Navigating through the CBSE Class 10 Science curriculum requires an in-depth understanding of ecosystem dynamics, food chains, food webs, trophic levels, Lindeman’s 10% law of energy transfer, biological magnification, stratospheric ozone depletion, and solid waste management. Chapter 13 of Class 10 Biology, “Our Environment”, explores the delicate ecological balance between biotic communities and abiotic physical environments. It investigates how solar energy flows unidirectionally through trophic levels while toxic synthetic pesticides accumulate progressively up the food chain; details the photochemical formation and destruction of the stratospheric ozone shield (O_3) by Chlorofluorocarbons (CFCs); and examines the environmental impacts of non-biodegradable plastics and biodegradable municipal wastes. To help students master every aspect of this high-weightage chapter, this comprehensive solutions guide offers textbook-accurate, highly structured, and pedagogically sound responses strictly aligned with the latest CBSE evaluation standards.
Every question presented in the official NCERT textbook—ranging from all three in-text question sets (Pages 257, 261, and 264) to the complete chapter-end exercises (Questions 1 to 9 on Pages 264–265)—has been solved with exhaustive detail. Short-answer conceptual responses follow the official 30–40 word limit, while 3-mark and 5-mark ecological problems use structured tables and clear energy flow schematics. Key scoring terms, official CBSE board year tags, and comparative charts have been highlighted to ensure students secure maximum marks in their CBSE Board Examinations.
Master Formula & Concept Summary Tables
1. The 10% Law of Energy Transfer Across Trophic Levels
| Trophic Level | Ecological Role / Category | Example Organisms | Energy Available (From 10,000 J Solar Incident Energy) |
|---|---|---|---|
| First Trophic Level (T_1) | Producers (Green Plants / Autotrophs) | Phytoplankton, Grass, Trees | 100 J (Captures only ~1% of incident solar energy) |
| Second Trophic Level (T_2) | Primary Consumers (Herbivores) | Zooplankton, Grasshoppers, Deer | 10 J (10% of 100 J transmitted) |
| Third Trophic Level (T_3) | Secondary Consumers (Small Carnivores) | Small Fish, Frogs, Birds | 1 J (10% of 10 J transmitted) |
| Fourth Trophic Level (T_4) | Tertiary Consumers (Top Carnivores) | Large Fish, Snakes, Tigers, Humans | 0.1 J (10% of 1 J transmitted) |
2. Comparison: Biodegradable vs. Non-Biodegradable Wastes
| Parameter / Basis | Biodegradable Waste | Non-Biodegradable Waste |
|---|---|---|
| Origin & Nature | Organic substances derived from plant and animal sources. | Synthetic, human-made chemical compounds and plastics. |
| Action of Micro-organisms | Broken down into simple inorganic nutrients by bacterial and fungal enzymes. | Cannot be decomposed; specific enzymes do not exist in nature. |
| Environmental Persistence | Persists for a short period; recycles back into the soil. | Persists for hundreds of years; accumulates in landfills and oceans. |
| Biological Magnification | Does not cause biomagnification in food chains. | Causes severe Biological Magnification of toxic pesticides (e.g., DDT). |
| Examples | Vegetable peels, animal dung, paper, cotton, wood, fallen leaves. | Polythene bags, nylon, plastics, glass bottles, DDT, radioactive waste. |
3. Chemistry of Stratospheric Ozone Formation and Depletion
| Reaction Step | Physical Agent / Chemical Reaction | Description / Significance |
|---|---|---|
| Step 1: Photodissociation | O₂ —(High Energy UV Radiation)→ O + O | High-energy solar ultraviolet radiation splits diatomic molecular oxygen into two nascent, highly reactive free oxygen atoms. |
| Step 2: Ozone Synthesis | O + O₂ → O₃ (Ozone Molecule) | A free oxygen atom combines with an intact oxygen molecule to form a triatomic Ozone (O_3) molecule in the stratosphere. |
| Step 3: Ozone Depletion | CFCs —(UV)→ Cl• then Cl• + O₃ → ClO• + O₂ | Synthetic Chlorofluorocarbons release active chlorine free radicals that catalytically destroy thousands of ozone molecules. |
NCERT In-Text Questions: Set 1 (Page No. 257)
Question 1 Why are some substances biodegradable and some non-biodegradable? [CBSE 2024, 2022]
Answer: The distinction is based on the specificity of biological enzymes secreted by decomposers (bacteria and fungi):
- Biodegradable Substances: Natural organic substances (like cellulose, protein, starch) possess chemical bonds that match the active sites of bacterial and fungal enzymes. Decomposers break them down into simple inorganic nutrients.
- Non-Biodegradable Substances: Human-made synthetic substances (like plastics, polythene, DDT) have complex synthetic chemical structures for which no natural enzymes exist in nature. Hence, microorganisms cannot decompose them.
Question 2 Give any two ways in which biodegradable substances would affect the environment. [CBSE 2024, 2020]
Answer: When present in large, unmanaged quantities, biodegradable substances affect the environment in two negative ways:
- Foul Odour and Air Pollution: Anaerobic decomposition of heaped organic waste releases obnoxious, toxic gases like Hydrogen Sulphide (H_2S) and Methane (CH_4), creating severe air pollution.
- Breeding Ground for Pathogens: Rotting garbage dumps serve as breeding grounds for flies, mosquitoes, and pathogenic bacteria, spreading infectious diseases like cholera, typhoid, and dengue.
Question 3 Give any two ways in which non-biodegradable substances would affect the environment. [CBSE 2024, 2023]
Answer:
- Biological Magnification: Toxic chemical pesticides (like DDT) and heavy metals cannot be metabolized or excreted. They enter aquatic and terrestrial food chains, accumulating in progressively higher concentrations at each trophic level, poisoning top consumers like humans.
- Soil and Water Pollution: Discarded plastics and polythene block drainage systems, choke aquatic life in rivers and oceans, and prevent rainwater from percolating into the ground, severely depleting groundwater recharge.
NCERT In-Text Questions: Set 2 (Page No. 261)
Question 1 What are trophic levels? Give an example of a food chain and state the different trophic levels in it. [CBSE 2024, 2020]
Answer: A trophic level is a distinct feeding position or step in a food chain through which transfer of food energy takes place in an ecosystem:
================================================================================ EXAMPLE OF A TERRESTRIAL GRASSLAND FOOD CHAIN: -------------------------------------------------------------------------------- 1. First Trophic Level (T₁ - Producers) : Grass / Green Plants 2. Second Trophic Level (T₂ - Primary Consumers) : Deer / Herbivores 3. Third Trophic Level (T₃ - Secondary Consumers) : Tiger / Carnivores Energy Flow Schematic: Grass (T₁) ────────► Deer (T₂) ────────► Tiger (T₃) ================================================================================
Question 2 What is the role of decomposers in the ecosystem? [CBSE 2024, 2022, 2020]
Answer: Decomposers (saprophytic bacteria and fungi) perform three vital ecological functions:
- Nutrient Recycling: They break down complex organic matter from dead plants and animal excreta into simple inorganic substances (carbon, nitrogen, phosphorus) that return to the soil, water, and air for reuse by producers.
- Natural Cleansing Agents: By decomposing dead bodies, they prevent the accumulation of rotting organic corpses, cleansing the environment naturally.
- Maintaining Ecosystem Balance: They complete biogeochemical cycles, ensuring a continuous supply of essential mineral raw materials for autotrophic photosynthesis.
NCERT In-Text Questions: Set 3 (Page No. 264)
Question 1 What is ozone and how does it affect any ecosystem? [CBSE 2024, 2023]
Answer: Ozone is a triatomic molecule made of three oxygen atoms (O_3):
- Dual Nature: At ground level (troposphere), ozone is a deadly toxic pollutant. However, in the upper atmosphere (stratosphere), it forms a protective Ozone Layer.
- Ecosystem Effect: The stratospheric ozone layer absorbs over 99% of the harmful solar Ultraviolet (UV-B) radiation.
- Consequences of Ozone Depletion: If the ozone layer is damaged, excessive UV radiation reaches Earth, causing skin cancer, cataract, and immune suppression in humans, destroying delicate phytoplankton (the base of aquatic food chains), and inhibiting photosynthesis and crop yields in terrestrial plants.
Question 2 How can you help in reducing the problem of waste disposal? Give any two methods. [CBSE 2024, 2020]
Answer: We can reduce the municipal waste disposal crisis through two practical methods:
- Source Segregation and Composting: Separate domestic waste at home into green bins (biodegradable) and blue bins (non-biodegradable). Convert kitchen and garden wastes into nutrient-rich organic manure (Compost) for gardening, reducing landfill burden.
- Practicing the 3 R’s (Reduce, Reuse, Recycle): Minimize single-use plastics by carrying reusable cloth/jute bags for shopping, reuse plastic containers for storage, and send paper, metals, and glass bottles to industrial recycling facilities.
NCERT Chapter-End Exercises (Page No. 264-265)
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: (c) Fruit-peels, cake and lime-juice and (d) Cake, wood and grass (Also (a) if raw natural leather is considered) Explanation: Plastic is completely non-biodegradable (making option b incorrect). Fruit-peels, cake, lime juice, wood, and grass are 100% natural organic substances easily broken down by microbial enzymes.
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 valid food chain represents a sequential feeding relationship where energy transfers from one trophic level to the next: Grass (Producer, T₁) → Goat (Herbivore, T₂) → Human (Omnivore/Carnivore, T₃)
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: All three habits conserve energy, reduce fossil fuel consumption, eliminate plastic pollution, and lower carbon emissions.
Question 4 What will happen if we kill all the organisms in one trophic level? [CBSE 2024, 2022]
Answer: If all organisms in a specific trophic level are eliminated, the entire ecological balance is severely disrupted:
- Starvation of Upper Trophic Level: The organisms in the immediate higher trophic level will face an acute food shortage, leading to massive starvation, population decline, or extinction.
- Uncontrolled Population Explosion of Lower Level: The organisms in the immediate lower trophic level will lose their natural predators, causing an uncontrolled population explosion that will overconsume and destroy the lower vegetative base.
- Ecosystem Collapse: The food web will break, leading to ecological instability and species loss.
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? [CBSE 2024, 2020]
Answer:
- Yes, the impact is different at different trophic levels:
- Removing Producers (T_1): Causes the most catastrophic immediate collapse because solar energy cannot enter the ecosystem, causing all herbivores and carnivores to starve to death.
- Removing Top Carnivores (T_4): Causes an explosion of herbivores, resulting in rapid overgrazing, deforestation, and soil desertification.
- No organism can be removed without damage: Every trophic level forms an interconnected link in the ecological food web. Removing any level invariably destabilizes energy flow and disrupts ecosystem equilibrium.
Question 6 What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem? [CBSE 2024, 2023, 2020]
Answer: Biological Magnification (Biomagnification) is the phenomenon of progressive increase in the concentration of non-biodegradable toxic chemicals (such as DDT, mercury, and lead) in the tissues of organisms at each successive trophic level of a food chain:
================================================================================ MAGNIFICATION PROFILE IN AN AQUATIC FOOD CHAIN: -------------------------------------------------------------------------------- 1. Water (Chemical Source) : 0.000003 ppm of DDT 2. Phytoplankton (Producers - T₁) : 0.04 ppm 3. Small Fish (Primary/Secondary Consumers - T₂) : 0.5 ppm 4. Large Fish (Tertiary Consumers - T₃) : 2.0 ppm 5. Fish-Eating Birds / Humans (Top Consumers - T₄): 25.0 ppm (HIGHEST) ================================================================================
- Variation Across Levels: Yes, the concentration is lowest at the first trophic level (producers) and reaches the absolute maximum at the highest trophic level (top carnivores/humans) because top consumers ingest large biomass from lower levels where non-excretable toxins accumulate.
Question 7 What are the problems caused by the non-biodegradable wastes that we generate? [CBSE 2024, 2020]
Answer: Non-biodegradable wastes create severe ecological hazards:
- Soil Infertility: Plastic bags and toxic chemicals smother soil microflora, prevent root aeration, and stop rainwater percolation, causing land degradation.
- Drainage Choking & Floods: Discarded plastics block municipal sewage drains, causing water-logging and urban flooding during rains.
- Threat to Stray Animals and Marine Life: Stray cattle ingest plastic bags containing food waste, which choke their digestive tracts and cause fatal intestinal blockages; marine animals die from ingesting microplastics.
- Toxic Chemical Leaching: Electronic waste (e-waste) and batteries leach heavy metals (lead, cadmium, mercury) into groundwater reserves.
Question 8 If all the waste we generate is biodegradable, will this have no impact on the environment? [CBSE 2024, 2022]
Answer: No, even if 100% of waste is biodegradable, it will still have severe adverse environmental impacts if generated in excessive, unmanaged quantities:
- Decomposition Bottleneck: Decomposers (bacteria) have a limited rate of decomposition. Piles of accumulated organic garbage will rot in the open.
- Severe Air & Water Pollution: Large-scale rotting generates foul smells, releases greenhouse gases (CH_4, CO_2), and when washed into water bodies, consumes dissolved oxygen (Eutrophication), suffocating fish and aquatic life.
- Epidemic Outbreaks: Massive garbage heaps attract vectors (flies, rats), triggering outbreaks of infectious diseases.
Question 9 Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage? [CBSE 2024, 2023, 2020]
Answer:
- Cause for Concern: Thinning of the stratospheric ozone layer allows high-energy Ultraviolet (UV-B) radiation to reach Earth, causing skin cancers, genetic mutations, cataract, weakened immune systems in humans, and destruction of oceanic phytoplanktons.
- International Protective Measures:
- In 1987, the United Nations Environment Programme (UNEP) successfully forged the Montreal Protocol, an international treaty that froze the production of Chlorofluorocarbons (CFCs) at 1986 levels.
- Globally, it is now legally mandatory for all industrial manufacturers to produce CFC-free refrigerators, air conditioners, and aerosol sprays, replacing them with ozone-friendly Hydrofluorocarbons (HFCs).
Frequently Asked Questions (FAQs) – Class 10 Biology Chapter 13
Question 1: What are the biotic and abiotic components of an ecosystem? [CBSE 2024] Answer:
- Biotic Components: All living organisms within an ecosystem, classified as Producers (plants), Consumers (animals), and Decomposers (bacteria/fungi).
- Abiotic Components: All non-living physical and chemical factors, including temperature, rainfall, wind, soil, minerals, sunlight, and humidity.
Question 2: State Lindeman’s 10% Law of Energy Transfer. [CBSE 2024, 2022] Answer: Lindeman’s law states that only approximately 10% of the total chemical energy present in the organic food of one trophic level is transferred and assimilated into the biomass of the next higher trophic level; the remaining 90% is lost as metabolic heat, respiration, and excretion.
Question 3: Why is energy flow in an ecosystem always unidirectional? [CBSE 2024, 2020] Answer: Solar energy captured by green autotrophs never reverts to the Sun, and energy passing from herbivores to carnivores cannot flow backward to plants. At each trophic step, energy is dissipated as unusable metabolic heat, making energy flow strictly one-way (unidirectional).
Question 4: Why are food chains generally limited to only 3 or 4 trophic levels? [CBSE 2024, 2023] Answer: According to the 10% law, energy decreases exponentially at each successive trophic step. After 3 to 4 trophic levels, the amount of remaining available energy is so minuscule that it cannot support another viable population of higher consumers.
Question 5: What is a Food Web? How is it different from a Food Chain? [CBSE 2023] Answer: A food chain is a single linear pathway of energy transfer; a Food Web is a complex, interconnected network of multiple food chains operating simultaneously in an ecosystem, providing alternative feeding pathways that enhance ecological stability.
Question 6: What was the main objective of the 1987 Montreal Protocol? [CBSE 2024] Answer: The Montreal Protocol was an international environmental treaty organized by UNEP to freeze and phase out the production and consumption of ozone-depleting substances like Chlorofluorocarbons (CFCs).
Question 7: What are the three R’s of waste management? [CBSE 2022] Answer:
- Reduce: Minimizing consumption and unnecessary waste generation.
- Reuse: Using items repeatedly for other purposes instead of throwing them away.
- Recycle: Processing waste materials (plastics, metals, glass, paper) into new products.
Question 8: Why is disposable plastic tableware being replaced by kulhads (clay cups) and paper cups? [CBSE 2024, 2020] Answer: Plastic cups are non-biodegradable and create landfill waste. Kulhads (clay cups) were tested, but making them on a massive scale led to the loss of fertile agricultural topsoil. Disposable paper cups made from biodegradable wood pulp are now preferred because they decompose naturally and can be recycled.
Question 9: What is Eutrophication? [CBSE 2023] Answer: Eutrophication is the excessive growth of algae (algal bloom) in water bodies caused by nutrient enrichment from agricultural fertilizer runoff and sewage. When the algae die, decomposing bacteria consume all dissolved oxygen, causing fish and aquatic life to suffocate.
Question 10: At which trophic level is pesticide concentration highest in a food chain? Why? [CBSE 2024] Answer: Pesticide concentration is highest at the top-most trophic level (e.g., human beings) due to biological magnification, as non-biodegradable toxins accumulate in body fat and cannot be excreted.
Question 11: How do producers capture solar energy? What percentage do they capture? [CBSE 2023] Answer: Terrestrial green plants capture approximately 1% of the total incident solar energy falling on their leaves and convert it into chemical energy through chlorophyll-driven photosynthesis.
Question 12: Why are artificial ecosystems like aquariums and crop fields called managed ecosystems? [CBSE 2022] Answer: They are human-made systems that lack natural biological self-sustainability; an aquarium requires regular artificial cleaning, oxygen pumping, and feeding because it lacks an active community of natural decomposers.
Question 13: What happens to the ozone molecule when it absorbs UV radiation? [CBSE 2024] Answer: When an ozone molecule (O_3) absorbs a high-energy UV photon, it dissociates into a stable oxygen molecule (O_2) and an energetic free oxygen atom (O), absorbing the harmful radiation before it reaches the ground.
Question 14: What is vermicomposting? [BOARD EXAM FAVORITE] Answer: Vermicomposting is the bio-technological process of converting biodegradable organic solid waste into nutrient-rich organic compost using specialized earthworms (such as red wigglers).
Question 15: Why is biological magnification dangerous for human health? [CBSE 2024] Answer: Humans occupy the top consumer position in diverse food chains. High accumulation of toxic pesticides and heavy metals leads to chronic organ failure, endocrine disruption, nervous system disorders, and cancer.
Mastering the NCERT Solutions for Class 10 Science Chapter 13, “Our Environment”, equips students with the ecological principles, energy flow calculations, and environmental conservation strategies required for the CBSE Board Examination. Review the 10% energy law calculations, the ozone formation chemistry, and the 15 board-level FAQs above to secure full marks in your environmental science evaluations.