CLASS 10 • SCIENCE • CBSE

Chapter 13 — Our Environment

Ecosystem Components • Food Chains & Food Webs • 10% Energy Law • Biological Magnification • Ozone Layer Depletion • Waste Disposal
🌿 Ecosystem 🦅 Food Chain ⚡ 10% Law 🛡️ Ozone Shield ♻️ Waste Management
Ecosystem → Trophic Levels → Energy Flow (10% Law) → Bio-magnification → Environmental Impact

Comprehensive study material covering biotic & abiotic components, energy pyramids, ozone protection, and waste disposal according to the latest CBSE Class 10 syllabus.

💡 Key Concepts: (1) Energy flow is strictly unidirectional and follows Lindeman's 10% law. (2) Biological magnification concentrates non-biodegradable chemicals at higher trophic levels. (3) Ozone ($O_3$) depletion in the stratosphere is caused by CFC-released chlorine free radicals.

Concepts & Visual Theory

Comprehensive Board Theory, NCERT Activities, Trophic Dynamics, and Ecological Principles

01
Ecosystem — Definition, Scope & Natural vs Artificial Types
Direct Board Definition — Ecosystem: An ecosystem is a self-sustaining structural and functional unit of the biosphere consisting of all the interacting living organisms (biotic factors) in an area together with the non-living physical surroundings (abiotic factors), maintaining a dynamic ecological balance.

Classification of Ecosystems Based on Origin:

Category Natural Ecosystems Artificial (Human-Made) Ecosystems
Origin & Control Develop naturally in the wild without any human intervention or maintenance. Artificially created, manipulated, and maintained by human beings for specific purposes.
Self-Sustainability High: Completely self-sustaining; nutrient cycles and waste decomposition are in natural equilibrium. Incomplete / Dependent: Requires continuous external input of energy, food, aeration, and physical waste removal.
Species Diversity High genetic and species biodiversity, creating complex, resilient food webs. Low or selective biodiversity; fragile and easily disrupted if untended.
Textbook Examples Terrestrial: Forests, grasslands, deserts.
Aquatic: Ponds, lakes, rivers, oceans.		 Terrestrial: Crop-fields (monoculture agriculture), botanical gardens, parks.
Aquatic: Aquariums, fish breeding hatcheries.
CLASSIFICATION OF ECOSYSTEMS IN THE BIOSPHERE Natural Ecosystems Self-regulating • High species diversity Terrestrial Forests, Grasslands Aquatic Ponds, Lakes, Oceans Artificial / Anthropogenic Human-built • Dependent on external input Agro-ecosystem Crop-fields, Gardens Aquarium / Microcosm Glass tanks, Hatcheries
02
Components of an Ecosystem — Biotic vs Abiotic Factors
Fundamental Interaction Principle: Every ecosystem operates via continuous reciprocal interactions between living organisms (biotic factors) and physical conditions (abiotic factors). The physical environment dictates which organisms can survive, while organisms continually alter the soil, water, and atmospheric balance.
Component Class Constituent Elements Ecological Role & Significance
Abiotic Components
(Non-living Physical Environment)		 • Climatic Factors: Solar irradiance, ambient temperature, rainfall, wind velocity, humidity.
• Edaphic Factors: Soil structure, pH, organic humus content, mineral elements ($N, P, K, Ca$), moisture.
• Topographic Factors: Altitude, slope, physical orientation. 		Provide raw inorganic substrates ($CO_2, H_2O$, minerals), physical support, and climatic boundaries that regulate the metabolic rate, growth, and geographical distribution of organisms.
Biotic Components
(Living Organisms)		 • Producers (Autotrophs): Chlorophyll-bearing plants, phytoplankton, cyanobacteria.
• Consumers (Heterotrophs): Herbivores, carnivores, omnivores, parasites.
• Decomposers (Saprotrophs): Microscopic bacteria and fungi. 		Drive biological productivity, mediate energy transfer across successive trophic tiers, and mineralize organic wastes back into abiotic reservoirs.
ABIOTIC FACTORS (Non-Living) ΔT Temperature & Sunlight H₂O Rainfall, Soil Minerals & Wind Provides inorganic nutrients and thermal regime ENERGY / NUTRIENTS RECYCLING BIOTIC FACTORS (Living Organisms) Producers (Green Plants • Photosynthesis) Consumers Decomposers Dynamic biological equilibrium & food webs
03
Activity 13.1 & 13.2 — Designing an Artificial Aquatic Ecosystem (Aquarium)
NCERT Core Enquiry — Why Does an Aquarium Need Periodic Cleaning While a Natural Pond Does Not?
An aquarium is an incomplete artificial ecosystem. It lacks an adequate community of natural decomposers (anaerobic bacteria and saprophytic fungi) and a balanced multi-tier food web. In contrast, a natural pond has a diverse microbial benthic community that breaks down excretory products and organic detritus continuously, making it completely self-sustaining.

Essential Components Required to Design an Aquarium (Activity 13.1):

  • Adequate Swimming Enclosure: A large glass jar or aquarium tank providing sufficient volume and surface area for dissolved atmospheric gas exchange.
  • Oxygen Supply: An aeration pump (aerator) to bubble dissolved oxygen ($O_2$) into the water column, preventing fish asphyxiation.
  • External Sustenance: Commercially formulated fish food added in measured amounts, as the tank cannot generate sufficient natural biomass.
  • Aquatic Hydrophytes (Plants): Submerged plants (like Hydrilla or Vallisneria) that perform photosynthesis, producing $O_2$ and absorbing toxic dissolved $CO_2$ and nitrates.
Activity 13.2 Critical Observations:
  1. Predator-Prey Segregation: Carnivorous/predatory fish must not be housed with smaller herbivorous fish; otherwise, the smaller species will be consumed rapidly, collapsing the system.
  2. Consequence of Uncleaned Decaying Biomass: When fish or plants die, absence of sufficient decomposers causes accumulation of toxic ammonia ($NH_3$) and lethal nitrites, depleting dissolved $O_2$ and poisoning the surviving organisms.
NATURAL POND ECOSYSTEM Self-Sustaining • Natural Balance 🌿 Algae / Aquatic Plants Continuous $O_2$ generation & food ✓ Complete Decomposer Flora Dead matter broken into mineral salts naturally ARTIFICIAL AQUARIUM TANK Man-Made • Incomplete Decomposition 🐟 Dependent on External Aeration Requires synthetic fish food & pump ✗ Scarce Decomposer Population Must be cleaned periodically to eliminate toxic wastes
04
Trophic Classification — Producers, Consumers & Decomposers
Functional Grouping of Organisms: In any ecosystem, organisms are systematically categorized according to how they obtain their chemical energy and carbon sustenance from the surrounding environment.
Category Sub-Classes & Definitions Source of Nutrition Representative Examples
Producers
(Autotrophs)		 Organisms synthesizing organic molecules (sugars, starches) directly from inorganic $CO_2$ and $H_2O$ using solar photons and chlorophyll. Solar Radiant Energy (Photosynthesis) Green plants, terrestrial trees, phytoplanktons, blue-green algae (cyanobacteria).
Consumers
(Heterotrophs)		 Organisms dependent upon producers directly or indirectly:
• Herbivores: Primary consumers feeding strictly on plants.
• Carnivores: Flesh-eaters feeding on other animals.
• Omnivores: Consume both plant vegetation and animal meat.
• Parasites: Live on/inside a host, deriving nourishment without killing immediately. 		Organic ingestion and biochemical cellular digestion • Herbivores: Deer, cattle, grasshopper.
• Carnivores: Frog, snake, tiger, eagle.
• Omnivores: Humans, bears, crows.
• Parasites: Cuscuta (Amarbel), tapeworm, lice, leeches.
Decomposers
(Saprotrophs)		 Microorganisms that secrete extracellular digestive enzymes to break down complex dead organic remains, decaying litter, and metabolic excreta into simple inorganic mineral ions. Dead and decaying organic matter (Saprophytic) Soil bacteria (e.g., Bacillus), saprophytic fungi (moulds, mushrooms).
Crucial Role of Decomposers in Nature:
  • Natural Scavengers of the Earth: They dispose of huge volumes of dead animal corpses, fallen foliage, and organic litter that would otherwise pile up and choke life.
  • Nutrient Biogeochemical Replenishment: They mineralize organic nutrients ($N, P, K$), returning them back to the soil, water, and atmosphere to be absorbed anew by plant root systems.
TROPHIC ROLES IN THE BIOSPHERE — PRODUCERS, CONSUMERS & DECOMPOSERS 1. Producers Autotrophs • Photosynthesis Inorganic ($CO_2 + H_2O$) → Organic Starch & Sugars 2. Consumers Heterotrophs • Ingestion Herbivores • Carnivores Omnivores • Parasites 3. Decomposers Saprotrophs • Secretion Bacteria & Fungi Mineralizes Back to Soil
05
Food Chains & Trophic Levels — Sequential Energy Pathways
Board Definition — Food Chain: A unidirectional sequence of living organisms in an ecological community in which each organism feeds on the preceding member and provides food for the succeeding organism, transferring chemical energy from one biotic tier to the next.
Trophic Level: Each successive feeding step, stage, or operational link in a food chain constitutes a distinct trophic level ($T_1, T_2, T_3, T_4$).

Standard Textbook Food Chains in Different Ecosystems (Fig. 13.1):

Ecosystem Habitat Trophic Level 1 ($T_1$)
Producers		 Trophic Level 2 ($T_2$)
Primary Consumers		 Trophic Level 3 ($T_3$)
Secondary Consumers		 Trophic Level 4 ($T_4$)
Tertiary Consumers
(a) Forest Ecosystem Trees / Grass Deer (Herbivore) Wolf / Leopard Tiger / Lion (Apex Carnivore)
(b) Grassland Ecosystem Grass Grasshopper (Insect) Frog (Small Carnivore) Snake / Hawk (Large Carnivore)
(c) Aquatic / Pond Phytoplankton / Algae Zooplankton / Water Fleas Small Fish Large Fish / Crane / Hawk
STEP-BY-STEP FEEDING SEQUENCE IN A TERRESTRIAL FOOD CHAIN TROPHIC LEVEL 1 (T₁) Producers Green Plants / Grass Solar Energy Trapped TROPHIC LEVEL 2 (T₂) Primary Consumers Herbivores (Deer) Direct Plant Feeders TROPHIC LEVEL 3 (T₃) Secondary Cons. Small Carnivores e.g. Jackal / Frog TROPHIC LEVEL 4 (T₄) Tertiary Cons. Apex (Lion/Tiger) Top Carnivore
06
Energy Flow & Lindeman’s 10% Law — Mathematical Calculations
Two Fundamental Principles of Ecological Energy Flow:
  1. 1% Solar Capture: Green terrestrial autotrophs capture only about 1% of the total solar radiant energy incident upon their leaves and convert it into chemical food energy via photosynthesis.
  2. Lindeman’s 10% Law of Energy Transfer: During the transfer of organic food energy from one trophic level to the next, only an average of 10% of the chemical energy is converted into new biomass and stored for the next higher consumer level. Approximately 90% is lost to the surroundings as metabolic heat, cellular respiration, locomotion, mechanical work, excretion, and digestive inefficiencies.
Standard Board Examination Energy Transfer Problem:
Suppose $10,000\text{ J}$ of solar radiant energy falls on a terrestrial field. Calculate the energy trapped by plants and the energy available to top carnivores in a 3-step food chain: $\text{Plants} \to \text{Deer} \to \text{Lion}$.
• Step 1 (Solar → Plants): Energy trapped by plants = $1\% \text{ of } 10,000\text{ J} = \mathbf{100\text{ J}}$ (at $T_1$).
• Step 2 (Plants → Deer): Energy passed to primary consumer (deer) = $10\% \text{ of } 100\text{ J} = \mathbf{10\text{ J}}$ (at $T_2$).
• Step 3 (Deer → Lion): Energy passed to secondary consumer (lion) = $10\% \text{ of } 10\text{ J} = \mathbf{1\text{ J}}$ (at $T_3$).
Result: Out of $10,000\text{ J}$ solar radiation, only $1\text{ J}$ reaches the apex lion!
ECOLOGICAL PYRAMID OF ENERGY — 10% LAW DYNAMICS TOTAL SOLAR RADIATION INCIDENT = 1,000,000 J (Plants capture only 1% = 10,000 J) T₁: PRODUCERS (Green Plants) — 10,000 Joules (100% Base) T₂: PRIMARY CONSUMERS (Herbivores) — 1,000 Joules (10%) T₃: SECONDARY CONSUMERS — 100 Joules (1%) T₄: 10 J (0.1%) 90% Energy Lost as Heat & Life Work
07
Why Food Chains Are Limited to 3–4 Steps & Number Pyramids
Core Board Question — Why Do Food Chains Rarely Exceed 4 or 5 Trophic Levels?
According to the 10% law, 90% of energy is dissipated into the environment at each trophic transition. By the time energy traverses 3 to 4 trophic tiers, the quantity of residual usable energy becomes so miniscule that it cannot support another population of apex consumers. A 5th or 6th consumer would expend more energy hunting and foraging than it could ever extract from its prey, rendering further steps bioenergetically unviable.

Pyramid of Numbers — Population Distribution across Trophic Levels:

  • Greatest Number of Individuals at the Base: Because energy availability is greatest at the producer level ($T_1$), ecosystems sustain the highest population density among autotrophs (grasses, phytoplankton, shrubs).
  • Progressive Reduction in Numbers: As one climbs to higher trophic levels, available energy plummets exponentially. Consequently, each successive trophic tier supports fewer individuals: $$\text{Number of Producers } (T_1) > \text{ Herbivores } (T_2) > \text{ Carnivores } (T_3) > \text{ Apex Carnivores } (T_4)$$
  • Example: A savanna grassland supports millions of blades of grass, thousands of zebras and antelopes, tens of hyenas, but only a small pride of lions.
WHY FOOD CHAINS TERMINATE AFTER 3 TO 4 STEPS T₁: Plants 10,000 J T₂: Deer 1,000 J (10%) T₃: Tiger 100 J (1%) Hypothetical T₅ < 0.1 J Usable UNSUSTAINABLE Loss of 90% energy at each step leaves insufficient energy after 4 trophic levels to sustain another apex tier.
08
Food Webs — Network of Interconnected Chains & Ecosystem Resilience
Board Definition — Food Web: In natural ecosystems, feeding relationships are rarely simple isolated straight lines. Each organism is consumed by two or more species, which are in turn consumed by several other predatory species. The resulting complex, interconnected, multidirectional mesh of interlocking food chains is known as a food web.
Characteristic Linear Food Chain Complex Food Web
Structure A straight, unbranched linear feeding sequence ($A \to B \to C \to D$). A complex network of interconnected, branching feeding pathways.
Alternative Prey None: If one intermediate species vanishes, all higher trophic tiers face direct starvation and collapse. Multiple Alternatives: If one prey species declines, predators switch to alternative prey species, maintaining ecosystem balance.
Ecosystem Stability Extremely fragile, vulnerable, and unstable in nature. Imparts high ecological resilience, adaptability, and long-term homeostasis.
FOOD WEB — INTERCONNECTED NETWORK PROVIDING ALTERNATIVE PREY PATHWAYS Green Plants / Grass (T₁) Grasshopper Rabbit Mice / Rat Seed-Eating Bird Frog Snake Hawk / Eagle (Apex)
09
Unidirectional Flow of Energy — Thermodynamics of the Biosphere
Two Definitive Inferences from the Energy Flow Diagram (Fig. 13.4):
  1. Strictly Unidirectional Nature: The solar radiant energy captured by photosynthetic autotrophs never reverts back to the Sun. Similarly, the biochemical energy transferred to herbivores does not return to autotrophs, nor does carnivore energy flow back to herbivores. Energy moves progressively upward in one single direction.
  2. Progressive Dissipation: The total usable energy available at each subsequent trophic level diminishes sharply due to non-recoverable heat lost to the environment (obeying the Second Law of Thermodynamics). Hence, nutrient minerals cycle cyclically, but energy flows strictly non-cyclically.
Feature Energy Flow in Ecosystem Nutrient (Mineral) Flow in Ecosystem
Directionality Unidirectional & Non-cyclic: Enters from Sun → flows through trophic tiers → permanently dissipates as heat. Cyclic & Recyclable: Cycles continuously between biotic living organisms and abiotic soil/water pools.
Ultimate Source Sun (continuous external thermonuclear input required). Lithosphere, hydrosphere, and atmosphere (finite atomic reserves on Earth).
Decomposer Role Decomposers release energy as metabolic heat into space (cannot recycle energy). Decomposers mineralize organic compounds into reusable inorganic nutrients.
UNIDIRECTIONAL ENERGY FLOW VS CYCLIC MINERAL FLOW SUN 1% Capture Producers Autotrophs Herbivores Primary Cons. Carnivores Top Predators ↓ Continuous 90% Heat Dissipation to Surroundings (Cannot Revert Back) ↓
10
Biological Magnification — Pesticide Accumulation in Food Chains
Board Definition — Biomagnification: The progressive accumulation and increasing concentration of toxic, non-biodegradable synthetic chemicals (such as pesticides, DDT, heavy metals) per unit biomass at each successively higher trophic level in a food chain is known as Biological Magnification.

Mechanism & Route of Entry into Human Diets:

  1. Agricultural Application: Farmers spray non-biodegradable pesticides, insecticides, and fungicides on crops to control agricultural blights and insects.
  2. Run-off into Soil & Water Bodies: Rain washes these persistent synthetic compounds into the soil and adjacent aquatic rivers, lakes, and ponds.
  3. Uptake by Producers: Terrestrial plants absorb the chemicals along with soil water and minerals; aquatic algae and phytoplanktons absorb them from water bodies.
  4. Bioaccumulation & Magnification: Because these synthetic molecules are chemically non-biodegradable and cannot be metabolized or excreted by animal organ systems, they become stored in fatty adipose tissues. When a herbivore eats thousands of contaminated plants, it absorbs all their toxins. When a carnivore eats hundreds of herbivores, the concentration multiplies exponentially.
  5. Apex Accumulation in Humans: Because humans occupy the top position in diverse food chains (consuming agricultural grains, fruits, vegetables, milk, and meat), the highest toxic pesticide concentrations accumulate inside human bodies. These toxins cannot be removed by washing, peeling, or cooking.
BIOLOGICAL MAGNIFICATION — TOXIC CONCENTRATION RISES UPWARD Water Body: Dissolved Non-Biodegradable Pesticide (DDT = 0.000003 ppm • Trace Level) Phytoplankton / Aquatic Plants (DDT = 0.04 ppm • 1,000x Increase) Small Fish (DDT = 0.5 ppm • Ingests Thousands of Plankton) Large Fish (DDT = 2.0 ppm) HUMANS (MAXIMUM • ~25 ppm) CONCENTRATION OF TOXINS INCREASES
11
Ozone Layer ($O_3$) — Formation, Protective Shield & CFC Depletion
Molecular Dual Identity of Ozone ($O_3$):
• At Ground / Tropospheric Level: Ozone is a pungent, highly toxic, lethal poison to living organisms and a dangerous respiratory pollutant.
• At Stratospheric High Altitude: Ozone forms an invaluable protective blanket that shields Earth’s surface by filtering out high-energy, lethal Ultraviolet (UV-B) radiations emitted by the Sun.

Photochemical Formation of Ozone in the Stratosphere:

High-energy UV photons strike diatomic molecular oxygen ($O_2$), dissociating it into highly reactive nascent free oxygen atoms ($O$). These free atoms instantly combine with molecular oxygen to generate ozone ($O_3$):

$$\text{Step 1: } O_2 \xrightarrow{\text{High Energy UV}} O + O \quad (\text{Photodissociation})$$ $$\text{Step 2: } O + O_2 \longrightarrow O_3 \quad (\text{Ozone Formation})$$

Hazard of Ozone Depletion & Montreal Protocol (1987):

  • Biological Hazard: Depletion allows high-frequency UV-B to penetrate, causing skin cancers, cataract blindness in the human eye cornea, suppressed immune function, and impaired photosynthesis in marine phytoplankton.
  • Chemical Culprit: Synthetic Chlorofluorocarbons (CFCs) widely used in aerosol propellants, refrigerants, and fire extinguishers. In the stratosphere, UV rays release reactive chlorine free radicals ($Cl^\bullet$), each capable of destroying up to 100,000 ozone molecules.
  • Global Policy — UNEP Montreal Protocol (1987): In 1987, the United Nations Environment Programme (UNEP) negotiated an unprecedented international accord to freeze CFC manufacturing at 1986 levels. Today, it is legally mandatory for all appliance manufacturers globally to produce 100% CFC-free refrigerators and air-conditioners (using alternatives like HFCs).
STRATOSPHERIC OZONE CYCLE & CFC-MEDIATED DEPLETION ☀ Sun UV Rays Step 1: Photolysis O₂ → O + O High energy UV splits molecular oxygen Step 2: Ozone Synthesis O + O₂ → O₃ Nascent oxygen joins diatomic oxygen Threat: CFC Breakdown Cl• + O₃ → ClO + O₂ Chlorine radical breaks 100,000 O₃ molecules!
12
Managing the Garbage — Biodegradable vs Non-Biodegradable & Railway Cup Case Study
Why Can Enzymes Not Break Down Plastics? (Biochemical Specificity):
Enzymes are biological protein catalysts that exhibit absolute substrate specificity (lock-and-key mechanism). Particular enzymes possess active sites configured to break down only specific chemical bonds found in natural organic molecules (such as peptide bonds in proteins or glycosidic bonds in starches). Microorganisms lack enzymes capable of cleaving the synthetic carbon-carbon and halogen linkages in human-made plastics. Hence, plastics remain undigested, persisting inertly for hundreds of years.
Parameter Biodegradable Waste Non-Biodegradable Waste
Definition Substances broken down into harmless, simple inorganic components by the biological action of bacteria, fungi, and saprophytes over time. Substances that cannot be cleaved or degraded by biological saprophytic action under ambient environmental conditions.
Degradation Time Rapid (few days to months). Extremely prolonged (hundreds of years; virtually non-perishable).
Environmental Impact If accumulated in heaps: causes foul odor, breeds vector mosquitoes/flies, emits greenhouse methane ($CH_4$). If composted properly: enriches soil humus. Clogs urban sewerage drains, causes soil compaction, chokes marine and stray livestock, releases carcinogenic dioxins when burnt, magnifies up food webs.
Examples Spoiled food, fruit and vegetable peels, waste paper, cotton/jute textiles, cow dung, leaves. Polythene bags, bakelite, PVC pipes, synthetic plastics, glass bottles, heavy metal batteries, pesticides (DDT).
NCERT Case Study — Disposable Cups in Indian Railways:
  1. Plastic Glasses: Introduced initially for hygiene. Rejected due to billions of non-biodegradable discarded cups causing colossal plastic pollution and drain blockage.
  2. Earthen Clay Cups (Kulhads): Proposed as a traditional eco-friendly alternative. Rejected on large-scale rollout because mass manufacturing extracted huge volumes of fertile topsoil, precipitating irreversible agricultural soil erosion.
  3. Disposable Paper Cups (Modern Solution): Manufactured from renewable cellulosic wood pulp. Completely biodegradable, recyclable, and requires no stripping of valuable agricultural topsoil.

Scientific Methods of Solid Waste Management:

1. Composting & Vermicomposting

Converting segregated kitchen organic waste and cattle dung into rich natural humus using soil earthworms (Eisenia fetida).

2. Sanitary Landfills

Compacting non-hazardous non-recyclable solid refuse into low-lying engineered pits sealed with clay/plastic liners to prevent groundwater leachate contamination.

3. High-Temperature Incineration

Thermal oxidation of hazardous bio-medical hospital waste at $> 1000^\circ\text{C}$ to reduce volume into non-infectious sterile ash.

4. Recycling & Source Segregation

Separating refuse at household sources into green (biodegradable) and blue (recyclable plastic/metal/glass) bins to recover raw manufacturing resources.

INDIAN RAILWAYS DISPOSABLE CUPS — COMPARATIVE ECOLOGICAL AUDIT 1. Plastic Cups ✗ Non-Biodegradable Persists hundreds of years Clogs urban sewers & drains 2. Clay Kulhads ✓ Biodegradable Earth ✗ Depletes fertile topsoil Severely erodes farmland 3. Paper Cups ✓ 100% Biodegradable Made from renewable pulp ✓ Ideal Sustainable Choice

NCERT In-Text Questions & Chapter Exercises

Complete official textbook solutions with comprehensive step-by-step scoring keys

NCERT In-Text Questions — Page 212
Page 212 • Q1

What are trophic levels? Give an example of a food chain and state the different trophic levels in it.

Answer:
Definition: Each step, level, or stage in a food chain where the transfer of food energy takes place from one organism to another is called a trophic level.

Example — Terrestrial Grassland Food Chain: $$\text{Grass } (T_1) \longrightarrow \text{ Grasshopper } (T_2) \longrightarrow \text{ Frog } (T_3) \longrightarrow \text{ Snake } (T_4)$$ Different Trophic Levels in this chain:
  1. First Trophic Level ($T_1$) — Producers: Grass (autotrophs; trap solar energy via photosynthesis).
  2. Second Trophic Level ($T_2$) — Primary Consumers: Grasshopper (herbivore; feeds directly on producers).
  3. Third Trophic Level ($T_3$) — Secondary Consumers: Frog (small carnivore; feeds on herbivores).
  4. Fourth Trophic Level ($T_4$) — Tertiary Consumers: Snake (larger carnivore; apex consumer in this chain).
Page 212 • Q2

What is the role of decomposers in the ecosystem?

Answer:
Decomposers (microscopic bacteria and fungi) perform vital ecological functions:
  1. Mineralization & Nutrient Recycling: They break down complex organic compounds present in dead plants, animal carcasses, and metabolic excreta into simple inorganic substances ($N, P, K$) that return to the soil and water, ensuring natural replenishment for plant root uptake.
  2. Natural Scavenging of the Earth: In their absence, dead organic remains and waste garbage would pile up endlessly, choking habitats and releasing toxic foul gases.
  3. Maintaining Ecological Equilibrium: By acting as the terminal link in nutrient cycles, they maintain continuous flow between biotic and abiotic compartments.
NCERT In-Text Questions — Page 214
Page 214 • Q1

Why are some substances biodegradable and some non-biodegradable?

Answer:
The distinction stems from the substrate specificity of biological enzymes:
  • Biodegradable Substances: Natural organic substances (such as fruit peels, paper, wood) have molecular structures and chemical bonds that can be recognized and cleaved by the specific catalytic enzymes secreted by soil decomposers (bacteria and fungi).
  • Non-Biodegradable Substances: Man-made synthetic materials (such as polythene, plastics, DDT) possess synthetic carbon-carbon linkages and halogen bonds not found in nature. Decomposers lack enzymes capable of breaking these unnatural bonds, leaving these substances persistent and unaffected by biological degradation.
Page 214 • Q2

Give any two ways in which biodegradable substances would affect the environment.

Answer:
1. Beneficial Effect (Soil Enrichment): Upon decomposition by microorganisms, biodegradable waste breaks down into natural humus and minerals, enhancing soil fertility and organic structure.
2. Adverse Effect (Sanitary Hazard if Untreated): When piled up in large unmanaged heaps, decaying biodegradable waste releases foul-smelling gases ($H_2S, NH_3$), generates greenhouse methane ($CH_4$), and serves as a breeding ground for flies, mosquitoes, and pathogenic microbes causing cholera, typhoid, and dengue.
Page 214 • Q3

Give any two ways in which non-biodegradable substances would affect the environment.

Answer:
1. Urban Drainage Choking & Soil Degradation: Discarded polythene bags and plastic wrappers choke underground drainage sewers, causing urban flooding. In the soil, they form impermeable barriers that impede rainwater percolation and restrict plant root aeration.
2. Biological Magnification: Non-biodegradable pesticides (e.g., DDT) and heavy metals cannot be metabolized or excreted by living bodies. They accumulate progressively at successive trophic levels, causing toxic damage to apex consumers and humans.
NCERT In-Text Questions — Page 216
Page 216 • Q1

What is ozone and how does it affect any ecosystem?

Answer:
Nature of Ozone: Ozone is a triatomic molecule ($O_3$) formed by three atoms of oxygen. While highly toxic and lethal at ground level (troposphere), it forms a vital protective layer in the upper stratosphere.

Ecological Effect:
  • Protective UV Shield: The stratospheric ozone layer absorbs lethal solar Ultraviolet-B (UV-B) radiation, shielding life on Earth.
  • Ecological Protection: If depleted, intense UV radiation penetrates to Earth's surface, causing skin cancers, cataract blindness, and immune suppression in animals, while destroying chlorophyll and killing marine phytoplanktons that form the base of aquatic food chains.
Page 216 • Q2

How can you help in reducing the problem of waste disposal? Give any two methods.

Answer:
Two effective, actionable methods to reduce the waste disposal crisis:
  1. Segregation of Domestic Waste at Source (Using 3 R's): Segregating household refuse into biodegradable (wet kitchen waste) and non-biodegradable (dry plastics, metals, glass). Biodegradable matter can be composted into manure at home, while clean plastics and paper can be sent to recycling facilities.
  2. Elimination of Single-Use Plastics: Carrying reusable cloth or jute bags for shopping instead of accepting plastic carry-bags, and adopting biodegradable paper cups or reusable metallic bottles.
NCERT End-of-Chapter Exercises — Pages 216–217
Exercise Q1 • MCQ

Which of the following groups contain only biodegradable items?
(a) Grass, flowers and leather
(b) Grass, wood and plastic
(c) Fruit-peels, cake and lime-juice
(d) Cake, wood and grass

Answer: Options (c) and (d) [and strictly (a), (c), (d)]:
• In option (a): Grass, flowers, and natural leather are all organic and biodegradable.
• In option (c): Fruit-peels, cake, and lime-juice are entirely biological organic items.
• In option (d): Cake, wood, and grass are all organic plant/food items broken down by microbes.
(Note: Plastic in option (b) is non-biodegradable. In official CBSE marking, both (c) and (d), as well as (a), are accepted as correct biodegradable sets).
Exercise Q2 • MCQ

Which of the following constitute a food-chain?
(a) Grass, wheat and mango
(b) Grass, goat and human
(c) Goat, cow and elephant
(d) Grass, fish and goat

Answer: (b) Grass, goat and human
Reason: A food chain requires a producer followed by sequential consumers ($T_1 \to T_2 \to T_3$). $$\text{Grass (Producer) } \longrightarrow \text{ Goat (Herbivore) } \longrightarrow \text{ Human (Omnivore)}$$ Options (a) and (c) contain only organisms of the same trophic level (all producers or all herbivores), and option (d) has no predatory sequence between fish and goat.
Exercise Q3 • MCQ

Which of the following are environment-friendly practices?
(a) Carrying cloth-bags to put purchases in while shopping
(b) Switching off unnecessary lights and fans
(c) Walking to school instead of getting your mother to drop you on her scooter
(d) All of the above

Answer: (d) All of the above
Reason: Carrying cloth bags prevents plastic waste; turning off electrical appliances conserves fossil fuel-generated electricity; walking saves fossil fuel and reduces vehicular greenhouse emissions.
Exercise Q4

What will happen if we kill all the organisms in one trophic level?

Answer:
Eliminating all individuals of one trophic level precipitates a severe ecological collapse across both higher and lower tiers:
  1. Starvation of Higher Trophic Levels: The organisms belonging to the next higher trophic level will suffer severe acute food shortages and face starvation or mass death (e.g., if all deer are killed, carnivores like tigers will starve).
  2. Explosive Overpopulation of Lower Trophic Levels: Organisms of the lower trophic level will multiply uncontrollably without predatory checks (e.g., without deer, grass may flourish initially, but without herbivores, vegetation equilibrium collapses; conversely, removing carnivores causes herbivore overgrazing, turning fertile pasture into desert).
  3. Destruction of Ecological Balance: The food chain breaks permanently, disrupting energy flow and ecosystem stability.
Exercise Q5

Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the organisms of any trophic level be removed without causing any damage to the ecosystem?

Answer:
1. Yes, the nature of the impact differs across trophic levels:
  • Removing Producers ($T_1$): Causes the most catastrophic immediate collapse. Solar energy cannot enter the ecosystem, leading to the total extinction of all herbivores and carnivores.
  • Removing Herbivores ($T_2$): Causes starvation of carnivores and unchecked overgrowth of producers.
  • Removing Carnivores ($T_3/T_4$): Causes uncontrolled herbivore population explosion, resulting in catastrophic overgrazing and desertification.
  • Removing Decomposers: Causes piling up of dead corpses, termination of nutrient biogeochemical recycling, and soil sterilization.
2. No organism can be removed without causing damage: Every trophic level is deeply interconnected through complex food webs. Removing any level invariably induces ecological imbalance and species disruption.
Exercise Q6

What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?

Answer:
Definition: Biological magnification is the progressive accumulation and increasing concentration of toxic, non-biodegradable chemicals (pesticides, heavy metals) per unit biomass at each successively higher trophic level in a food chain.

Variation Across Trophic Levels:
Yes, the levels are distinctly different and increase progressively upward:
  • Lowest concentration is found in the physical abiotic surroundings and base producers ($T_1$).
  • Because these chemicals cannot be metabolized or excreted, each consumer retains all the toxins ingested from hundreds of organisms in the preceding tier.
  • Consequently, the concentration multiplies at each ascending trophic level, reaching its absolute maximum at the highest (apex) trophic level occupied by top carnivores and humans.
Exercise Q7

What are the problems caused by the non-biodegradable wastes that we generate?

Answer:
Non-biodegradable wastes (plastics, glass, metals, pesticides) create multi-faceted environmental hazards:
  1. Blockage of Municipal Drains: Plastic bags and wrappers choke sewage channels, resulting in stagnant gutter water, foul odors, and urban waterlogging during monsoons.
  2. Soil Infertility & Compaction: Plastic sheets buried in soil form impervious barriers, hindering rainwater percolation, suffocating earthworms, and arresting root development.
  3. Harm to Stray Animals & Marine Fauna: Cattle ingest plastic bags mixed with food waste, blocking their alimentary canals and causing painful death. Discarded fishing nets and microplastics kill marine life.
  4. Air Pollution from Open Burning: Incinerating plastic refuse in open air releases carcinogenic dioxins, furans, and toxic chlorinated fumes into the atmosphere.
  5. Biological Magnification: Pesticides (DDT) and heavy metals leach into aquifers, poisoning food chains.
Exercise Q8

If all the waste we generate is biodegradable, will this have no impact on the environment?

Answer:
No, it will still have significant environmental impacts if not managed scientifically:
  1. Overwhelming Microbial Capacity: Large volumes of biodegradable waste cannot be decomposed instantaneously. Huge rotting mounds accumulate in neighborhoods.
  2. Foul Odor & Vector Diseases: Anaerobic microbial decay produces noxious, foul-smelling gases like hydrogen sulphide ($H_2S$) and ammonia ($NH_3$). These heaps become prime breeding grounds for flies, rats, and mosquitoes, spreading epidemics (cholera, dengue, dysentery).
  3. Aquatic Eutrophication: If biodegradable organic waste enters water bodies, decomposers consume massive amounts of dissolved oxygen ($O_2$) during decomposition, raising the Biochemical Oxygen Demand (BOD) and causing mass fish suffocation.
  4. Greenhouse Gas Emissions: Anaerobic decomposition in open landfills generates copious quantities of methane ($CH_4$), a potent greenhouse gas contributing to global warming.
Conclusion: Even 100% biodegradable waste requires timely, organized composting and waste-to-energy biogas processing.
Exercise Q9

Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?

Answer:
1. Why it is a Cause for Serious Concern: The stratospheric ozone layer acts as an essential shield filtering out lethal, high-frequency Ultraviolet-B (UV-B) rays from the Sun. Depletion of this barrier causes severe biological harm:
  • Human Health: Elevated UV exposure triggers skin cancers (melanoma), cellular mutations, corneal cataract blindness, and weakened immune defense systems.
  • Ecosystem Collapse: UV radiation penetrates surface ocean waters, killing photosynthetic phytoplanktons that form the base of marine food webs and stunting plant growth.
2. Global Steps Taken to Limit Damage:
  • UNEP Montreal Protocol (1987): The United Nations Environment Programme brokered an international treaty freezing chlorofluorocarbon (CFC) production and consumption at 1986 levels.
  • Global Ban on CFC Refrigerants: Mandatory phase-out of CFCs in refrigerators, aerosol sprays, and air-conditioning units, replacing them with ozone-safe alternative hydrofluorocarbons (HFCs). All appliance manufacturers globally must produce CFC-free equipment.

Board-Style Practice / PYQs

High-frequency CBSE board questions • Reveal answers on demand

1 MARK | CBSE 2024, 2020

Why is the flow of energy in an ecosystem always unidirectional?

Answer

Energy captured by autotrophs (producers) from sunlight does not revert back to the solar input, and energy passed to herbivores does not pass back to autotrophs. As it moves progressively through trophic levels, it is lost as heat to the environment and cannot be reused.

1 MARK | CBSE 2023, 2019

Which chemical is primarily responsible for the depletion of the ozone layer in the stratosphere?

Answer

Chlorofluorocarbons (CFCs), synthetic chemicals used in refrigerants, air-conditioners, and fire extinguishers.

2 MARKS | CBSE 2024, 2018

State Lindeman's 10% law of energy transfer. If $10,000J$ of energy is available to producers, calculate the energy available to the tertiary consumer.

Answer

10% Law: Only 10% of the energy entering a trophic level is stored as biomass and made available to the next higher trophic level.

• Producers ($T_1$): $10,000\text{ J}$
• Primary Consumers (Herbivores, $T_2$): $10\% \text{ of } 10,000 = 1,000\text{ J}$
• Secondary Consumers (Carnivores, $T_3$): $10\% \text{ of } 1,000 = 100\text{ J}$
• Tertiary Consumers (Top Carnivores, $T_4$): $10\% \text{ of } 100 = \mathbf{10\text{ J}}$

2 MARKS | CBSE 2023, 2017

What is Biological Magnification? Why does the concentration of toxic chemicals accumulate maximally in human beings?

Answer

Biological Magnification is the progressive increase in the concentration of non-biodegradable, toxic chemical substances (such as pesticides and heavy metals) at successive trophic levels of a food chain.

Because these chemicals cannot be metabolised or excreted by organisms, they get concentrated at each feeding stage. Since humans occupy the top-most position in multiple food chains, the maximum concentration of toxins accumulates in human tissues.

3 MARKS | CBSE 2024, 2020

Explain how the ozone shield is formed in the stratosphere. Write the chemical reactions involved and state the harmful effects of its depletion.

Answer

Formation: High-energy UV radiations split molecular oxygen ($O_2$) into free oxygen atoms ($O$). These nascent oxygen atoms combine with molecular oxygen to form ozone ($O_3$):

O₂ ──UV──→ O + O
O + O₂ → O₃ (Ozone)

Harmful Effects of Depletion: Increased UV-B radiation reaching Earth's surface causes skin cancers, cataract in eyes, damage to immune systems, and reduced crop yields by killing phytoplanktons.

3 MARKS | CBSE 2022, 2016

Differentiate between Biodegradable and Non-Biodegradable wastes. Give two examples of each and explain how non-biodegradable wastes pollute the environment.

Answer

• Biodegradable Wastes: Substances that can be broken down into simpler non-toxic substances by decomposers (bacteria/fungi). Examples: Vegetable peels, animal dung, paper.
• Non-Biodegradable Wastes: Substances that cannot be broken down by biological processes and persist indefinitely. Examples: Plastics, glass, DDT, metal cans.

Environmental Impact: (1) Cause soil and water pollution, (2) Clog drainage systems, (3) Undergo biological magnification entering the food chain, (4) Ingestion by cattle causes internal intestinal blockages.

5 MARKS | CBSE 2024, 2019

(a) Construct a 4-step terrestrial food chain and aquatic food chain.
(b) Why is the number of trophic levels in a food chain strictly limited to 3 or 4?
(c) What are decomposers? What would happen if all decomposers were completely eliminated from an ecosystem?

Answer

(a) Terrestrial Food Chain: $\text{Grass} \to \text{Grasshopper} \to \text{Frog} \to \text{Snake}$
Aquatic Food Chain: $\text{Phytoplankton} \to \text{Zooplankton} \to \text{Small Fish} \to \text{Large Fish}$

(b) Limitation of Trophic Levels: According to the 10% law, only 10% energy transfers to each successive trophic level while 90% is lost. Beyond 3 to 4 trophic levels, the amount of remaining usable energy becomes too minuscule to sustain the life processes of another consumer level.

(c) Decomposers: Microorganisms (bacteria and fungi) that decompose complex dead plant/animal remains into simple inorganic minerals. If eliminated:
1. Dead bodies and organic wastes will pile up everywhere.
2. Nutrient recycling (carbon, nitrogen, phosphorus) to soil and atmosphere will cease, leading to soil exhaustion and eventual collapse of plant growth and entire ecosystems.

PYQ+ Coming Soon

Year-wise PYQs, case-based competency questions, assertion-reasoning, and high-yield ecosystem blueprint analysis will be added here.

Quick Revision Points

Fast recall summary for quick revision before exams

  • Energy Flow: Unidirectional (Sun → Autotrophs → Heterotrophs). Energy cannot revert back.
  • 10% Law: 10% energy transferred to next level; limits food chains to 3–4 trophic levels.
  • Bio-magnification: Max pesticide/toxin accumulation occurs at the top trophic level (Humans).
  • Ozone Formation: $O_2 \xrightarrow{\text{UV}} O + O$; then $O + O_2 o O_3$. Protects against skin cancer and cataracts.
  • Montreal Protocol (1987): International agreement to freeze CFC production at 1986 levels.
  • Waste Management: Segregation at source, recycling, composting, and reducing single-use plastics.

Chapter Test

3 levels • instant score

Q1

Which of the following constitutes a food chain?

Q2

Which organism forms the first trophic level in an ecosystem?

Q3

The percentage of solar energy captured by green plants in a terrestrial ecosystem is:

Q4

Ozone layer resides in which layer of the atmosphere?

Q1

If 100 J of energy is available at the producer level, how much reaches a secondary consumer?

Q2

Which trophic level will have the maximum concentration of non-biodegradable pesticides?

Q3

Depletion of ozone layer causes increased incidence of:

Q4

Which group contains only biodegradable items?

Q1

In 1987, UNEP succeeded in forging an agreement to freeze CFC production. It was:

Q2

Disposal of waste by high-temperature burning without smoke is known as:

Q3

Decomposers in an ecosystem are mainly:

Q4

Why is an artificial aquarium an ecosystem?