Question 1
Paper 4 short-answer style1 markName the original source of energy for almost all ecosystems. (1 mark)
Model answer
The Sun (light energy).
Why this scores
One mark for 'the Sun / light'.
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Detailed notes on Organisms and their Environment for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
From sun to ecosystem. Producers capture light; energy flows through consumers, decreasing at each step. Only ~10% reaches the next level.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Sun → producers → consumers → decomposers. One-way flow.
The chain of energy:
Why FLOW not CYCLE.
Compare with NUTRIENT CYCLES (carbon, nitrogen, water): these atoms ARE recycled — the same atoms in your body have been in countless other organisms.
Worked qualitative. Why is the SUN's energy so important to LIFE on Earth?
Cambridge tip. Always say energy 'FLOWS' (not 'cycles'). Cambridge marks the term.
Respiration heat, excretion, uneaten parts, undigested parts.
Only ~10% of the energy at one trophic level passes to the next. The other ~90% is LOST in several ways:
1. Respiration → HEAT.
2. Excretion.
3. Not all parts eaten.
4. Not all eaten parts digested.
Why food chains are short.
Worked qualitative. Why is it 'more efficient' to eat plants directly than to eat meat?
Cambridge tip. Cambridge often gives a calculation: '5000 kJ at producers, what's at primary consumers if 10% efficient?' Answer: 500 kJ. Memorise the 10% rule.
Energy pyramid always upright. Number pyramids can be inverted.
Pyramid of NUMBERS.
Pyramid of BIOMASS.
Pyramid of ENERGY.
Worked qualitative. A single oak tree (1 producer) supports thousands of caterpillars (many primary consumers). Draw the pyramid of numbers — what shape is it?
Cambridge tip. Memorise: NUMBER pyramid CAN invert; BIOMASS sometimes; ENERGY NEVER inverts.
Verbatim phrases and definitions Cambridge mark schemes credit.
Energy flow appears Paper 4 (5-7 marks: identify trophic levels, explain energy loss, draw pyramid). Examiner reports flag students saying energy 'cycles' and confusing pyramid types.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (19.1); 0610/42 Oct/Nov 2024 — Q20 (energy flow); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on energy flow, written exactly the way a tutor would explain them at the board.
Question
Trace the flow of energy through an ecosystem, starting from its source.
Step-by-step solution
Step 1
The ultimate source of energy is the Sun (light energy).
Step 2
Producers (green plants and algae) capture light energy by photosynthesis, storing it as chemical energy in glucose.
Step 3
Producers are eaten by primary consumers (herbivores), which are eaten by secondary and then tertiary consumers (carnivores) — energy is transferred along the chain.
Step 4
Energy leaves the ecosystem mainly as heat from respiration; it flows through, it does not cycle.
Answer
Sun → producers (photosynthesis) → primary consumers → secondary/tertiary consumers. Energy flows through and is lost as heat; it does not cycle.
Question
State whether each is a producer, a consumer or a decomposer: (a) a grass plant; (b) a rabbit; (c) a mushroom fungus growing on a dead log.
Step-by-step solution
Step 1
(a) A grass plant makes its own food by photosynthesis → producer.
Step 2
(b) A rabbit eats other organisms → consumer.
Step 3
(c) A fungus breaking down dead matter → decomposer.
Answer
(a) Producer; (b) consumer; (c) decomposer.
Examiner tip
Producers make food (photosynthesis); consumers eat other organisms; decomposers break down dead material.
Question
Explain why only a small percentage of the energy at one trophic level passes to the next.
Step-by-step solution
Step 1
Much of the energy is used in respiration and is eventually lost as heat to the surroundings (especially in mammals and birds keeping warm).
Step 2
Energy is lost in waste — in the faeces (undigested food) and in urine (e.g. urea).
Step 3
Not all parts are eaten (e.g. bones, roots), so the energy in those parts is not passed on.
Step 4
So only a small fraction of the energy is used for growth and so is available to the next level.
Answer
Energy is lost as heat from respiration, in faeces and urine, and in parts that are not eaten — so only the energy stored as growth passes to the next level.
Question
Explain why a pyramid of energy always has its widest bar at the producer level and is always upright.
Step-by-step solution
Step 1
A pyramid of energy shows the amount of energy at each trophic level.
Step 2
The producers have the most energy, because they capture it directly from the Sun.
Step 3
At each step up the chain, energy is lost (as heat, waste and uneaten parts), so each level has less energy than the one below.
Step 4
Because energy always decreases up the chain, the pyramid is always upright (never inverted).
Answer
Producers have the most energy (from the Sun), and energy decreases at each level because it is lost, so the pyramid is always widest at the producers and never inverted.
Question
Explain why food chains rarely have more than four or five trophic levels.
Step-by-step solution
Step 1
At each trophic level, a large proportion of the energy is lost (as heat, waste and uneaten parts).
Step 2
So only a small fraction of the energy passes to the next level.
Step 3
After several levels, so little energy remains that it cannot support another level of consumers.
Step 4
There is not enough energy to sustain a population of animals beyond about the fourth or fifth level, so food chains are short.
Answer
So much energy is lost at each level that after a few levels too little remains to support another population of consumers — so food chains are short.
Examiner tip
Link the repeated energy loss to there being too little energy left to support higher levels.
Question
Explain why a given area of land can feed more people if they eat crops directly than if they eat animals that were fed on those crops.
Step-by-step solution
Step 1
Energy is lost at every trophic level — only a small fraction passes from one level to the next.
Step 2
If people eat crops directly, they are primary consumers — only one energy transfer has happened, so more of the original energy is available.
Step 3
If people eat animals that ate the crops, there is an extra trophic level, so much more energy has been lost (as heat, waste, uneaten parts of the animals).
Step 4
So the same area of land produces more food energy when crops are eaten directly, and can feed more people.
Answer
Energy is lost at each level, so eating crops directly (one transfer) wastes less energy than eating animals fed on crops (an extra transfer). The land therefore feeds more people if they eat the crops.
Examiner tip
The key idea: a shorter food chain loses less energy, so more food energy is available to people.
High-scoring sample answers for energy flow on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Name the original source of energy for almost all ecosystems. (1 mark)
Model answer
The Sun (light energy).
Why this scores
One mark for 'the Sun / light'.
State what is meant by a producer and how it obtains its energy. (2 marks)
Model answer
A producer is an organism that makes its own food (e.g. a green plant). It obtains its energy by capturing light energy from the Sun and using it in photosynthesis.
Why this scores
Two marks: makes its own food; uses light energy in photosynthesis.
Explain why energy is lost as it passes from one trophic level to the next. (3 marks)
Model answer
Much of the energy is used in respiration and lost as heat to the surroundings. Energy is also lost in waste materials such as faeces and urine. In addition, not all parts of an organism are eaten (e.g. bones, roots), so the energy in them is not passed on. So only the energy stored as growth is available to the next level.
Why this scores
Three marks from: respiration/heat loss; loss in faeces/urine; parts not eaten; (parts not digested).
Explain why a pyramid of energy is always upright (never inverted). (4 marks)
Model answer
A pyramid of energy shows the amount of energy at each trophic level. The producers have the most energy, because they capture it directly from the Sun by photosynthesis. At each step up the chain, energy is lost (as heat from respiration, in waste, and in uneaten parts), so each level always has less energy than the level below it. Because the energy always decreases as you go up the chain, the pyramid is always upright and can never be inverted.
Why this scores
Four marks: pyramid shows energy at each level; producers have most; energy lost at each level; so each level has less / always upright.
Explain why food chains rarely contain more than four or five trophic levels. (5 marks)
Model answer
At each trophic level, a large amount of the energy is lost — used in respiration and lost as heat, lost in waste such as faeces and urine, and contained in parts that are not eaten. As a result, only a small proportion of the energy passes to the next level. After this happens at several levels, so little of the original energy remains that there is not enough to support a further population of consumers. Therefore food chains are limited to about four or five trophic levels.
Why this scores
Five marks: energy lost at each level (heat/respiration; waste; uneaten parts); only a small fraction passes on; after several levels too little energy remains; not enough to support another level.
Explain why a given area of farmland can support more people if they eat the crops grown on it, rather than eating animals that have been fed on those crops. (6 marks)
Model answer
Energy is transferred along a food chain, but at each trophic level a large amount of energy is lost — in respiration as heat, in waste materials, and in parts that are not eaten, so only a small proportion of the energy passes to the next level. If people eat the crops directly, they are acting as primary consumers, so the energy passes through only one transfer (from the crop to the person), and most of the energy captured by the crops is still available. If, instead, the crops are fed to animals and the people eat the animals, there is an extra trophic level in the chain: a large amount of energy is lost when the animals respire, move and produce waste, so much less of the original energy reaches the people. Therefore, for the same area of farmland, far more food energy is available when the crops are eaten directly, so the land can feed more people. This is why a more plant-based diet is more efficient in terms of land and energy.
Why this scores
Up to 6 marks: energy lost at each level (heat/waste/uneaten); eating crops = one transfer/primary consumer; eating animals = extra level; more energy lost with the extra level; so more food energy from eating crops; land feeds more people.
Definitions to memorise and the exact keywords mark schemes credit for energy flow answers — sharpened from recent examiner reports for the 2026 0610 sitting.
An organism that makes its own food (e.g. a green plant or alga). It captures light energy by photosynthesis.
An organism that eats other organisms. Primary consumer = eats producers; secondary = eats primary consumers; and so on.
A feeding position in a food chain: producers are the first trophic level, primary consumers the second, and so on.
An organism (mainly bacteria and fungi) that breaks down dead organisms and waste, recycling the nutrients.
A diagram showing the energy at each trophic level. Always upright, because energy decreases up the chain.
The traps other students keep falling into on energy flow questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Nutrients cycle, so students assume energy does too.
How to avoid it
Energy FLOWS through (one way) — it enters as light and leaves as heat. Only nutrients (carbon, nitrogen) cycle.
Why it happens
Pyramids of numbers can be inverted.
How to avoid it
A pyramid of ENERGY is always upright, because energy decreases at every level.
Why it happens
Producers are the start of the chain.
How to avoid it
Producers do not make energy — they CAPTURE light energy from the Sun and convert it into chemical energy in food.
The things students keep getting wrong in this sub-topic, answered.