BiologyISC Class 12 · Ecosystem

Energy Flow and Food Chains: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

Energy enters ecosystems as sunlight; less than 50 per cent of it is photosynthetically active radiation (PAR), and plants capture only 2-10 per cent of PAR. Energy then flows one way from producers to herbivores to carnivores through grazing and detritus food chains, and by the 10 per cent law only about a tenth passes to each next trophic level.

Understand Energy Flow and Food Chains

Source of energy. Except for deep-sea hydrothermal ecosystems, the sun is the only source of energy. Of incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR). Plants and photosynthetic bacteria capture only 2-10 per cent of PAR, and this small amount sustains the entire living world.

Unidirectional flow. Energy flows one way: sun → producers → consumers. It is not recycled, because at every transfer much is lost as heat (respiration). Ecosystems obey the first law of thermodynamics (energy is transformed, not created) and the second law (they need a constant energy supply to counter increasing disorder).

Producers and consumers.

Producers: green plants (herbaceous and woody plants on land; phytoplankton, algae and higher plants in water).
Primary consumers (herbivores): insects, birds and mammals on land; molluscs in water.
Secondary consumers (primary carnivores), tertiary consumers (secondary carnivores).

Trophic levels. The place an organism occupies according to its feeding relationship: producers 1st, herbivores 2nd, carnivores 3rd, and so on. A trophic level is a functional level, not a species: a sparrow is a primary consumer when it eats seeds and fruits and a secondary consumer when it eats insects and worms.

Two types of food chain.

	Grazing food chain (GFC)	Detritus food chain (DFC)
Starts with	living producers (green plants)	dead organic matter (detritus)
Example	grass → goat → man	dead leaves → earthworm (detritivore) → bird; fungi and bacteria
Organisms	herbivores, carnivores	decomposers (saprotrophs: mainly fungi and bacteria) that secrete digestive enzymes and absorb the products
Main conduit of energy	in aquatic ecosystems	in terrestrial ecosystems (a much larger fraction of energy flows through it)

The two are linked: some DFC organisms are prey for GFC animals, and omnivores such as cockroaches and crows feed in both. These natural interconnections form a food web.

Standing crop. The mass of living material present in a trophic level at a given moment is its standing crop, measured as biomass or number per unit area. Biomass in dry weight is more accurate (water content varies).

The 10 per cent law. Only about 10 per cent of the energy at one trophic level is transferred to the next; the rest is used in respiration, lost as heat or passes to decomposers. So energy decreases at successive trophic levels, and the number of levels in a grazing food chain is restricted (usually producer, herbivore, primary carnivore, secondary carnivore). Detritus food chains are not limited in the same way because decomposers feed on all dead matter.

Energy arithmetic. Energy at level n = energy at producers × (0.1)^(n − 1). Working backwards, multiply by 10 for each step down.

Diagram checklist (energy flow). Sun → producers (box) → herbivores → carnivores → top carnivores, each box smaller; downward arrows from every box labelled 'heat lost in respiration'; side arrows from every box to 'decomposers (detritus)'; label PAR at the start and '≈10%' on each transfer arrow.

Energy Flow and Food Chains: key facts

PAR and capture: PAR Photosynthetically Active Radiation
Share of sunlight less than 50 per cent
Captured by plants 2-10 per cent of PAR; Photosynthetically Active Radiation.
10% law: $E_{n} = E_{1} \times (0.1)^{n - 1}$; E₁ = energy at producers; n = trophic level.
Ecological efficiency: $efficiency = \frac{E _{n + 1}}{E _{n}} \times 100$; About 10% on average.
Food chains: Grazing food chain starts with living plants; main energy route in water
Detritus food chain starts with dead organic matter; main energy route on land
Food web chains linked, e.g. by crows and cockroaches feeding in both; Linked into food webs.

PAR	Photosynthetically Active Radiation
Share of sunlight	less than 50 per cent
Captured by plants	2-10 per cent of PAR

Grazing food chain	starts with living plants; main energy route in water
Detritus food chain	starts with dead organic matter; main energy route on land
Food web	chains linked, e.g. by crows and cockroaches feeding in both

How to answer Energy Flow and Food Chains questions

ISC asks the trophic level with most energy, the expansion of PAR, GFC versus DFC, the 10 per cent law and calculations of energy at different levels.

1Use exact terms: 'producers' or 'first trophic level', not 'green plants' alone.
2Expand abbreviations exactly: PAR = Photosynthetically Active Radiation.
3For 10 per cent sums, count the steps between levels and divide (or multiply) by 10 for each.
4State that energy flow is unidirectional and explain with heat loss at every step.
5For GFC versus DFC, compare starting point, organisms and which ecosystem relies on each.
6Explain why food chains are short using the 10 per cent law.

Energy Flow and Food Chains in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Eating lower in the food chain

Suppose a hostel kitchen has grain containing 10 000 kcal and must choose between serving the grain to students directly or feeding it to chickens and serving the chicken meat.

Compare the energy reaching the students and explain.

Try it first, then show the solutionSolution

Eating grain: students are primary consumers; by the 10 per cent law about 10 000 × 0.1 = 1000 kcal is passed on.
Feeding chickens first: chickens get about 1000 kcal and the students, now secondary consumers, about 1000 × 0.1 = 100 kcal.
So the direct route passes about 10 times more energy.
Each extra trophic level loses about 90 per cent as heat and life processes.

Answer: About 1000 kcal versus 100 kcal; eating at a lower trophic level is about ten times more energy-efficient.

Memory hook: Every step up, one zero off.

Tricks to remember Energy Flow and Food Chains

Short memory aids. Say each one aloud once, then use it on a question the same day.

One zero per step
Moving up one trophic level knocks one zero off the energy (10 per cent law).
Use it when: Quick energy sums.
PAR = Plants' Active Range
Think 'the light plants can use', but write Photosynthetically Active Radiation.
Use it when: Remembering what PAR is (spell out correctly).
Green chain grazes, brown chain rots
GFC starts green (living plants); DFC starts brown (dead matter).
Use it when: Telling the two food chains apart.

Energy Flow and Food Chains: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Where the energy is, and what plants can use · 2 marks · Short answerBoard level

In the food chain grass → deer → tiger: (a) which trophic level holds the most energy, and why? (b) What is PAR, how much of the sunlight does it make up, and how much of it do plants capture?

Try it first, then show the solutionSolution

(a) The producers (grass, first trophic level): all energy enters through them, and about 90 per cent is lost at each later transfer (1 mark).
(b) PAR is Photosynthetically Active Radiation, the part of sunlight plants can use; it is less than 50 per cent of incident solar radiation, and plants capture only 2-10 per cent of it (1 mark).

Answer: Producers (first trophic level); Photosynthetically Active Radiation, under half of sunlight, of which plants capture 2-10 per cent.

Example 2: GFC and DFC · 3 marks · Short answerBoard level

Distinguish between grazing and detritus food chains (three points).

Try it first, then show the solutionSolution

GFC begins with living green plants; DFC begins with dead organic matter (detritus) (1 mark).
GFC involves herbivores and carnivores; DFC involves decomposers (saprotrophs: fungi, bacteria) and detritivores (1 mark).
GFC is the main energy route in aquatic ecosystems; DFC carries most energy in terrestrial ecosystems (1 mark).

Answer: Living versus dead start; consumers versus decomposers; aquatic versus terrestrial main route.

Example 3: Energy down the chain · 3 marks · Short answerHarder board level

Producers in a meadow fix 36 500 J of energy. Using the 10 per cent law, calculate the energy available to herbivores, primary carnivores and secondary carnivores.

Try it first, then show the solutionSolution

Herbivores: 36 500 × 0.1 = 3650 J (1 mark).
Primary carnivores: 3650 × 0.1 = 365 J (1 mark).
Secondary carnivores: 365 × 0.1 = 36.5 J (1 mark).

Answer: 3650 J, 365 J, 36.5 J.

Example 4: Working backwards · 3 marks · Short answerHarder board level

A hawk at the fourth trophic level receives 6 kJ of energy. How much energy must the producers at the base have fixed, by the 10 per cent law? Why are food chains rarely longer than four or five levels?

Try it first, then show the solutionSolution

Fourth level is three steps above producers: 6 × 10 × 10 × 10 = 6000 kJ (2 marks).
About 90 per cent is lost at each step, so after a few steps too little energy remains to support another level (1 mark).

Answer: 6000 kJ; energy runs out after a few transfers.

Example 5: A sparrow's trophic level · 2 marks · Short answerHarder board level

Why is a trophic level called a functional level rather than a species? Explain with the sparrow, and state where humans fit.

Try it first, then show the solutionSolution

A species can occupy more than one trophic level at the same time: a sparrow is a primary consumer when it eats seeds and fruits, and a secondary consumer when it eats insects and worms (1 mark).
Humans are omnivores: primary consumers when eating plants and secondary or tertiary consumers when eating meat or fish (1 mark).

Answer: Level depends on what is eaten; sparrow and humans occupy several levels.

Example 6: Efficiency from field data · 4 marks · Short answerHOTS / competency

Suppose in a grassland the producers fix 22 500 kJ m⁻² yr⁻¹ and the herbivores assimilate 1800 kJ m⁻² yr⁻¹. (a) Calculate the transfer efficiency. (b) Is it close to the 10 per cent law? (c) Give two reasons why most energy is not passed on.

Try it first, then show the solutionSolution

(a) 1800/22 500 × 100 = 8% (1 mark).
(b) Yes; it is of the same order as 10 per cent (the law is an average) (1 mark).
(c) Much plant energy is used in the plants' own respiration and lost as heat; parts are not eaten (roots, wood) or not digested and pass to decomposers as detritus (2 marks).

Answer: 8%, close to 10%; respiration and uneaten or undigested material.

Where students lose marks in Energy Flow and Food Chains

The slips examiners see most often, and what to write instead.

Mistake: Vague answer for the trophic level with most energy
In 2025 examiners reported green plants, autotrophs or photosynthetic organisms written instead of producers, and some wrote top or tertiary level.
Do this: Write 'producers (first trophic level)'.
Usually costs: 1 mark.
Mistake: Invented expansions of PAR
Examiners (2025) saw 'Photosynthetic Aided Respiration' and 'Photosynthetically Active Region'.
Do this: Photosynthetically Active Radiation.
Usually costs: 1 mark.
Mistake: Counting steps wrongly in 10% sums
Applying the 10% cut once too many or too few times is common.
Do this: Number the levels: from level 1 to level n there are n − 1 transfers.
Usually costs: 1 mark.
Mistake: Saying energy is recycled
Nutrients are recycled; energy is not.
Do this: Energy flow is unidirectional; it ends as heat.
Usually costs: 1 mark.

Exam tips

Show each 10 per cent step in a line, with units.
Write 'unidirectional' and give the heat-loss reason.
Say which food chain dominates in aquatic and terrestrial ecosystems.
Give the sparrow as the standard example of one species at two trophic levels.
Learn the syllabus abbreviations: PAR, GPP, NPP.

Practice questions on Energy Flow and Food Chains

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The percentage of PAR captured by plants is about:
(a) 50-60% (b) 2-10% (c) 20-30% (d) 90%
Hint
Small fraction.
Answer
(b) 2-10%.
Q2 · 2 marks · Short answerBoard level
What is standing crop? Why is dry weight preferred for measuring biomass?
Hint
Water varies.
Answer
Standing crop is the mass (or number) of living material in a trophic level at a particular time per unit area. Dry weight is more accurate because water content varies greatly between organisms and with conditions.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Energy flow in an ecosystem is unidirectional.
Reason (R): At each trophic level a large part of the energy is lost as heat and cannot be reused by producers.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Heat cannot be photosynthesised.
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
In the chain grass → grasshopper → frog → snake → hawk, grass fixes 14 500 J. How much energy reaches the snake and the hawk?
Hint
Snake is 3 steps up.
Answer
Snake (4th level): 14 500 × 0.1³ = 14.5 J. Hawk (5th level): 1.45 J.
Q5 · 2 marks · Short answerHarder board level
How are grazing and detritus food chains linked to form a food web? Give two examples of linking organisms.
Hint
Prey and omnivores.
Answer
Some organisms of the detritus food chain (such as earthworms) are prey for animals of the grazing food chain, and omnivores like cockroaches and crows feed in both, interconnecting the chains into a web.
Q6 · 2 marks · Short answerHarder board level
If incident sunlight on a field is 1 000 000 kcal m⁻² yr⁻¹, what is the maximum PAR, and what range of energy do plants capture?
Hint
Under 50%, then 2-10%.
Answer
PAR is less than 50%, so at most 500 000 kcal m⁻² yr⁻¹. Plants capture 2-10% of PAR: about 10 000-50 000 kcal m⁻² yr⁻¹ (at most).
Q7 · 3 marks · Short answerHOTS / competency
Suppose a lake is polluted so that most fish die, but the phytoplankton remain. Predict how energy flow through the grazing and detritus chains will change in the next few weeks.
Hint
Dead fish become detritus.
Answer
Grazing food chain beyond the zooplankton is cut, so less energy flows to higher consumers; zooplankton may increase without fish predation. Dead fish add a large amount of detritus, so energy flow through the detritus food chain (bacteria, fungi) rises sharply; their decomposition uses up oxygen.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose in a forest food web: oak leaves → caterpillars → tits → sparrowhawks, and fallen oak leaves → earthworms → blackbirds.
(a) Identify the grazing and the detritus chain. (1)
(b) Which chain is likely to carry more energy in this forest? Why? (1)
(c) If oak leaves fix 50 000 kJ, how much reaches sparrowhawks by the 10% law? (1)
Hint
Terrestrial ecosystem.
Answer
(a) Grazing: oak → caterpillars → tits → sparrowhawks; detritus: fallen leaves → earthworms → blackbirds. (b) The detritus chain: in terrestrial ecosystems most plant material is not eaten alive but falls as litter. (c) Sparrowhawks are at the 4th level: 50 000 × 0.1³ = 50 kJ.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Expand PAR.Show answer
Photosynthetically Active Radiation.
2.What fraction of PAR do plants capture?Show answer
2-10 per cent.
3.Which food chain is the major energy conduit in aquatic ecosystems?Show answer
The grazing food chain.
4.What are saprotrophs?Show answer
Decomposers (mainly fungi and bacteria) that feed on dead organic matter.
5.What does the 10 per cent law state?Show answer
Only about 10 per cent of energy passes from one trophic level to the next.
6.Producers fix 36 500 J; energy at primary carnivores?Show answer
365 J.

Energy Flow and Food Chains: frequently asked questions

Why is energy flow unidirectional?

Energy passes from the sun to producers to consumers and is lost as heat at each step; heat cannot be reused by producers, so energy does not cycle.

What is the 10 per cent law?

Only about 10 per cent of the energy at one trophic level is transferred to the next; the rest is used or lost as heat.

What is the difference between grazing and detritus food chains?

A grazing chain starts with living plants and herbivores; a detritus chain starts with dead organic matter and decomposers.

Why are food chains short?

Because about 90 per cent of energy is lost at each transfer, little remains after four or five levels.

What is a trophic level?

The position of an organism in a food chain based on its source of food, such as producer, primary consumer or secondary consumer.