BiologyCBSE Class 12 · Ecosystem

Energy Flow, Food Chains and Trophic Levels

Q: Why is energy flow unidirectional?

Energy passes from the sun to producers and then to consumers, and is lost as heat at each step; it cannot flow back to the sun or producers.

Q: 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 in respiration or lost as heat.

Q: What is the difference between grazing and detritus food chains?

A grazing food chain starts with living plants; a detritus food chain starts with dead organic matter broken down by decomposers.

Q: What is a trophic level?

The position of an organism in a food chain based on its source of food: producers (first), herbivores (second), carnivores (third) and so on.

Q: Why are food chains short?

Because only about 10 per cent of energy passes to each higher level, too little remains after four or five levels to support another population.

Updated 15 October 2026 · Fact-checked

Energy flows one way in an ecosystem: from the sun to producers and then to consumers, and is lost as heat at each step. Plants capture only 2-10 per cent of photosynthetically active radiation. Energy moves through grazing and detritus food chains, linked into food webs, and only about 10 per cent passes from one trophic level to the next.

Notice energy only moves upward and outward: about 10% passes to the next level while the rest is lost as heat, so energy never cycles.

Illustration drawn for QuestionBuddy.

Understand Energy Flow and Food Chains

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

Unidirectional flow and thermodynamics. All organisms depend on producers directly or indirectly, so energy flows one way: sun → producers → consumers. Energy is not created or destroyed (first law) but at each transfer much is lost as heat; ecosystems also obey the second law: they need a constant supply of energy to build the molecules they need and counter the universal tendency towards disorder. Energy does not cycle; it must be supplied continuously.

Producers and consumers.

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

Two kinds of food chains.

Grazing food chain (GFC): begins with living producers. Example: grass → goat → man (producer → primary consumer → secondary consumer).
Detritus food chain (DFC): begins with dead organic matter (detritus). It is made of decomposers: heterotrophs, mainly fungi and bacteria, also called saprotrophs, which secrete digestive enzymes that break down dead and waste materials into simple inorganic substances and absorb them.
In aquatic ecosystems the GFC is the major conduit of energy; on land, a much larger fraction flows through the DFC.
The two chains are linked: some DFC organisms are prey for GFC animals, and some animals (cockroaches, crows) are omnivores. Such interconnected chains form a food web.
No energy stays in an organism forever: it is passed to a consumer, or the organism dies, beginning the detritus chain.

Trophic levels. An organism's position in the food chain by its source of food:

1st: producers; 2nd: herbivores (primary consumers); 3rd: carnivores (secondary consumers); 4th: top carnivores (tertiary consumers).
The amount of energy decreases at successive trophic levels.
Standing crop: the mass of living material at each trophic level at a particular time, measured as biomass (fresh or dry weight, which is more accurate because water content varies) or as number per unit area.

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 detritus. This limits the number of trophic levels in a grazing food chain (usually producer, herbivore, primary carnivore, secondary carnivore).

Flow chart: sun → (less than 50% PAR) → producers fix 2-10% of PAR → herbivores (~10%) → primary carnivores (~10%) → secondary carnivores (~10%); at every level: heat loss + dead matter → decomposers (DFC).

Draw and label: energy flow through trophic levels. Checklist: a row of boxes decreasing in size from left to right: sun, producers, herbivores, carnivores, top carnivores; thick arrows between boxes getting thinner; a curved arrow up from each box labelled 'heat (respiration)'; arrows down from each box to a long box 'decomposers (detritus)'. Labels: solar radiation, PAR, producers (T1), herbivores (T2), carnivores (T3), top carnivores (T4), heat loss, decomposers, 10% transfer.

Energy Flow and Food Chains: key facts

Energy capture: $incident \to< 50% PAR \to 2 - 10% of PAR fixed by plants$; PAR = photosynthetically active radiation.
10 per cent law: $E_{n + 1} \approx 0.1 \times E_{n}$; So E at level k ≈ E(producers) × 0.1^(k−1).
Trophic levels: $T_{1} producers \to T_{2} herbivores \to T_{3} carnivores \to T_{4} top carnivores$; Energy decreases at each successive level.
Food chains: $GFC: grass \to goat \to man; DFC: detritus \to decomposers$; On land, more energy flows through the DFC.
Standing crop: $biomass (dry weight) or number per unit area at a given time$; Dry weight is more accurate than fresh weight.

How to answer Energy Flow and Food Chains questions

Energy-flow questions are about the direction of flow, food chain types, trophic levels and 10 per cent calculations.

1State that energy flow is unidirectional (sun → producers → consumers) and that energy is lost as heat at each step.
2For energy capture, apply: PAR is less than 50% of incident light; plants fix 2-10% of PAR.
3For the 10 per cent law, multiply by 0.1 for each step up the chain (or by 10 going down).
4Assign trophic levels by what an organism eats, not by its size; omnivores can occupy more than one level.
5Distinguish GFC (starts with living plants) from DFC (starts with dead matter; decomposers are saprotrophs).
6Explain why chains are short: by the fourth or fifth level too little energy remains to support a population.

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.

10 per cent law and the length of food chains

Suppose a grassland receives 1,000,000 kJ of sunlight per m² per year. Assume 50 per cent is PAR and the grass fixes 2 per cent of the PAR. A farmer wonders why a hawk population there is so small compared with grasshoppers.

Calculate the energy fixed by grass and the energy reaching grasshoppers, frogs, snakes and hawks (grass → grasshopper → frog → snake → hawk), using the 10 per cent law.

Try it first, then show the solutionSolution

PAR = 50% of 1,000,000 = 500,000 kJ; grass fixes 2% of PAR = 10,000 kJ m⁻² yr⁻¹.
Grasshoppers (T2): 10% of 10,000 = 1000 kJ; frogs (T3): 100 kJ.
Snakes (T4): 10 kJ; hawks (T5): 1 kJ.
Only 1 kJ (0.0001% of the sunlight) reaches hawks, so the land can support very few hawks; a sixth level would get only 0.1 kJ, which is why chains are short.

Answer: Grass 10,000 kJ; grasshoppers 1000; frogs 100; snakes 10; hawks 1 kJ per m² per year.

Memory hook: Each step up keeps one-tenth; the top gets crumbs.

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-way street
Energy flows one way (sun to consumers); nutrients go round in circles.
Use it when: Unidirectional flow.
Half, then a sliver
Less than half of sunlight is PAR; plants catch only 2-10% of that.
Use it when: PAR figures.
Divide by ten
Up one level = divide by 10.
Use it when: 10 per cent law sums.

Energy Flow and Food Chains: solved examples

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

Example 1: 10 per cent law · 1 mark · MCQBoard level

If producers in a food chain fix 20,000 J of energy, how much energy is available to the secondary consumers according to the 10 per cent law?

Try it first, then show the solutionSolution

Primary consumers: 10% of 20,000 = 2000 J.
Secondary consumers: 10% of 2000 = 200 J.

Answer: 200 J.

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

Differentiate between the grazing food chain and the detritus food chain. Which is the major route of energy flow on land and in water?

Try it first, then show the solutionSolution

GFC begins with living producers (e.g. grass → goat → man) (1 mark).
DFC begins with dead organic matter (detritus) and involves decomposers (fungi, bacteria) that break it down by secreting enzymes (1 mark).
In aquatic ecosystems the GFC is the major conduit; in terrestrial ecosystems a much larger fraction of energy flows through the DFC (1 mark).

Answer: GFC starts with living plants, DFC with detritus; GFC dominates in water, DFC on land.

Example 3: Why energy flow is unidirectional · 2 marks · Very short answerHarder board level

Explain why energy flow in an ecosystem is unidirectional and why ecosystems need a constant input of energy.

Try it first, then show the solutionSolution

Energy passes from the sun to producers and then to consumers; at each step much is lost as heat, which cannot be reused by producers, so it cannot flow back (1 mark).
Following the second law of thermodynamics, ecosystems need a constant energy supply to synthesise molecules and counter the tendency towards disorder (1 mark).

Answer: Heat lost at each level cannot return; second law demands constant energy input.

Example 4: Energy budget · 3 marks · Short answerHarder board level

Suppose 2,000,000 J of sunlight falls on a pond surface. Less than 50 per cent is PAR; assume exactly 40 per cent, and that phytoplankton fix 5 per cent of PAR. Find the energy fixed and the energy reaching small fish that eat zooplankton that eat phytoplankton.

Try it first, then show the solutionSolution

PAR = 0.40 × 2,000,000 = 800,000 J (1 mark).
Fixed by phytoplankton = 0.05 × 800,000 = 40,000 J (1 mark).
Zooplankton = 4000 J; small fish (T3) = 400 J (1 mark).

Answer: 40,000 J fixed; 400 J reaches the small fish.

Example 5: Trophic levels and omnivores · 3 marks · Short answerHOTS / competency

In the web: grass → rabbit → fox; grass → grasshopper → frog → snake; crows eat grasshoppers and also grains. Assign trophic levels to rabbit, frog, snake and crow, and explain the crow's position.

Try it first, then show the solutionSolution

Rabbit: T2 (primary consumer); frog: T3 (secondary consumer) (1 mark).
Snake: T4 (tertiary consumer) (1 mark).
Crow: T2 when it eats grains and T3 when it eats grasshoppers; as an omnivore it occupies more than one trophic level, which links chains into a food web (1 mark).

Answer: Rabbit T2, frog T3, snake T4, crow T2 and T3 (omnivore).

Example 6: Decomposers · 2 marks · Very short answerHarder board level

Why are decomposers called saprotrophs? How do they obtain their food?

Try it first, then show the solutionSolution

Sapro means to decompose: they get energy and nutrients by degrading dead organic matter (detritus); mainly fungi and bacteria (1 mark).
They secrete digestive enzymes that break down dead and waste materials into simple inorganic substances, which they then absorb (1 mark).

Answer: They feed on dead matter by external digestion with secreted enzymes and absorption.

Where students lose marks in Energy Flow and Food Chains

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

Mistake: Saying energy cycles
Energy and nutrients are confused.
Wrong working
Energy is recycled in the ecosystem.
Right working
Energy flows one way and is lost as heat; nutrients are recycled.
Do this: Energy flows, matter cycles.
Usually costs: 1 mark.
Mistake: 10% of the wrong base
Students take 10% of sunlight for herbivores.
Wrong working
Herbivores get 10% of the sunlight.
Right working
Herbivores get about 10% of the energy fixed by producers.
Do this: Apply 10% from one trophic level to the next.
Usually costs: 1 mark.
Mistake: Mixing up PAR figures
Two percentages are swapped.
Wrong working
Plants capture 50% of PAR.
Right working
PAR is less than 50% of incident radiation; plants capture 2-10% of PAR.
Do this: Half is PAR; a sliver is captured.
Usually costs: 1 mark.
Mistake: Calling carnivores 'primary consumers'
Consumer and carnivore numbering differ by one.
Wrong working
A fox eating rabbits is a primary consumer.
Right working
The fox is a secondary consumer (primary carnivore).
Do this: Count consumers from herbivores: herbivore = primary consumer.
Usually costs: 1 mark.

Exam tips

Quote PAR (<50%) and plant capture (2-10% of PAR) exactly.
Give one example each of GFC and DFC with arrows.
Show 10 per cent calculations step by step for each trophic level.
Mention omnivores (crows, cockroaches) when explaining food webs.
Define standing crop and say why dry weight is used.

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
Of the incident solar radiation, the photosynthetically active radiation (PAR) is:
(a) about 100% (b) less than 50% (c) 2-10% (d) 10%
Hint
Plants fix 2-10% of PAR.
Answer
(b) less than 50%.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): In terrestrial ecosystems, a larger fraction of energy flows through the detritus food chain than through the grazing food chain.
Reason (R): Much of the plant biomass on land is not eaten by herbivores and enters the detritus pool.
(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
Leaf litter.
Answer
(a).
Q3 · 1 mark · Very short answerBoard level
Why is dry weight preferred to fresh weight for measuring biomass?
Hint
Water content.
Answer
Water content varies a lot between organisms and with conditions; dry weight measures only the organic matter, so it is more accurate and comparable.
Q4 · 3 marks · Short answerHOTS / competency
Suppose a lake's top predator, a large fish, is found to receive 50 kJ m⁻² yr⁻¹ and is at the fourth trophic level. Using the 10 per cent law, how much energy did producers fix? If producers fixed 3 per cent of PAR, how much PAR fell on the lake?
Hint
Multiply by 10 for each step down.
Answer
T4 = 50 → T3 = 500 → T2 = 5000 → T1 (producers) = 50,000 kJ m⁻² yr⁻¹. PAR = 50,000/0.03 ≈ 1,666,667 kJ m⁻² yr⁻¹ (about 1.67 × 10⁶).
Q5 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a forest study shows: of the leaves produced each year, herbivores eat 8%, and the rest falls as litter; earthworms, fungi and bacteria break down the litter; some birds eat earthworms; hawks eat these birds.
(a) Which food chain carries most energy here? (1)
(b) Write a food chain linking the litter to hawks and give each member's trophic role. (1)
(c) How does this example show that GFC and DFC are interconnected into a food web? (2)
Hint
92% litter.
Answer
(a) The detritus food chain (92% of leaf production enters litter). (b) Leaf litter (detritus) → earthworm (detritivore) → bird (predator/consumer) → hawk (top carnivore). (c) Earthworms belong to the DFC but are eaten by birds, which also belong to grazing chains (e.g. eating insects that eat leaves); hawks eat these birds, so energy from both chains merges at higher levels, forming a food web.
Q6 · 2 marks · Very short answerHarder board level
Why can a detritus food chain support a long series of decomposers even though grazing chains are limited to 4-5 levels? Discuss.
Hint
Dead matter from every level.
Answer
The detritus pool receives dead matter from every trophic level (plants, herbivores, carnivores) and waste, so decomposers draw on a large and continuous energy source; they also break matter down in many small steps. Grazing chains are limited because energy is cut to about 10% at each step from a single starting source.

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.What fraction of PAR do plants capture?Show answer
2-10 per cent.
2.Which food chain carries more energy on land?Show answer
Detritus food chain.
3.Give a grazing food chain from NCERT.Show answer
Grass → goat → man.
4.What is standing crop?Show answer
The mass of living material at a trophic level at a particular time.
5.Energy reaching T3 if producers fix 10,000 J?Show answer
100 J.
6.What are decomposers also called?Show answer
Saprotrophs.

Energy Flow and Food Chains: frequently asked questions

Why is energy flow unidirectional?

Energy passes from the sun to producers and then to consumers, and is lost as heat at each step; it cannot flow back to the sun or producers.

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 in respiration or lost as heat.

What is the difference between grazing and detritus food chains?

A grazing food chain starts with living plants; a detritus food chain starts with dead organic matter broken down by decomposers.

What is a trophic level?

The position of an organism in a food chain based on its source of food: producers (first), herbivores (second), carnivores (third) and so on.

Why are food chains short?

Because only about 10 per cent of energy passes to each higher level, too little remains after four or five levels to support another population.