrogen gas

D ammonium ions → nitrite ions → nitrate ions

Nitrification starts from ammonium and works upwards through nitrite to nitrate. The sequence that begins with nitrogen gas has smuggled fixation into the front of it. The one that ends with nitrogen gas is denitrification running the other way. Protein to urea is deamination.

Question 11

A carbon atom is in a molecule of glucose in a leaf. Which route could return it to the air fastest?

A the leaf falls, decomposes, and the decomposers respire

B the plant respires the glucose itself

C the leaf is eaten, the animal dies, and its remains form a fossil fuel that is later burned

D the glucose is converted to cellulose and stored in the cell wall

The plant is respiring right now, day and night, so B can return that atom within minutes. A works but needs the leaf to fall and decompose first. C is the slowest route in the whole cycle — millions of years. D is not a return route at all: it locks the carbon up in the plant.

Question 12

Which statement about the two cycles is correct?

A in both cycles, the energy as well as the atoms is recycled

B plants can absorb both carbon dioxide and nitrogen gas directly from the air

C the carbon cycle recycles atoms but the nitrogen cycle does not

D in both cycles the atoms are used again and again, while the energy that travelled with them leaves as heat

The answer about atoms being used again and again is the sentence that links these cycles back to 19.1 and it is worth being able to write from memory. Absorbing nitrogen gas directly from the air is the misconception the nitrogen cycle exists to correct: carbon dioxide yes, nitrogen gas absolutely not. Saying only the carbon cycle recycles atoms invents a difference that is not there — both are nutrient cycles and both go round.

19.5 Populations and the Sigmoid Curve ▼

▶ Watch: Populations and the Sigmoid Growth Curve

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

Three Definitions That Nest Inside Each Other

These three are asked as one-mark recall questions and they are free marks, provided you keep the qualifying phrases. Every word in each definition is there to exclude something.

population → community → ecosystem

Population = a group of organisms of one species, living in the same area, at the same time. Three conditions, all needed: rabbits in a field this year are one population; rabbits in that field and rabbits in Scotland are not. Community = all of the populations of different species in an ecosystem. So a community is every living thing, and nothing that is not living. Ecosystem = a unit containing the community of organisms and their environment, interacting together. The extra words are the whole difference from a community — an ecosystem includes the soil, the water, the air and the temperature, and the word “interacting” matters too.

The one-word difference examiners look for

Community and ecosystem are distinguished by the non-living part. If your definition of an ecosystem does not mention the environment, the physical surroundings or the non-living factors, it is a definition of a community and it will not score.

What Controls How Fast a Population Grows

The syllabus limits this to four factors, so learn the four and resist the urge to add more.

Factor	How it acts
Food supply	plentiful food means individuals grow, survive and reproduce, so the population rises. As numbers rise the food per individual falls, and growth slows.
Competition	individuals compete with each other for food, water, space, light, shelter and mates. The more crowded the population, the harder the competition and the more individuals fail to reproduce or die.
Predation	predators remove individuals. A large prey population supports more predators, which then reduce the prey again — so the two rise and fall in a linked cycle, with the predator peak always after the prey peak.
Disease	a dense, crowded population lets a pathogen spread easily from host to host, so disease has a far greater effect at high population density than at low.

Notice what these four have in common

Every one of them gets worse as the population gets bigger. Less food each, more competition, more predators attracted, disease spreading faster. That is not a coincidence — it is the reason a population cannot go on growing for ever, and it is the whole explanation of the shape of the curve you are about to meet.

The Sigmoid Curve

Put a few yeast cells into a flask of nutrient broth, seal it, and count them every couple of hours. Add nothing; remove nothing. The graph you get is S-shaped — sigmoid means S-shaped — and it has four named phases.

Lag, exponential, stationary, death. The names are marks; so are the reasons.

Supplement

Explaining each phase — the part that carries the marks

1. Lag phase

The number barely changes. The organisms are adjusting to their new conditions: taking up water, growing in size, and producing the enzymes they need to use the particular nutrients available. Few of them are dividing yet, and there are so few individuals that even a doubling is invisible on the scale of the graph. It is a phase of preparation, not of failure.

2. Exponential (log) phase

Now the number rises steeply and the graph gets steeper as it goes. There is plenty of food, plenty of space and plenty of oxygen, there is little competition and there is no build-up of toxic waste — so nothing is limiting, the birth rate is far higher than the death rate, and the population doubles in a fixed time. Every organism that divides produces two more that can divide, which is why the line curves upwards rather than running straight.

3. Stationary phase

The line levels off. The population has reached the largest size the environment can support: food is running short, competition is intense, waste products have built up to levels that harm the organisms, and space has run out. Now the crucial sentence: the birth rate equals the death rate. Organisms are still reproducing and still dying, in large numbers — the number simply does not change because the two rates match.

4. Death phase

The line falls. The food supply is exhausted and the accumulated waste products are toxic, so the death rate is now greater than the rate of reproduction, and the population declines. In a sealed flask it will eventually approach zero. In a natural habitat where resources are renewed, the population usually oscillates around the stationary level instead.

The two sentences that decide your mark on the stationary phase

Weak: “The population stops growing because there is no more food.”

Strong: “The rate of reproduction equals the death rate, because food is short, waste has built up and competition is intense, so the population size stays constant.”

The commonest misconception at this phase is that nothing is happening — that reproduction has stopped. It has not. Two large, opposite rates are cancelling, and saying so is what earns the mark.

Worked example Using the yeast graph above: (a) state the population at 10 hours and at 20 hours. (b) Calculate the mean rate of increase between those times, in cells per cm³ per hour. (c) Explain why the rate of increase between 20 and 24 hours is so much lower. [4]

(a) Read the graph, do not estimate loosely

At 10 hours the count is 600 cells per cm³; at 20 hours it is 5000. Read from the plotted point down to the axis with a ruler edge or the edge of your answer booklet.

(b) Rate = change ÷ time

(5000 − 600) ÷ (20 − 10) = 4400 ÷ 10 = 440 cells per cm³ per hour. Give the unit; a naked number can lose a mark on a rate question.

Between 20 and 24 hours the culture is in the stationary phase. The food supply is running short and toxic waste products have accumulated, and competition between the cells is intense, so the death rate has risen to match the rate of division and the number stops increasing.

(a) 600 and 5000 cells per cm³. (b) 440 cells per cm³ per hour. (c) Stationary phase: food short, waste toxic, competition intense, so births and deaths now balance.

What a limiting factor is doing here

In the exponential phase nothing is limiting, so the population grows at its maximum possible rate. From the stationary phase onwards something is — food, space or the concentration of waste — and whichever runs short first sets the ceiling. It is exactly the same logic you met with limiting factors in photosynthesis: the factor in shortest supply controls the rate, and increasing anything else changes nothing.

Check Yourself: 19.5 Populations and the Sigmoid Curve

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Which is the correct definition of a population?

A a group of organisms of one species, living in the same area, at the same time

B all the organisms living in one area

C a group of organisms of one species and their environment

D the number of organisms an area can support

All three conditions are needed — one species, same area, same time. “All the organisms living in one area” is a community. The answer with “their environment” has smuggled the environment in, which makes it closer to an ecosystem. Losing this mark is a waste, because it is pure recall.

Question 2

What is the difference between a community and an ecosystem?

A a community contains only animals; an ecosystem contains plants as well

B a community is smaller than an ecosystem in area

C a community is all the populations of different species; an ecosystem is that community together with its non-living environment, interacting

D an ecosystem contains one species; a community contains many

The whole distinction is the non-living part. B is about size, which is not the difference at all — a rock pool and a rainforest are both ecosystems. If your answer to a definition question does not mention the environment, you have defined a community.

Question 3

Which is not one of the four factors Cambridge names as affecting the rate of population growth?

A the number of trophic levels in the food chain

B predation

C disease

D food supply

The four are food supply, competition, predation and disease. Everything else you can think of — temperature, oxygen, space — is either part of competition or outside the objective. Sticking to the named four keeps your answers on the mark scheme.

Question 4

During which phase of a sigmoid curve is the population growing at its maximum rate?

A lag

B exponential (log)

C stationary

D death

Exponential, because nothing is limiting: food, space and oxygen are all plentiful and waste has not built up. Note the wording — “rate of growth”, not “size”. The population is largest in the stationary phase but growing fastest in the exponential one, and questions swap those two words deliberately.

Question 5

Why does the number of organisms stay constant during the stationary phase?

A because the rate of reproduction equals the death rate

B because the organisms have stopped reproducing

C because no organisms are dying yet

D because the food supply has been completely used up

A flat line means two large rates cancelling, not two rates of zero. “Stopped reproducing” is the classic misconception and it is what most students write. Food that is completely used up belongs to the death phase — if the food were completely gone the number would be falling, not level.

Question 6

Why is there a lag phase at all?

A because there is not enough food at the start

B because predators are present at the start and are removed later

C because the organisms are adjusting to the conditions, growing and producing the enzymes they need before they divide

D because waste products have already built up

At the start there is more food per organism than at any other time, which rules out “not enough food at the start”. The delay is biological preparation — enzyme production above all. Waste that has already built up describes the end of the curve, not the beginning; in a sealed flask nothing has had time to accumulate yet.

Question 7

A population of yeast falls after 26 hours in a sealed flask. Which pair of reasons is correct?

A predators have arrived, and the temperature has fallen

B the yeast cells have grown larger, so fewer fit in the flask

C the yeast has stopped reproducing, and the flask has run out of space

D the food supply is exhausted, and toxic waste products have accumulated

Food out, waste in — those are the two the mark scheme lists for a sealed culture. “Predators have arrived” imports predators into a sealed flask, which is impossible. The answer about stopped reproduction and space is only half right: reproduction has slowed but has not stopped, and running out of space would flatten the curve rather than send it downwards.

Question 8

A population of lynx and a population of hares are counted over 60 years. The two curves rise and fall in a repeating cycle. What would you expect?

A the lynx peak comes before the hare peak

B the lynx peak comes shortly after the hare peak

C the two peaks occur at exactly the same time

D the lynx population is always larger than the hare population

Predator follows prey, always: plenty of hares means the lynx are well fed and rear more young, so the lynx rise after the hares. Then the lynx eat enough hares to reduce them, and the lynx fall in turn. D contradicts everything in 19.3 — there is far too little energy at the top for the predator to outnumber its prey.

Question 9

Why does disease have a greater effect on a dense population than on a sparse one?

A because pathogens reproduce faster in warm weather

B because dense populations have no immune system

C because dense populations contain weaker individuals

D because individuals are closer together, so a pathogen passes from host to host more easily

Transmission is about contact, and contact is about crowding. The “weaker individuals” answer rests on the empty word “weaker” again — though a crowded population is also short of food, and undernourished animals do resist infection less well, which is a fair second mark if you say that rather than “weaker”.

Question 10

A population rises from 200 to 1800 in 8 hours. What is the mean rate of increase?

A 225 per hour

B 200 per hour

C 1600 per hour

D 9 per hour

(1800 − 200) ÷ 8 = 1600 ÷ 8 = 200 per hour. Option A is the standard slip — dividing the final value by the time instead of the change in value. C forgets to divide at all, and D divides the two populations into each other.

Question 11

Rabbits are introduced to an island with grass and no predators. Which curve shape is most likely over the following years?

A a straight line rising for ever

B a flat line, because there are no predators to cause change

C a sigmoid curve, levelling off when food and space become limiting

D a steady fall from the start

Removing predators removes only one of the four factors. Food supply, competition and disease all still act, and all three bite harder as numbers rise, so growth must eventually level off. The straight line rising for ever is the “nothing will stop them” misconception; the flat line treats predation as the only control there is.

Question 12

Which statement about the exponential phase is correct?

A the population increases by the same number of individuals each hour

B the population is at its largest size

C the death rate is zero

D the population doubles in a fixed time, because nothing is limiting growth

Exponential means multiplying by a constant factor, not adding a constant number — which is exactly why the line curves upwards instead of running straight, and why “the same number of individuals each hour” is wrong. “The death rate is zero” overstates it: organisms do die, there are simply far more being produced. “At its largest size” confuses fastest growth with largest size, which happens later.

19.6 Exam Technique and the Vocabulary That Scores ▼

The Six Sentences

If you learn nothing else from this topic, learn these six. Between them they cover the majority of the marks, and each one is a sentence you can write down verbatim.

An arrow in a food chain means “is eaten by” and shows the direction in which energy is transferred.
Nutrients are recycled; energy is not. Energy enters as light and leaves as heat.
A producer makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. (It does not make energy, and it does not get food from the soil.)
A pyramid of numbers can be the wrong shape because it takes no account of the size of the organisms.
Energy is lost between trophic levels in respiration as heat, in undigested material in the faeces, in excretion, and in the parts that are not eaten.
In the stationary phase the rate of reproduction equals the death rate.

Words That Cost Marks, and What to Write Instead

Do not write	Write	Why
“plants make energy”	plants make organic nutrients; the energy is transferred	energy is never created
“energy is used up / destroyed / lost”	energy is transferred to the environment as heat	“lost” alone is often allowed, but “destroyed” never is
“energy is recycled”	nutrients are recycled	the single most heavily penalised error here
“the arrow shows what it eats”	the arrow means is eaten by	reverses every chain you draw
“plants take in food from the soil”	roots absorb water and mineral ions; food is made in the leaves	mineral ions are not food and carry no energy
“10 % is always transferred”	use the figures given; the percentage varies	it is a rough generalisation, not a rule
“a pyramid of biomass can never be inverted”	a pyramid of energy can never be inverted	biomass is a snapshot and can be inverted
“nitrogen-fixing bacteria turn nitrate into nitrogen”	that is denitrification; fixation goes air → soil	the two run in opposite directions
“in the stationary phase nothing is reproducing”	births and deaths are equal	a flat line is a balance, not a stop
“the introduced species was stronger”	it had no natural predators in that habitat	“stronger” explains nothing

How to Read the Command Word

State / name — one word or one short phrase. No explanation, no time spent.
Describe — say what happens. On a graph this means quoting figures: “rises from 200 to 5000 between 4 and 20 hours, then levels off”.
Explain — say why. Every sentence needs a “because”. A description will not score on an explain question.
Suggest — you are being asked to apply what you know to something unfamiliar. There is usually more than one acceptable answer, so a sensible, biologically-argued response will score even if it is not the one on the mark scheme.
Discuss — two sides. Advantages and disadvantages, or the argument and its limits.
Construct / draw — you will produce a diagram, and the layout itself carries marks: producer first or at the bottom, arrows the right way, bars to scale, every part labelled.

A checklist for every graph or table in this topic

1. Read both axis labels and both units before anything else. 2. Note where the graph changes shape, and name the phase or the event. 3. When you describe, quote two figures and the time or place they came from. 4. When you calculate, write the division down. 5. Give the unit in your final answer.

Where Topic 19 Meets the Rest of the Course

Challenge papers on this topic get their difficulty by reaching backwards, so know where the joins are. Photosynthesis (Topic 6) is the energy input, and limiting factors reappear in the sigmoid curve. Respiration (Topic 12) is where the heat is lost, at every level. Nutrition and digestion (Topic 7) explains why so much leaves in the faeces — a herbivore cannot digest cellulose. Active transport (Topic 3) is how a root absorbs nitrate ions against the concentration gradient. Deamination in the liver (Topic 13) is named in the nitrogen cycle by Cambridge itself. And selection (Topic 18) explains why a native species has no defence against a predator it has never met.

🧬 Apply It: Three Situations Worth Thinking Through

Each of these puts Topic 19 up against something you have already learned. Read it, decide what you would write, and only then open the answer.

A sealed glass sphere contains sea water, a few small shrimps, some algae, a little gravel and a bubble of air. It is left on a windowsill in the light. Nothing is ever added to it and nothing is ever taken out, and it stays alive for years.

Explain how this is possible, and state the one thing that must keep entering the sphere.

▼

What is going round

The nutrients. Carbon: the algae photosynthesise, taking carbon dioxide from the water; the shrimps eat the algae; both respire and return carbon dioxide; decomposing bacteria in the gravel break down the waste and dead material and respire too. Nitrogen: the shrimps excrete nitrogen-containing waste, decomposers release ammonium ions, nitrifying bacteria produce nitrate, and the algae absorb it and make protein again. The same atoms are used over and over.

What cannot go round

Energy. Every organism in the sphere respires, and every respiration transfers energy to the surroundings as heat, which leaves through the glass and can never be recovered. Nothing in the sphere can re-make it.

The answer

Light must keep entering. It is the only input, and it is the reason the sphere sits on a windowsill. Put it in a cupboard and the algae stop photosynthesising, the whole system runs down and everything in it dies — not because the atoms ran out, but because the energy did.

Biology Connection

This sphere is the sentence “nutrients cycle, energy flows” built out of glass. If you can explain the sphere, you can answer almost any question in 19.1, 19.3 and 19.4, because they are all the same idea seen from different angles.

Two neighbouring fields have grown the same crop for ten years. Field A is ploughed deeply every autumn and has drainage pipes under it. Field B is flat, undrained, and holds standing water for weeks after heavy rain. Field A yields roughly twice as much. Soil tests show field A contains 42 mg of nitrate per kg of soil and field B only 9 mg per kg. The two fields receive the same rainfall, sunlight and fertiliser.

Explain the difference in nitrate, and calculate how many times greater the nitrate concentration is in field A.

▼

Do the number first — it is the quickest mark on the page

42 ÷ 9 = 4.7 times greater (to two significant figures). Always answer the calculation part before the explanation; it takes ten seconds and cannot be argued with.

Identify the one variable that differs

Rainfall, sunlight, fertiliser and crop are all the same — the question has controlled them deliberately. The only difference is how much oxygen is in the soil, and drainage and ploughing are both ways of getting air into it.

Two bacterial processes, pulling in opposite directions

In field A there is plenty of oxygen, so nitrifying bacteria are active and convert ammonium ions through nitrite to nitrate. Decomposers also work faster in aerated soil, supplying more ammonium in the first place. In field B the water fills the air spaces, so 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 altogether.

Close it off with the yield

More nitrate in field A means the roots can absorb more, so the crop makes more amino acids and proteins, so it grows more and yields more. Note that the fertiliser is not wasted in field B so much as converted and lost.

Biology Connection

Every applied nitrogen-cycle question turns on oxygen. Aerated soil favours nitrification and decomposition; waterlogged soil favours denitrification. Learn that single sentence and you can answer any version of this question, whether it is about ploughing, drainage, flooding or compacted ground.

A biologist studying a shallow lake finds that a species of small fish introduced thirty years ago now makes up most of the fish biomass. She also finds that the pyramid of numbers for the lake is a normal pyramid shape, but the pyramid of biomass is inverted at the base, with less algae than there is zooplankton. A student concludes that the biomass measurement must be an error, because “there has to be more producer than consumer”.

Say whether the student is right, and explain what the two pyramids together are telling the biologist.

▼

The student is wrong, and the reason is the word “snapshot”

A pyramid of biomass measures the dry mass present at one moment. Algae are microscopic, reproduce extremely quickly and are grazed almost as fast as they are produced, so very little algal material is standing there at any instant. Over a whole year, though, the algae produce far more material than the zooplankton ever do.

What would settle it

A pyramid of energy, measured in kJ per m² per year. Because it counts everything produced across the year rather than what is present on one afternoon, it includes the rapid turnover of the algae, and it is guaranteed to come out the right way up — energy is lost at every transfer, so a level can never contain more than the level below supplied it.

Why the numbers pyramid looks normal

Because algal cells are individually tiny but there are astronomically many of them, so counting gives a wide base. That is the mirror image of the oak-tree case: there, one enormous organism made the base too narrow. Both show the same weakness — a pyramid of numbers takes no account of size.

Biology Connection

Whenever two pyramids disagree, the disagreement is the information. Ask what each one measures and what it ignores — numbers ignores size, biomass ignores time, energy ignores nothing but is horrible to measure. An examiner asking you to compare two pyramids is asking you to notice exactly that.

Check Yourself: 19.6 Exam Technique and Vocabulary

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

A question says “Describe the change in the population between 4 and 16 hours.” What should your answer contain?

A what happened, with figures quoted from the graph

B the reasons for the change, in terms of food and waste

C a prediction of what will happen next

D the name of the phase only

Describe means say what happened, and on a graph it means numbers: “rises from 40 to 3900 cells per cm³”. Giving the reasons for the change answers an explain question, and writing the explanation instead of the description is one of the commonest ways to score zero on a question you fully understand.

Question 2

Which answer would gain full marks for “explain why the fox population fell”?

A “The fox population fell sharply after year 3.”

B “The rabbits were removed, so the foxes lost a major source of food and fewer survived to reproduce.”

C “There were fewer foxes because the fox population decreased.”

D “Foxes are predators at the top of the food web.”

Only B contains a cause and a consequence linked by “so”. A describes rather than explains, C is circular, and D is a true but irrelevant fact. If your sentence has no “because” or “so” in it, it is probably not an explanation.

Question 3

Which phrase should never appear in an answer about energy in an ecosystem?

A “the energy is destroyed”

B “transferred to the environment as heat”

C “released in respiration”

D “stored as chemical energy in organic nutrients”

Energy cannot be destroyed, and an examiner reading that word will stop reading the sentence. The other three are all standard mark-scheme phrasings worth copying into your own answers.

Question 4

A question asks you to “construct a food chain” from a paragraph of prose. What must you do first?

A identify the top predator and work backwards

B decide whether the organisms are herbivores or carnivores

C count how many trophic levels the passage mentions

D identify the producer, because every chain must start with one

Find the producer and the left-hand end is fixed; everything else follows from it. Working backwards from the top predator is exactly how reversed arrows happen, because you end up writing the chain in the order you thought of it.

Question 5

Which is the best answer to “state the trophic level of the owl” in the chain wheat → mouse → owl?

A carnivore

B predator

C secondary consumer / third trophic level

D tertiary consumer

A and B are true statements about the owl that do not answer the question asked — a trophic level is a position. D is the arrow-counting slip. Read the command and the noun in the question, then answer in the same vocabulary.

Question 6

A question is worth [4] and says “Discuss the advantages and disadvantages of using a pyramid of energy.” What is the safest structure?

A four advantages

B a labelled diagram with no writing

C a definition of a pyramid of energy, then one advantage

D two or three advantages and at least one disadvantage

“Discuss” requires both sides, so an answer with no disadvantage caps itself however good the advantages are. Here the disadvantage is the practical one: the data are difficult and slow to collect because energy must be measured over a long period.

Question 7

Which correction is right?

A “decomposers recycle energy” should become “decomposers recycle heat”

B “decomposers recycle energy” should become “decomposers recycle nutrients”

C “decomposers recycle energy” is already correct

D “decomposers recycle energy” should become “decomposers produce energy”

Nutrients cycle, energy flows. Nothing recycles heat, and nothing produces energy, so A and D swap one error for another. This is the correction examiners ask for most often in this topic.

Question 8

You are given a table of energy values and asked to calculate an efficiency. Which is the safest way to write your answer?

A the final percentage only, clearly underlined

B a sentence describing the transfer, with no numbers

C the division written out, then the answer with a percentage sign

D the two energy values copied from the table

Showing the division protects you: if the arithmetic slips you still earn the method mark, and if you have transposed a figure the examiner can see what you meant. It costs about four seconds.

Question 9

Which statement is the correct use of the word “suggest” in a Cambridge question?

A it means the answer is a guess and any answer is accepted

B it means only recall is needed

C it means the question is optional

D it means you must apply what you know to an unfamiliar situation, and more than one biologically sound answer may be credited

“Suggest” is the examiner telling you that this is not in the textbook and you are expected to reason. It is where challenge papers live, and the response to it is to write a proper biological argument rather than to freeze because you do not recognise the organism.

Question 10

Which is the most complete answer to “explain why this pyramid of numbers is inverted at the base”?

A “Because the tree is very big.”

B “Because there are more insects than trees.”

C “Because 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.”

D “Because the pyramid was drawn to the wrong scale.”

C gives the rule and the instance, which is how a two-mark version is split. B restates the diagram without explaining it — it is a description dressed up as a reason. D blames the drawing when the drawing is correct.

Question 11

A four-mark question asks for the effects of removing one species from a food web. What is the best way to organise the answer?

A describe the whole web from the producers upwards

B one sentence for what ate it, one for what it ate, and one for the other prey of its predators — each with a reason

C name every organism whose numbers will change

D state that the whole web will collapse

Up, down, sideways — three directions, three reasoned sentences, and you have hit every mark point available. C lists without explaining, and D is the answer people give when they have not looked at the web at all; a web with alternative food sources rarely collapses.

Question 12

You have two minutes left. Question 6(c) asks you to name the four phases of a sigmoid curve [2] and question 7(b) asks you to explain the advantages of a pyramid of energy over a pyramid of biomass [4]. What should you do?

A write the four phase names first, then use whatever is left on 7(b)

B answer 7(b) first because it is worth more

C write a plan for 7(b) and leave 6(c)

D check the answers you have already written instead

Marks per second is the only measure that matters at the end of a paper, and four words you already know are the fastest two marks on the page. B is what most people do and it regularly costs the easy marks, because a long answer expands to fill whatever time it is given.