NCERT Solutions Class 10 Science Chapter 15 Our Environment

Master your CBSE board exam preparations with these authoritative NCERT Solutions Class 10 Science Chapter 15 Our Environment. This chapter holds a vital place in the Class 10 Science curriculum under the Natural Resources and Biology unit, carrying consistent weightage of 3 to 5 marks in the annual CBSE board examination. Our comprehensive, step-by-step NCERT Science Class 10 solutions cover every in-text question and end-of-chapter exercise, enriched with precise definitions, energy flow calculations, photochemical equations, and board-level presentation tips to help you secure a perfect score.

Chapter Overview

Chapter 15 of Class 10 Science, Our Environment, explores the intricate relationships between living organisms and their physical surroundings. The chapter is structured into three fundamental themes:

• Ecosystem and Its Components: Understanding biotic factors (producers, consumers, decomposers) and abiotic factors (temperature, rainfall, wind, soil, minerals).
• Flow of Energy and Food Chains: Trophic levels, unidirectional energy flow, Raymond Lindeman's 10% law of energy transfer, complex food webs, and the critical phenomenon of biological magnification (biomagnification).
• Environmental Problems and Human Impact: Depletion of the stratospheric ozone layer by chlorofluorocarbons (CFCs), the role of the 1987 UNEP Montreal Protocol, and scientific management of municipal solid waste (biodegradable vs. non-biodegradable waste).

NCERT In-Text Questions and Step-by-Step Solutions

In-Text Questions (Page 257)

Question 1: What are trophic levels? Give an example of a food chain and state the different trophic levels in it.

Answer:

Trophic Levels: The distinct sequential feeding steps or levels in a food chain at which transfer of energy and nutrients takes place from one group of organisms to another are called trophic levels.

Example of a Terrestrial Food Chain:

Grass → Grasshopper → Frog → Snake → Eagle / Hawk

Trophic Level	Ecological Role	Organism in Example
First Trophic Level ($T_1$)	Autotrophs / Producers	Grass (Synthesizes organic food via photosynthesis)
Second Trophic Level ($T_2$)	Primary Consumers (Herbivores)	Grasshopper (Feeds directly on grass)
Third Trophic Level ($T_3$)	Secondary Consumers (Small Carnivores)	Frog (Feeds on insects/herbivores)
Fourth Trophic Level ($T_4$)	Tertiary Consumers (Larger Carnivores)	Snake (Feeds on secondary consumers)
Fifth Trophic Level ($T_5$)	Apex / Quaternary Consumers (Top Carnivores)	Hawk / Eagle

Question 2: What is the role of decomposers in the ecosystem?

Answer:

Decomposers are microorganisms, including saprophytic bacteria and fungi, that break down dead remains and waste products of organisms. Their crucial ecological roles include:

1. Mineral Recycling and Nutrient Cycling: Decomposers break down complex organic substances into simple inorganic substances that return to the soil and water, making them available once again to autotrophs.
2. Natural Cleansing of the Biosphere: By decomposing dead animal carcasses and fallen plant debris, they prevent the accumulation of organic waste and keep the environment clean.
3. Maintaining Soil Fertility: The humus formed during the decomposition process enriches the soil with essential minerals (phosphorus, nitrates, potassium) and enhances its moisture retention capacity.

In-Text Questions (Page 261)

Question 1: Why are some substances biodegradable and some non-biodegradable?

Answer:

Substances differ in their chemical structures and the specificity of natural biological catalysts (enzymes):

• Biodegradable Substances: These substances are natural organic compounds containing bonds (such as ester, glycosidic, or peptide bonds) that can be easily recognized, cleaved, and metabolized by the specific digestive enzymes synthesized by saprophytic bacteria and fungi (e.g., vegetable peels, paper, wood, cow dung, cotton).
• Non-Biodegradable Substances: These substances are man-made synthetic polymers or heavy chemical compounds (e.g., polythene, DDT, bakelite, nylon) characterized by strong synthetic covalent bonds. Since microorganisms do not possess specific enzymes capable of breaking down these unnatural linkages, these substances cannot be degraded and persist in the environment for hundreds of years.

Question 2: Give any two ways in which biodegradable substances would affect the environment.

Answer:

1. Generation of Foul Odour and Greenhouse Gases: Uncontrolled microbial decomposition of large heaps of biodegradable organic matter generates foul-smelling gases like hydrogen sulphide ($H_2S$) and greenhouse gases like methane ($CH_4$) and carbon dioxide ($CO_2$).
2. Vector Breeding and Water Eutrophication: Accumulation of biodegradable waste attracts flies, mosquitoes, and pathogenic microbes, causing vector-borne diseases. If dumped into water bodies, their rapid aerobic breakdown consumes dissolved oxygen, resulting in eutrophication and asphyxiation of aquatic organisms.

Question 3: Give any two ways in which non-biodegradable substances would affect the environment.

Answer:

1. Biological Magnification: Toxic synthetic non-biodegradable chemicals such as pesticides (e.g., DDT) enter aquatic and terrestrial food chains, remain unmetabolized and unexcreted, and accumulate in progressively higher concentrations at each successive trophic level, causing severe reproductive and neurological disorders in top consumers.
2. Soil and Drainage System Degradation: Non-biodegradable plastics choke urban storm drains and sewage pipelines, causing severe waterlogging. In the soil, plastic sheets create an impermeable barrier that blocks water percolation, depletes groundwater recharge, and inhibits root respiration of plants.

In-Text Questions (Page 264)

Question 1: What is ozone and how does it affect any ecosystem?

Answer:

Ozone ($O_3$) is a triatomic molecule made of three oxygen atoms. While ozone is a deadly poison at the ground/tropospheric level, it forms a protective blanket in the upper atmosphere (stratosphere):

• Ecosystem Protection: The stratospheric ozone layer absorbs lethal high-energy ultraviolet (UV) radiations (specifically UV-B) emitted by the Sun.
• Consequences of Depletion: If UV radiation reaches the Earth's surface unimpeded, it damages DNA, causes skin cancer, cataract, and immune system suppression in humans. In ecosystems, it destroys delicate marine phytoplankton (the primary producers of oceans), disrupting marine food chains and global carbon sequestration.

Question 2: How can you help in reducing the problem of waste disposal? Give any two methods.

Answer:

1. Segregation of Waste at Source: Separating household garbage into dry recyclable waste (plastic, glass, paper, metal) and wet biodegradable waste (vegetable peels, leftover food) ensures efficient processing. Wet waste can be converted into organic compost/vermicompost, while dry waste can be routed to municipal recycling facilities.
2. Adopting the 3Rs Principle (Reduce, Reuse, Recycle): Replacing single-use plastic bags with reusable cloth or jute bags, refusing over-packaged consumer goods, and repurposing household containers substantially decreases the aggregate volume of municipal solid waste destined for landfills.

NCERT Chapter End Exercise Solutions

Question 1: 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 (a), (c), and (d) all contain exclusively biodegradable items.

Detailed analysis:

• (a) Grass, flowers, leather: Grass and flowers are plant matter; leather is processed animal skin. All are organic and biodegradable.
• (b) Grass, wood, plastic: Plastic is a synthetic non-biodegradable polymer. Hence, group (b) is incorrect.
• (c) Fruit-peels, cake, lime-juice: All are organic biological derivatives and biodegradable.
• (d) Cake, wood, grass: Cake is food material, wood is cellulose-based plant tissue, grass is natural vegetation; all are biodegradable.

Question 2: 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

Explanation: A food chain requires a sequential predator-prey relationship where energy is transferred across trophic levels. In option (b), grass is the producer (autotroph), goat is the primary consumer (herbivore) that feeds on grass, and human is the secondary consumer (omnivore/carnivore) that consumes the goat.

Question 3: 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

Explanation: Cloth bags curb non-biodegradable plastic pollution; switching off electrical appliances conserves electrical energy and reduces fossil fuel combustion at power plants; walking eliminates tailpipe greenhouse gas and particulate emissions.

Question 4: What will happen if we kill all the organisms in one trophic level?

Answer:

Removing or killing all organisms of a particular trophic level creates severe ecological imbalances known as a trophic cascade:

1. Starvation of the Next Trophic Level: The organisms of the immediately higher trophic level will face an acute shortage of food and will either starve to death or migrate, eventually leading to their population collapse.
2. Overpopulation of the Lower Trophic Level: The organisms of the immediately lower trophic level will experience zero predation pressure, causing their population to multiply exponentially. This overpopulation depletes their foundational food resources (e.g., overgrazing of vegetation if herbivores explode).
3. Disruption of Food Web Dynamics: The collapse of one pathway forces predators to switch exclusively to alternative prey species, destabilizing multiple interconnected feeding channels across the ecosystem.

Question 5: 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:

Yes, the specific nature and immediate manifestations of the impact will vary depending on which trophic level is eliminated:

• Removal of Producers ($T_1$): Total collapse of the entire ecosystem, as solar energy cannot be captured, depriving all subsequent heterotrophic levels of sustenance.
• Removal of Primary Consumers ($T_2$): Autotrophs grow unchecked while secondary carnivores face immediate starvation.
• Removal of Apex Predators ($T_3/T_4$): Causes runaway population explosions of lower-order carnivores and herbivores, leading to habitat degradation.
• Removal of Decomposers: Nutrient cycling halts completely; corpses and organic waste accumulate indefinitely, suffocating the biosphere.

Conclusion: No organism or trophic level can be removed without causing damage. Every species occupies a specific ecological niche in the food web, and removing any component inevitably degrades ecological stability.

Question 6: What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?

Answer:

Biological Magnification (Biomagnification): The progressive increase in the concentration of non-biodegradable toxic substances (such as DDT, mercury, lead, and other pesticides) in the living tissues of organisms at each successive trophic level of a food chain is defined as biological magnification.

Mechanism and Variation across Levels:

• Non-biodegradable pesticides washed into water bodies are absorbed by aquatic autotrophs (phytoplankton).
• Because these toxins cannot be metabolized, broken down by enzymes, or rapidly excreted, they accumulate within fatty tissues.
• A primary consumer ingests a vast number of autotrophs over its lifespan, concentrating the toxin. In turn, secondary and tertiary consumers consume multiple lower-level organisms.
• Yes, the concentration is different at each level: It increases progressively from the base to the apex of the food chain. Consequently, top carnivores and humans (who occupy the highest trophic level in many food chains) suffer the maximum concentration of these toxic contaminants.

Question 7: What are the problems caused by the non-biodegradable wastes that we generate?

Answer:

1. Soil Degradation and Groundwater Blockage: Synthetic plastic wastes do not decay, preventing natural aeration and percolation of rainwater into underground aquifers.
2. Choking of Sewerage Systems: Plastic bags and debris clog drainage pipelines, causing urban flooding and filthy stagnant water breeding mosquitoes.
3. Bioaccumulation and Toxicity: Toxic industrial pollutants and agricultural pesticides enter the food chain, causing biomagnification and chronic toxicity in animals and humans.
4. Threat to Wildlife and Livestock: Stray cattle and wild animals frequently ingest littered plastic bags with food remnants, leading to lethal intestinal blockage and death.
5. Air Pollution via Incineration: Burning non-biodegradable polymers releases highly carcinogenic gases, such as dioxins and furans, into the atmosphere.

Question 8: If all the waste we generate is biodegradable, will this have no impact on the environment?

Answer:

No, it will still have significant adverse impacts if not managed scientifically. An excess of biodegradable waste causes:

1. Decomposition Overload and Air Pollution: Immense heaps of rotting organic waste release offensive odours and substantial quantities of potent greenhouse gases ($CH_4$ and $CO_2$), accelerating climate change.
2. Aquatic Hypoxia: If massive amounts of biodegradable waste wash into rivers and lakes, saprophytic bacterial decomposition consumes excessive dissolved oxygen (elevated Biochemical Oxygen Demand - BOD), suffocating fish and aquatic fauna.
3. Epidemics and Public Health Crises: Piles of unmanaged decaying organic waste serve as breeding hubs for flies, rats, and harmful bacteria, spreading typhoid, cholera, and dysentery.

Question 9: Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?

Answer:

Cause for Concern: The stratospheric ozone layer filters out lethal solar Ultraviolet-B (UV-B) radiation. Depletion of this protective shield exposes life forms on Earth to unfiltered UV rays, leading to:

• High incidence of skin carcinomas (melanoma) and premature aging in humans.
• Severe eye damage, including cataracts and permanent retinal injury.
• Weakened immune system responses in living organisms.
• Destruction of marine phytoplankton, causing catastrophic collapse of oceanic food chains and reduced global photosynthesis.

Mitigation Steps:

1. UNEP Montreal Protocol (1987): In 1987, the United Nations Environment Programme (UNEP) formulated a globally binding international treaty to freeze and phase out the production of Chlorofluorocarbons (CFCs) at 1986 levels.
2. Adoption of Ozone-Friendly Alternatives: Refrigerator and air-conditioner manufacturers globally transitioned to CFC-free and hydrofluorocarbon (HFC/HFO) refrigerants.

Important Formulas, Laws, and Scientific Principles

1. Lindeman's 10% Law of Energy Transfer (1942)

Statement: In any food chain, only about 10% of the chemical energy available at a given trophic level is transferred and incorporated into organic biomass at the next higher trophic level. The remaining 90% is lost to the environment as metabolic heat during respiration, digestion, locomotion, and reproduction.

2. Photochemical Formation of Ozone in Stratosphere

High-energy ultraviolet (UV) radiation splits molecular oxygen ($O_2$) into free oxygen atoms (nascent oxygen). These highly reactive free atoms instantly combine with intact molecular oxygen to form ozone ($O_3$):

$$\text{Step 1: } O_2 \xrightarrow{\text{High Energy UV}} O + O \quad (\text{Photodissociation})$$

$$\text{Step 2: } O + O_2 \longrightarrow O_3 \quad (\text{Ozone Molecule})$$

3. Characteristics of Energy Flow in an Ecosystem

• Unidirectional Flow: Energy captured by autotrophs does not revert to the Sun, and energy passing to herbivores never returns to autotrophs.
• Progressive Dissipation: Energy diminishes progressively at each step due to metabolic heat loss, which restricts typical food chains to 3 or 4 trophic levels. Beyond 4 levels, the residual energy is insufficient to sustain viable consumer populations.

Common Mistakes and Tips for CBSE Board Exams

Board Exam Relevance and Marking Distribution

In the CBSE Class 10 Board Examinations, Chapter 15 Our Environment carries 3 to 5 marks. Questions from this chapter are highly structured and typically include:

• 1-Mark MCQs / Assertion-Reason: Identifying biodegradable sets, trophic level sequence, or UNEP treaty year (1987).
• 2-Mark Short Answer Questions: Distinguishing between biodegradable and non-biodegradable wastes, role of decomposers, or ozone chemistry equations.
• 3-Mark Conceptual / Numerical Questions: Calculating energy transfer using the 10% law with a given food chain, or explaining biological magnification with examples.
• Case-Based Questions (4 Marks): Real-world scenarios on waste disposal management (e.g., Kulhads vs. disposable plastic cups on Indian Railways) and ozone layer recovery trends.

More NCERT Solutions and Practice

Achieving mastery in Class 10 Science requires consistent problem-solving and rigorous practice with official board-style questions. Utilize our comprehensive, chapter-wise CBSE NCERT solutions and access curated sample papers, assertion-reason question banks, and case-study tests for Class 10 Science at qptool.theorify.in to accelerate your board exam readiness!