CBSE Class 10 Science Chapter 13 Notes — Our Environment

What is an Ecosystem?

An ecosystem is a community of living organisms (biotic components) interacting with each other and with their non-living environment (abiotic components) as a system.

Types of Ecosystems

• Natural ecosystems: Forest, grassland, lake, ocean, desert
• Artificial ecosystems: Crop field, aquarium, garden

Components of an Ecosystem

Biotic components:

Abiotic components: Sunlight, temperature, water, soil, air, minerals

Food Chains and Food Webs

Food Chain

A food chain shows the linear sequence of organisms through which nutrients and energy flow.

General form: Producer → Primary consumer → Secondary consumer → Tertiary consumer

Example (terrestrial): Grass → Grasshopper → Frog → Snake → Hawk

Example (aquatic): Phytoplankton → Zooplankton → Small fish → Large fish → Shark

Trophic Levels

Each level in a food chain is called a trophic level:

• 1st trophic level: Producers (plants)
• 2nd trophic level: Primary consumers (herbivores)
• 3rd trophic level: Secondary consumers (small carnivores)
• 4th trophic level: Tertiary consumers (large carnivores)

The 10% Law of Energy Transfer

At each trophic level, only 10% of the energy is passed on to the next level. The remaining 90% is lost as heat during metabolic activities (respiration, movement, etc.).

Implication: A food chain can support fewer individuals at higher trophic levels. Also, it is more energy-efficient for humans to eat plant food (first trophic level) than animal food.

Why food chains have limited levels: By the 4th or 5th level, so little energy remains that it cannot sustain another trophic level.

Food Web

In nature, organisms don't eat just one food — they eat many things. This creates a complex network of interconnected food chains called a food web.

A food web is more stable than a single food chain — if one organism is removed, other food chains can still function.

Biological Magnification (Biomagnification)

Certain chemicals (especially non-biodegradable ones like DDT, mercury, lead) accumulate in the fatty tissues of organisms and are NOT excreted.

When a consumer eats many prey organisms, it accumulates the chemical from all of them. At each higher trophic level, the concentration increases dramatically.

Example — DDT in an aquatic ecosystem:

Consequences:

• Affects reproduction in top predators (thin egg shells in birds → population decline)
• Humans at the top of food chains accumulate the most pesticide
• Can cause cancer, hormonal disruption

Biodegradable vs Non-Biodegradable Waste

Environmental Impact of Non-Biodegradable Waste

• Plastic: Clogs drains, harms animals that eat it, leaches chemicals into soil/water
• Electronic waste (e-waste): Contains heavy metals (mercury, lead, cadmium) that poison soil and groundwater
• Pesticides: Biomagnify through food chains
• Nuclear waste: Highly radioactive; persists for thousands of years

Ozone Layer

What is Ozone?

Ozone (O₃) is a molecule made of three oxygen atoms. The ozone layer is found in the stratosphere (about 15–35 km above Earth's surface).

Function of Ozone Layer

Absorbs and blocks harmful ultraviolet (UV) radiation from the sun before it reaches the Earth's surface.

Ozone Depletion

The ozone layer is being depleted mainly by:

Cl + O₃ → ClO + O₂ (repeated cycle)

• CFCs (Chlorofluorocarbons): Used in old refrigerators, air conditioners, aerosol sprays, foam packaging
• When CFCs reach the stratosphere, UV radiation breaks them down, releasing chlorine (Cl) radicals
• One chlorine atom can destroy thousands of ozone molecules:

Effects of Ozone Depletion

• Increased UV radiation reaching Earth's surface
• Skin cancer and cataracts in humans
• Damage to immune system
• Harm to plankton (marine food web disrupted)
• Damage to crops and forests

Ozone Hole

The thinning of the ozone layer, most pronounced over Antarctica, is called the ozone hole. It was discovered in 1985.

International response: The Montreal Protocol (1987) — international treaty to phase out CFCs and other ozone-depleting substances. The ozone layer is slowly recovering.

Environmental Issues and Management

Waste Management — The 3 Rs

1. Reduce: Use less; buy only what is needed 2. Reuse: Use items multiple times (glass bottles, cloth bags) 3. Recycle: Process waste materials to make new products (paper, aluminium, glass)

Pollution and Its Effects

• Plastic pollution: Clogs drains, enters ocean, harms marine life
• Acid rain: Caused by SO₂ and NOₓ from burning fossil fuels → acidic rain damages buildings, forests, lakes
• Greenhouse effect and global warming: CO₂, CH₄, N₂O trap heat → warming of Earth → climate change
• Water pollution: Industrial waste, sewage, fertilisers/pesticides → eutrophication (algal bloom depletes O₂ in water → aquatic life dies)

Why Decomposers are Important

Decomposers (bacteria, fungi) break down dead plants and animals into simple inorganic substances. This returns nutrients to the soil and keeps the ecosystem's nutrient cycle going. Without decomposers, nutrients would be locked in dead organisms forever and the ecosystem would collapse.

Board Exam Tips

• 5–6 marks from this chapter
• Food chain with trophic levels — draw and label a 4–5 step food chain
• 10% law — explain with example (only 10% energy passes to next level)
• Biological magnification — define + give an example with increasing concentrations
• Ozone depletion causes and effects — 3-mark question; mention CFCs specifically
• Biodegradable vs non-biodegradable — give 3 examples of each
• "Why should we use cloth bags instead of plastic bags?" — environmental answer
• Decomposers' role in ecosystem — always mention nutrient recycling

📚 Lakhmir Singh Science Class 10 — Environment Chapter

📝 Oswaal CBSE Question Bank — Our Environment Questions

NCERT Exercise Solutions

Intext Questions

Page 260, Q1: What are trophic levels? Give an example of a food chain and state the different trophic levels in it.

Example food chain: Grass → Grasshopper → Frog → Snake → Hawk

Page 260, Q2: What is the role of decomposers in the ecosystem?

Chapter-End Exercise

Q1. What are the advantages of using cloth bags over polythene bags? List any four.

Q2. Differentiate between biodegradable and non-biodegradable substances. Give two examples of each.

Biodegradable substances complete natural nutrient cycles; non-biodegradable substances accumulate and cause long-term environmental damage.

Q3. What is biological magnification? How does it affect humans at the top of a food chain?

How it works: The substance is not broken down; it is stored in fatty tissues. Each organism eats many organisms of the lower level, concentrating more of the toxin.

Example: Water → Plankton → Fish → Large fish → Humans Low DDT → × 10 → × 100 → × 1000 → × 10,000

Effect on humans (top predator):

• Highest concentration of toxin accumulates in their body
• Can cause cancer, reproductive disorders, immune suppression, neurological damage
• DDT was banned in many countries due to these effects (thinning of eggshells in birds, reproductive failure)

Q4. What is ozone? What does it do in the upper atmosphere?

In the upper atmosphere (stratosphere, 15–35 km above Earth): 1. UV shield: Ozone absorbs harmful ultraviolet (UV) radiation from the sun, preventing it from reaching Earth's surface 2. Protects life: UV radiation causes skin cancer, cataracts, immune suppression, and harm to plants and marine organisms 3. Formation and destruction: Ozone is naturally formed and destroyed in a cycle involving UV light; it maintains a stable equilibrium

Ozone depletion — caused by chlorofluorocarbons (CFCs) from refrigerants and aerosols. CFC molecules break down ozone, thinning the ozone layer especially over the poles ("ozone hole"). This allows more UV to reach Earth.

Q5. What will happen to energy at each trophic level in a food chain? Explain the 10% rule.

• Only about 10% of the energy is transferred to the next level
• The remaining 90% is used by the organism for its own metabolism (respiration, movement) and is lost as heat

This is called the 10% rule (or 10% energy transfer rule), proposed by Lindeman.

Example (starting with 10,000 J at producers):

• Producers (T₁): 10,000 J
• Primary consumers (T₂): 1,000 J (10%)
• Secondary consumers (T₃): 100 J
• Tertiary consumers (T₄): 10 J

This is why food chains rarely have more than 4–5 trophic levels — by the 5th level, almost no energy remains. It also explains why eating plant-based food is more energy-efficient than eating meat.

Q6. What are the causes and effects of ozone layer depletion?

• CFCs (Chlorofluorocarbons) — from refrigerators, air conditioners, aerosol sprays; primary culprit
• Halons — used in fire extinguishers
• Nitrous oxide (N₂O) — from agricultural fertilisers
• Carbon tetrachloride (CCl₄) — industrial solvent

Effects:

• Increased UV-B radiation reaching Earth's surface
• Higher rates of skin cancer and cataracts in humans
• Immune suppression — reduced ability to fight diseases
• Harm to marine ecosystems — UV damages phytoplankton (base of ocean food chains)
• Reduced crop yields — damages plant DNA and reduces photosynthesis
• The Montreal Protocol (1987) — international agreement to phase out ozone-depleting substances