Class 10 Science Chapter 15 Question Answer | Our Environment | English Medium

Class 10 Science Chapter 15 Question Answer | Our Environment | English Medium | ASSEB

Chapter 15 — Our Environment

Welcome to HSLC Guru! This chapter explores the environment around us — the living and non-living components, how energy and matter flow through ecosystems, the threats to our atmosphere such as ozone depletion, and how we can manage waste responsibly. Through this lesson, ASSEB Class 10 Science learners will gain a clear understanding of ecosystems, food chains, biological magnification, the ozone layer, and effective waste-management practices that help protect Earth’s fragile balance.

Summary

An ecosystem is a self-sustaining unit of nature where living organisms (the biotic components — plants, animals, microorganisms) interact with their non-living surroundings (the abiotic components — air, water, soil, sunlight, temperature). Ecosystems may be natural (forest, pond, ocean) or artificial (garden, aquarium, crop field). Within every ecosystem, organisms are linked through feeding relationships called food chains. A food chain shows the linear flow of energy from one organism to another, beginning with producers (green plants), passing through primary consumers (herbivores), then secondary consumers (small carnivores), and finally tertiary consumers (top carnivores). Decomposers such as bacteria and fungi break down dead organic matter, returning nutrients to the soil. Several interconnected food chains form a food web.

Each step or feeding level in a food chain is called a trophic level. According to the 10% Law of energy transfer proposed by Lindeman, only about 10% of the energy available at one trophic level is passed on to the next; the remaining 90% is lost as heat, used in life processes, or remains unutilised. This is why food chains are usually short, with only 3–4 trophic levels. The flow of energy is unidirectional — from the Sun to producers, then to consumers and decomposers. Harmful chemicals such as pesticides (DDT) entering the food chain become more concentrated at each successive trophic level — a phenomenon called biological magnification or biomagnification. Graphical representations of trophic structure include the pyramid of energy (always upright) and the pyramid of biomass (usually upright, sometimes inverted as in a pond ecosystem).

The ozone layer, made up of triatomic oxygen molecules (O₃), lies in the upper atmosphere (stratosphere) and acts as a protective shield against harmful ultraviolet (UV) radiation from the Sun. Ozone is formed when high-energy UV rays split molecular oxygen (O₂) into free atoms, which then combine with O₂ to form O₃. However, since the 1980s, the ozone layer has been damaged by human-made chemicals known as chlorofluorocarbons (CFCs), used in refrigerators, air conditioners, aerosol sprays and fire extinguishers. CFCs release chlorine atoms that destroy ozone molecules. To control this, the United Nations Environment Programme (UNEP) succeeded in framing the Montreal Protocol in 1987, an international agreement to freeze and reduce CFC production worldwide.

The waste produced by human activities is called garbage. Substances that can be broken down by biological processes are biodegradable (vegetable peels, paper, cotton, dung), while those that cannot are non-biodegradable (plastic, glass, metals, DDT). The huge accumulation of garbage threatens the environment, so proper management is essential. Effective methods include segregation of waste at source into biodegradable and non-biodegradable bins, recycling of paper, plastic, glass and metal, composting of kitchen and garden waste into manure, and the production of biogas from cattle dung and organic waste. Common waste-disposal methods include landfilling, incineration, sewage treatment, vermicomposting and recycling. Responsible disposal — guided by the principle of “Reduce, Reuse, Recycle” — is the key to a healthy environment.

Textbook Question Answers

One Mark Questions

Q1. What is an ecosystem?

Answer: An ecosystem is a self-sustaining functional unit of nature where living organisms interact among themselves and with their non-living physical environment.

Q2. Name the two components of an ecosystem.

Answer: The two components are biotic components (living things) and abiotic components (non-living things).

Q3. What are producers? Give one example.

Answer: Producers are green plants that prepare their own food through photosynthesis. Example — grass, mango tree.

Q4. Define trophic level.

Answer: Each step or level in a food chain at which energy transfer takes place is called a trophic level.

Q5. What is the 10% law of energy transfer?

Answer: Only 10% of the energy from one trophic level is transferred to the next; the remaining 90% is lost as heat and through life processes.

Q6. What is biological magnification?

Answer: The progressive increase in the concentration of harmful chemicals at successive trophic levels of a food chain is called biological magnification.

Q7. What is the chemical formula of ozone?

Answer: The chemical formula of ozone is O₃ (triatomic oxygen).

Q8. Expand CFC. Where is it used?

Answer: CFC stands for Chlorofluorocarbon. It is used in refrigerators, air conditioners, aerosol sprays and fire extinguishers.

Q9. When was the Montreal Protocol signed?

Answer: The Montreal Protocol was signed in 1987 to freeze the production of CFCs and protect the ozone layer.

Q10. Give one example each of a biodegradable and a non-biodegradable substance.

Answer: Biodegradable — vegetable peels; Non-biodegradable — plastic.

Two / Three Mark Questions

Q1. Differentiate between biotic and abiotic components with examples.

Answer: Biotic components are the living parts of an ecosystem such as plants, animals and microorganisms. Abiotic components are the non-living physical and chemical factors such as sunlight, temperature, air, water and soil. Both interact to keep the ecosystem balanced.

Q2. What is a food web? How is it different from a food chain?

Answer: A food chain is a linear sequence of organisms through which food and energy pass from one to another. A food web is a network of many interconnected food chains in an ecosystem. Food webs make ecosystems more stable because organisms have alternative food sources.

Q3. Why are decomposers important in an ecosystem?

Answer: Decomposers such as bacteria and fungi break down dead plants and animals into simpler substances. They return essential nutrients to the soil, helping recycle minerals, and prevent the accumulation of dead organic matter — keeping the ecosystem clean and balanced.

Q4. Explain why energy flow in an ecosystem is unidirectional.

Answer: The energy captured by producers from the Sun flows in one direction — from producers to herbivores, then to carnivores and finally to decomposers. At every trophic level, most of the energy is lost as heat (90%) and cannot be reused, so energy never flows back to the Sun. Hence energy flow is unidirectional.

Q5. What is ozone? Why is the ozone layer important to us?

Answer: Ozone (O₃) is a triatomic form of oxygen present in the stratosphere. The ozone layer absorbs harmful ultraviolet (UV) radiation from the Sun, protecting us from skin cancer, cataracts, immune-system damage and crop loss. It acts as a natural shield for life on Earth.

Q6. Differentiate between biodegradable and non-biodegradable wastes with two examples each.

Answer: Biodegradable wastes can be decomposed by microorganisms — examples: paper, fruit peels. Non-biodegradable wastes cannot be decomposed naturally — examples: plastic, glass. Biodegradable wastes do not pollute the environment if managed properly, but non-biodegradable wastes accumulate and harm soil, water and animals.

Five / Six Mark Questions

Q1. Describe the components of an ecosystem with suitable examples.

Answer: An ecosystem consists of two main components — biotic and abiotic. The biotic component includes all living organisms, classified as producers (green plants), consumers (herbivores, carnivores, omnivores) and decomposers (bacteria, fungi). The abiotic component includes non-living factors like sunlight, temperature, water, air, soil and minerals. Both interact to maintain the flow of energy and the cycling of nutrients. For example, in a pond ecosystem, water, sunlight and dissolved minerals form the abiotic part, while algae, fishes, frogs and bacteria form the biotic part. The harmonious interaction between these components keeps the ecosystem self-sustaining.

Q2. Explain food chain, food web and trophic levels with examples.

Answer: A food chain is the sequence of organisms through which energy and nutrients are transferred. Example — Grass → Grasshopper → Frog → Snake → Hawk. A food web is formed when several food chains are interconnected, providing alternate routes for energy flow. Each step in a food chain is a trophic level: the first trophic level is occupied by producers, the second by primary consumers (herbivores), the third by secondary consumers (small carnivores) and the fourth by tertiary consumers (top carnivores). Decomposers act on dead bodies at every level. Trophic levels show how energy moves from the Sun upward and how interdependent organisms are within an ecosystem.

Q3. What is the 10% law of energy transfer? Why are food chains usually short?

Answer: The 10% law, given by Lindeman (1942), states that during the transfer of energy from one trophic level to the next, only about 10% is stored in the form of body mass and is available to the next level. The remaining 90% is used up in respiration, growth, movement, reproduction and lost as heat. Because of this enormous energy loss at each step, very little energy is left for organisms at higher trophic levels. As a result, food chains are usually limited to 3–4 trophic levels — beyond that, the available energy becomes too small to support another level of consumers. This explains why top carnivores are far fewer in number than producers.

Q4. What is ozone depletion? Discuss its causes, consequences and the steps taken to control it.

Answer: Ozone depletion is the gradual thinning of the ozone layer in the stratosphere caused mainly by chlorofluorocarbons (CFCs) and other ozone-depleting substances. The chlorine atoms released from CFCs break down ozone (O₃) molecules into oxygen (O₂), reducing the layer’s protective capacity. The major consequences include increased UV radiation reaching the Earth, leading to skin cancer, cataracts, weakening of the immune system, harm to phytoplankton, reduced crop yields and disturbed climatic patterns. To control this problem, the United Nations Environment Programme (UNEP) framed the Montreal Protocol in 1987, an international treaty to freeze and gradually phase out the production and use of CFCs. Today, eco-friendly substitutes such as HFCs are used in refrigerators and air conditioners, and the ozone layer is slowly recovering.

Q5. How can we manage garbage effectively? Discuss with reference to segregation, recycling, composting and biogas.

Answer: Effective garbage management is essential to maintain a clean and healthy environment. (i) Segregation: Waste must be separated at source into biodegradable (green bin) and non-biodegradable (blue bin) categories so that it can be processed properly. (ii) Recycling: Materials such as paper, plastic, glass and metal are reprocessed into new products, conserving resources and reducing pollution. (iii) Composting: Kitchen and garden waste is decomposed by microbes (or earthworms in vermicomposting) to produce nutrient-rich manure used in agriculture. (iv) Biogas: Cattle dung, kitchen waste and sewage are decomposed in anaerobic conditions in a biogas plant to produce methane, a clean fuel, while the slurry is used as fertiliser. Other waste-disposal methods include landfilling, incineration and sewage treatment. Adopting the principle of “Reduce, Reuse, Recycle” is the most sustainable solution.

Additional Questions

Multiple Choice Questions (MCQs)

Q1. Which of the following is an abiotic component?

(a) Bacteria (b) Sunlight (c) Frog (d) Algae

Answer: (b) Sunlight

Q2. The first trophic level in a food chain is occupied by —

(a) Carnivores (b) Decomposers (c) Producers (d) Herbivores

Answer: (c) Producers

Q3. The 10% law of energy transfer was given by —

(a) Darwin (b) Lindeman (c) Mendel (d) Tansley

Answer: (b) Lindeman

Q4. Ozone is made up of how many oxygen atoms?

(a) 1 (b) 2 (c) 3 (d) 4

Answer: (c) 3

Q5. Which of the following is non-biodegradable?

(a) Paper (b) Cow dung (c) Plastic (d) Vegetable peels

Answer: (c) Plastic

Q6. The Montreal Protocol was signed in —

Q7. Which gas is mainly responsible for ozone depletion?

(a) CO₂ (b) CH₄ (c) CFC (d) SO₂

Answer: (c) CFC

Q8. Increase in concentration of harmful chemicals along a food chain is called —

(a) Eutrophication (b) Biomagnification (c) Bioremediation (d) Biodegradation

Answer: (b) Biomagnification

Q9. Which of these is a decomposer?

(a) Grass (b) Goat (c) Fungi (d) Tiger

Answer: (c) Fungi

Q10. Which of the following is an example of an artificial ecosystem?

(a) Forest (b) Aquarium (c) Pond (d) Ocean

Answer: (b) Aquarium

Fill in the Blanks

Q1. The non-living components of an ecosystem are called __________.

Answer: abiotic components

Q2. Only __________ % of energy is transferred from one trophic level to the next.

Answer: 10

Q3. The chemical formula of ozone is __________.

Answer: O₃

Q4. __________ are organisms that break down dead bodies into simpler substances.

Answer: Decomposers

Q5. The international agreement to phase out CFCs is called the __________.

Answer: Montreal Protocol

True or False

Q1. Ozone is found in the troposphere.

Answer: False (it is found in the stratosphere)

Q2. Plastic is a non-biodegradable substance.

Answer: True

Q3. Energy flow in an ecosystem is bidirectional.

Answer: False (it is unidirectional)

Q4. Vermicomposting uses earthworms to decompose organic waste.

Answer: True

Q5. Producers occupy the highest trophic level.

Answer: False (they occupy the first trophic level)

Glossary

Term	Meaning
Ecosystem	A self-sustaining unit of nature where living and non-living components interact.
Biotic Components	Living parts of an ecosystem — plants, animals, microorganisms.
Abiotic Components	Non-living parts — air, water, soil, sunlight, temperature.
Producer	Green plants that prepare their own food by photosynthesis.
Consumer	Organism that depends on others for food (herbivore, carnivore, omnivore).
Decomposer	Microorganism that breaks down dead matter into simpler substances.
Food Chain	Linear sequence of organisms through which energy flows.
Food Web	Network of interconnected food chains in an ecosystem.
Trophic Level	Each step in a food chain at which energy transfer occurs.
10% Law	Only 10% of energy is transferred from one trophic level to the next.
Biological Magnification	Increase in concentration of harmful chemicals along a food chain.
Pyramid of Energy	Graphical representation of energy at each trophic level (always upright).
Pyramid of Biomass	Graph showing total dry weight of organisms at each trophic level.
Ozone (O₃)	Triatomic oxygen layer in the stratosphere that absorbs UV rays.
CFC	Chlorofluorocarbon — chemical that destroys the ozone layer.
Montreal Protocol	International agreement of 1987 to phase out CFCs.
Biodegradable Waste	Waste that can be decomposed by microorganisms (e.g. paper, peels).
Non-biodegradable Waste	Waste that cannot be decomposed naturally (e.g. plastic, glass).
Segregation	Separation of waste at source into different categories.
Composting	Conversion of organic waste into manure by microbes.
Vermicomposting	Composting using earthworms.
Biogas	Methane-rich fuel produced from organic waste in anaerobic conditions.
Recycling	Reprocessing used materials into new products.
Landfill	Land area used for dumping and burying solid waste.
Incineration	Burning of waste at high temperatures to reduce its volume.