Is it correct to say: Saying only 10% of energy is lost at each trophic level?

About 10% is passed on to the next level; most of the rest is lost (as heat, in digestion, in doing work).

Is it correct to say: Thinking energy flows in a cycle like nutrients?

Energy flow is unidirectional. Energy captured by autotrophs never returns to the sun, and energy passed to herbivores never returns to plants.

Is it correct to say: Saying biological magnification is highest in producers?

The concentration of non-biodegradable chemicals increases at each trophic level and is highest at the top — in humans.

Is it correct to say: Believing ozone is useful everywhere?

Ozone in the upper atmosphere shields the Earth from UV radiation, but at ground level ozone is a deadly poison.

Is it correct to say: Writing that the 1987 agreement banned CFCs completely?

The 1987 UNEP agreement froze CFC production at 1986 levels. Making CFC-free refrigerators is now mandatory worldwide.

Is it correct to say: Drawing arrows in a food chain from the eater to the eaten?

Arrows show the direction of energy flow, so they point from the food to the organism that eats it (grass → grasshopper).

📄 Formula sheet (PDF)

Our Environment

Ecosystem and its components, food chains and food webs, trophic levels, the 10 per cent law, biological magnification, the ozone layer and its depletion, and biodegradable vs non-biodegradable waste — NCERT Class 10 Science Ch 13

📐 14 formulas✏️ 4 examples🎯 6 practice⚖️ Unit V Natural Resources: 5 marks🏫 CBSE📚 Class 10✓ 2025–26 syllabus

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Board Exam Tips

→In energy questions, multiply by 10/100 for every step up the food chain. Count the steps, not the organisms, and give the answer in joules or kilojoules.
→Write both steps of ozone formation as equations: O₂ splits into two O atoms under UV light, then O + O₂ gives O₃.
→For biological magnification, name the cause (non-biodegradable chemicals such as pesticides), the process (accumulation at each trophic level) and the result (highest concentration in humans at the top).
→Explain why food chains have only 3 or 4 steps using the 10 per cent law — very little usable energy remains after four trophic levels.
→Keep a list of biodegradable and non-biodegradable examples ready, and say why the difference matters for the environment.

📐 Formulas(14)

Components of an Ecosystem

Ecosystem = biotic components + abiotic components

Biotic Components by Nutrition

Producers (autotrophs); Consumers (herbivores, carnivores, omnivores, parasites); Decomposers

A Food Chain

Grass \to Grasshopper \to Frog \to Snake \to Eagle

Trophic Levels

T_{1} : producers \to T_{2} : herbivores \to T_{3} : small carnivores \to T_{4} : large carnivores

Energy Captured by Green Plants

Food energy \approx 1% of sunlight falling on leaves

Ten Per Cent Law★ Board fav

E_{n + 1} = \frac{10}{100} \times E_{n}

Symbol	Meaning
$E_{n}$	Energy available at trophic level n (J or kJ)
$E_{n + 1}$	Energy available at the next trophic level (J or kJ)

Energy at the nth Trophic Level

E_{n} = E_{1} \times (\frac{1}{10})^{n - 1}

Symbol	Meaning
$E_{1}$	Energy available at the producer level (J or kJ)
$n$	Trophic level number (producers are n = 1)

Length of Food Chains

Usual food chain = 3 or 4 trophic levels

Number of Organisms at Each Level

Producers > herbivores > carnivores (number of individuals, generally)

Unidirectional Flow of Energy★ Board fav

Sun \to autotrophs \to herbivores \to carnivores

Biological Magnification★ Board fav

Concentration of non-biodegradable chemicals: T_{1} < T_{2} < T_{3} < \dots

Formation of Ozone★ Board fav

O_{2} UV O + O; O + O_{2} \to O_{3}

Ozone Depletion and CFCs

CFCs \Rightarrow ozone loss (from the 1980s); UNEP 1987: freeze CFC production at 1986 levels

Biodegradable and Non-biodegradable

Biodegradable: broken down by biological processes; Non-biodegradable: not broken down, persist

✏️ Solved Examples

1Solved Exampleeasy2 steps

The producers in a food chain have 20,000 J of energy. How much energy will be available to the organisms at the fourth trophic level?

The fourth trophic level is three steps above the producers.

E_{4} = E_{1} \times (\frac{1}{10})^{3}

2Solved Exampleboard3 steps

In the food chain grass → grasshopper → frog → snake → eagle, the frogs receive 100 J of energy. How much energy was present in the grass, and how much reaches the eagle? (3 marks)

Assign trophic levels: grass T₁, grasshopper T₂, frog T₃, snake T₄, eagle T₅.

3Solved Exampleboard4 steps

How is ozone formed in the upper atmosphere? Why is its depletion a cause for concern, and what step was taken in 1987?

High-energy UV radiation splits oxygen molecules into free atoms.

O_{2} UV O + O

4Solved ExampleHOTS3 steps

Sunlight carrying 1,000,000 J falls on the leaves of plants in a field. Using NCERT's values, find the energy available at each trophic level up to the fifth, and use the result to explain why food chains are usually only 3 or 4 steps long.

Green plants convert about 1% of the sunlight falling on leaves into food energy.

E_{1} = \frac{1}{100} \times 1000000 = 10000 J

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Saying only 10% of energy is lost at each trophic level
✓About 10% is passed on to the next level; most of the rest is lost (as heat, in digestion, in doing work).
2
Thinking energy flows in a cycle like nutrients
✓Energy flow is unidirectional. Energy captured by autotrophs never returns to the sun, and energy passed to herbivores never returns to plants.
3
Saying biological magnification is highest in producers
✓The concentration of non-biodegradable chemicals increases at each trophic level and is highest at the top — in humans.
4
Believing ozone is useful everywhere
✓Ozone in the upper atmosphere shields the Earth from UV radiation, but at ground level ozone is a deadly poison.
5
Writing that the 1987 agreement banned CFCs completely
✓The 1987 UNEP agreement froze CFC production at 1986 levels. Making CFC-free refrigerators is now mandatory worldwide.
6
Drawing arrows in a food chain from the eater to the eaten
✓Arrows show the direction of energy flow, so they point from the food to the organism that eats it (grass → grasshopper).

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
If 10,000 J of energy is available at the producer level, how much is available to the secondary consumers (third trophic level)?
Q2
Organisms at the fourth trophic level receive 5 J of energy. How much energy was available at the producer level?
Q3
Write the two equations that show how ozone is formed.
Q4
In the food chain grass → goat → man, which organism will have the highest concentration of a pesticide sprayed on the grass, and why?
Q5
About what percentage of the sunlight falling on leaves is converted into food energy by green plants in a terrestrial ecosystem?
Q6
Why is the flow of energy in an ecosystem said to be unidirectional?

📝 Notes

Our Environment

This chapter looks at the environment as a set of interacting ecosystems, and then at how human activities — pesticide use, CFCs and the garbage we produce — affect it. Only one idea is numerical, the 10 per cent law, but it explains most of the chapter.

Ecosystem and food chains

An ecosystem is made of biotic components (producers, consumers, decomposers) and abiotic components (temperature, rainfall, wind, soil, minerals). Feeding links form food chains, and each step is a trophic level. Food chains branch and connect into food webs.

The 10 per cent law in numbers

Green plants convert about 1% of the sunlight falling on their leaves into food energy. After that, each level passes on about 10%:

$E_n = E_1 \times (1/10)^{n-1}$

Count the steps between the two levels before calculating. Since the energy shrinks so fast, food chains usually have only 3 or 4 steps, and there are generally more individuals at the lower trophic levels.

Biological magnification

Pesticides that are not degradable enter the food chain through plants and accumulate at each trophic level. The concentration is lowest in producers and highest at the top. Since humans are at the top of most food chains, we receive the maximum amount.

Ozone and waste

Ozone forms in the upper atmosphere when UV radiation splits $\text{O}_2$ into oxygen atoms, which then join $\text{O}_2$ to make $\text{O}_3$ . CFCs damage this layer, which is why the 1987 UNEP agreement froze CFC production. Separate waste into biodegradable and non-biodegradable when you answer garbage-management questions, and explain that non-biodegradable materials persist and can harm the ecosystem.