Pearson Edexcel International GCSE in Biology · 4BI1

Energy Transfer Along a Food Chain

About nine tenths of the energy disappears at every step. Here is exactly where it goes — and what that means for how the world is fed.

Topic 4 · Ecology and the environment — one of 8 lessons in this topic, and one of 68 in Biology.

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

  • 4.8 — The transfer of substances and energy along a food chain
  • 4.9 — Why only about 10% of energy is transferred between trophic levels

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Energy enters a community in one way only: sunlight captured by producers in photosynthesis. From that moment it travels along the food chain, and at every transfer most of it is lost. Roughly 10 % of the energy in one trophic level ends up in the next — which means about 90 % does not, and the interesting question is where it goes.

The three routes energy takes when it does NOT reach the next level

  1. RESPIRATION. Every organism respires continuously to release energy for movement, growth, active transport and — in mammals and birds — for maintaining body temperature. That energy is eventually transferred to the surroundings as heat, and heat cannot be recaptured. This is by far the largest loss.
  2. EGESTION. Not everything eaten is digested. Material that cannot be broken down passes through the gut and is egested as faeces, taking its energy with it.
  3. EXCRETION. Waste products of metabolism, chiefly urea, still contain energy and are removed in urine.
  4. There is a fourth route worth knowing but often forgotten: not all of the level below is EATEN in the first place. Roots, bones, bark and organisms that die uneaten never enter the next level at all.

Notice that none of the energy is destroyed. It has been transferred to the surroundings, mostly as heat, or is sitting in faeces and dead material where decomposers will get it instead of the next consumer. From the point of view of the food chain it is lost, because it is no longer available to be eaten — but it has not vanished.

Think of it like a wage passing through several people's hands

Someone earns a hundred pounds and spends ninety on rent, food and heating. Ten pounds is left to pass on. Whoever receives that ten spends nine of it on the same things, and passes on one. Nobody has stolen anything and nothing has disappeared — it has been spent on running the person, which is precisely what respiration is. Two things follow immediately. The chain cannot be long, because there is nothing left after four or five hands. And you get vastly more from a wage by taking it early in the sequence, which is the argument for eating plants.

Worked example — the standard calculation

Producers in a field trap 87 000 kJ/m²/year. The primary consumers that eat them contain 7500 kJ/m²/year. Calculate the percentage of the producers' energy transferred to the primary consumers.

  1. Percentage transferred = energy in the higher level ÷ energy in the lower level × 100.
  2. = 7500 ÷ 87 000 × 100.
  3. = 8.62 %.

Answer: 8.6 % — close to the 10 % rule of thumb, which is an approximation rather than a law.

Two consequences follow from that percentage, and both are examined. The first is that FOOD CHAINS ARE SHORT — usually four or five levels at most. After four transfers only about a thousandth of the original energy remains, which is not enough to support a population of predators large enough to breed. There is no rule forbidding a sixth level; there is simply nothing left to feed it.

The second is the argument about feeding people, and it is worth setting out carefully because it is easy to state badly. If a field of crops is eaten directly by humans, one transfer has occurred and roughly a tenth of the producers' energy reaches us. If the crops are fed to cattle and we eat the cattle, two transfers have occurred and roughly a hundredth reaches us. So the same land supports far more people on a plant-based diet than on a meat-based one — which is a statement about energy, not about nutrition or about what anyone ought to eat.

It is also worth knowing why that argument has limits, because the strongest answers acknowledge them. Much grazing land is too steep, too poor or too dry to grow crops on, so cattle there are converting grass that people cannot eat into food that they can, rather than competing with them for it. Animals also supply protein, iron and vitamin B12 in forms that are harder to obtain otherwise. The energy argument is genuine and important, and it is not the whole argument.

One last point about what travels alongside the energy. Substances move along the chain too — the carbon, nitrogen and other elements in each organism's molecules — but unlike energy they are RECYCLED. Decomposers break down faeces and dead organisms and return mineral ions to the soil, where producers take them up again. Energy flows through a community once and leaves as heat; matter goes round and round. That difference is the reason the next lesson is about cycles.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 4.9 — where the energy goes, and why it never reaches the next level

  • RESPIRATION — energy released for movement, growth, active transport and maintaining body temperature, eventually transferred to the surroundings as HEAT. The largest loss by far
  • EGESTION — undigested material passes out as faeces, taking its energy with it
  • EXCRETION — metabolic waste, chiefly urea, still contains energy and is removed in urine
  • NOT ALL OF THE LEVEL IS EATEN — roots, bones and bark are left, and organisms that die uneaten pass to decomposers instead
  • The result is that only about 10 % of the energy in one trophic level reaches the next
  • None of it is destroyed: it is transferred to the surroundings or to decomposers, and is simply no longer available to be eaten

Calculating percentage energy transfer — the routine

  1. Identify the energy in the HIGHER trophic level and the energy in the level BELOW it.
  2. Percentage transferred = energy in the higher level ÷ energy in the lower level × 100.
  3. Check the units match — both are usually kJ/m²/year — and do not mix a figure per year with one per day.
  4. Worked example: 7500 ÷ 87 000 × 100 = 8.6 %.
  5. To go across TWO levels, apply the percentage twice: about 10 % of 10 % is about 1 %.

Energy against matter along a food chain — the distinction the next lesson depends on

A. ENERGY FLOWS THROUGH and is not recycled. It enters as sunlight, is transferred along the chain, and leaves as heat to the surroundings. It must be constantly resupplied by the Sun.

B. MATTER IS RECYCLED. Decomposers break down faeces and dead organisms, returning mineral ions to the soil for producers to absorb again — so the same atoms are used repeatedly.

The two consequences of the 10 % figure, stated the way marks are given

Food chains are short
After four or five transfers, too little energy remains to support a further population — usually only four or five trophic levels are possible.
Fewer organisms at each level
Less energy available means less biomass can be supported, which is why pyramids of biomass and energy are never inverted.
Eating plants feeds more people
Each transfer loses about 90 %, so eating crops directly involves one transfer rather than two, and the same land supports far more people.
The honest limits of that argument
Much grazing land cannot grow crops, so livestock there convert grass people cannot eat into food they can; animal products also supply protein, iron and vitamin B12.

Learn this definition · Trophic efficiency

The percentage of the energy in one trophic level that is transferred to the next. It is about 10 % as a rule of thumb, calculated as energy in the higher level ÷ energy in the lower level × 100.

Why mammals and birds transfer even less than 10 %

A. A MAMMAL OR BIRD maintains a constant body temperature, so a large part of the energy released by respiration is used for that and transferred to the surroundings as heat.

B. A FISH OR INSECT does not, so more of the energy it takes in goes into growth — which is one reason fish farming converts feed into food more efficiently than cattle do.

Not this: Energy is lost at each trophic level, meaning it is destroyed.

This: It is transferred elsewhere — mostly to the surroundings as heat from respiration, and to decomposers in faeces and dead material. It is lost to the FOOD CHAIN, not from existence.

Model answer [5 marks]

Explain why only about 10 % of the energy in one trophic level is transferred to the next. [5]

Organisms respire continuously, releasing energy for movement, growth and, in mammals and birds, for maintaining body temperature, and this energy is eventually transferred to the surroundings as heat. Some of the food eaten cannot be digested and is egested as faeces, which still contains energy. Metabolic waste such as urea is excreted in urine and also contains energy. In addition, not all of the organisms at a level are eaten — parts such as roots and bones are left, and organisms that die uneaten pass to decomposers. Only the energy remaining in the tissues actually eaten and digested is available to the next trophic level.

Model answer [3 marks]

Explain why a food chain rarely has more than five trophic levels. [3]

Only about 10 % of the energy at each level is transferred to the next, so after several transfers very little of the original energy remains. After four transfers roughly one thousandth of the producers' energy is left. This is not enough to support a population of organisms large enough to survive and reproduce, so a further trophic level cannot be sustained.

Model answer [4 marks]

Explain why a given area of land can feed more people if crops are eaten directly than if they are fed to cattle. [4]

About 90 % of the energy at each trophic level is lost through respiration, egestion and excretion, so only around 10 % passes to the next level. If people eat the crops directly there is only one transfer, from producer to human, and about a tenth of the energy trapped by the crops reaches them. If the crops are fed to cattle and the cattle are eaten there are two transfers, so only about a hundredth reaches them. The same area of land therefore supports far more people when the crops are eaten directly.

Mark-losing trap. Name the THREE routes: respiration as heat, egestion in faeces, excretion in urine. 'Energy is lost' scores nothing.

Mark-losing trap. Respiration is the LARGEST loss, and the energy leaves as heat to the surroundings.

Mark-losing trap. Energy is not destroyed. Say 'transferred to the surroundings' or 'passed to decomposers'.

Mark-losing trap. Energy FLOWS THROUGH and is lost as heat; matter is RECYCLED by decomposers. Never say energy is recycled.

Mark-losing trap. Divide the HIGHER level by the LOWER one. A transfer percentage can never exceed 100.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

  1. Grade 6 · State [1 mark] — Approximately what percentage of the energy in one trophic level is transferred to the next?
  2. Grade 7 · Identify [1 mark] — Which process accounts for the largest loss of energy between one trophic level and the next?
  3. Grade 8 · Explain [5 marks] — Explain why only about 10 % of the energy in the grass eaten by cattle ends up in the cattle's tissues. Select every statement that belongs in a full-mark explanation.
  4. Grade 9 · Calculate [3 marks] — Producers in a field trap 90 000 kJ/m²/year. The primary consumers contain 7200 kJ/m²/year, and the secondary consumers 650 kJ/m²/year. Calculate the percentage of the primary consumers' energy that is transferred to the secondary consumers.
  5. 9+ · Evaluate [5 marks] — A campaigner says: 'Since about 90 % of energy is lost at each trophic level, all farmland should be used to grow crops for people to eat directly, and livestock farming should end everywhere.' Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who showed that living things burn their food, and the one who followed what does NOT get lost along the chain. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Antoine Lavoisier — the one who showed that living things burn their food

This page's largest energy loss is respiration, and Lavoisier is the reason we know what respiration is. Working with Laplace, he placed a guinea pig in a chamber packed with ice, measured the water that melted, and compared it with the heat released when charcoal was burned to produce the same amount of carbon dioxide. The figures matched closely enough to establish that an animal is running a slow combustion — which is exactly why the energy in food ends up as heat in the surroundings rather than in the next organism along the chain.

  • “How did melting ice let you measure the heat an animal produces?”
  • “Why did you compare an animal with a burning piece of charcoal?”
  • “Where does the energy in an animal's food end up?”
  • “What did your measurements not manage to explain?”

Rachel Carson — the one who followed what does NOT get lost along the chain

Energy falls by about ninety per cent at every transfer along a chain, but a substance that the body cannot break down or excrete does the opposite — it accumulates, because a consumer eats many of the level below and keeps what each one carried. Carson assembled the evidence for that with DDT, tracing it up chains into birds of prey at concentrations tens of thousands of times higher than in the water. The two patterns are opposite consequences of the same arithmetic, and understanding one makes the other obvious.

  • “Why does a chemical become more concentrated as energy becomes scarcer?”
  • “Which organisms in a chain are most at risk, and why?”
  • “How did you gather evidence for something spread across a whole continent?”
  • “What made a chemical persist instead of breaking down?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Hermann von Helmholtz in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course