Our Environment Class 10 NCERT Science Guide

Ecosystem and its Biotic and Abiotic Components

An ecosystem is a self-sustaining structural and functional unit of the biosphere where living organisms interact among themselves and with their surrounding physical environment. In your class 10 science our environment study, ecosystems are classified into two main categories: natural ecosystems (like forests, ponds, and oceans) and artificial or man-made ecosystems (like crop fields, gardens, and aquariums).

An ecosystem consists of two key components:

1. Biotic Components: These include all living entities. They are further divided into producers (green plants and blue-green algae that synthesize organic food using photosynthesis), consumers (animals that depend directly or indirectly on producers, classified as herbivores, carnivores, parasites, and omnivores), and decomposers (microorganisms like bacteria and fungi that break down dead remains and organic waste).
2. Abiotic Components: These are the non-living physical factors like temperature, sunlight, rainfall, soil, minerals, wind, and humidity. These abiotic parameters dictate the distribution and growth of biotic communities.

The Step-by-Step Flow of Energy in an Ecosystem

1. Solar Energy Capture — Green plants (producers) trap approximately 1% of the total solar energy falling on their leaves and convert it into chemical energy through the process of photosynthesis.
2. First Trophic Level Transfer — When herbivores consume plants, a vast amount of energy is lost as heat to the surroundings. Some energy goes into digestion, respiration, growth, and reproduction. Only about 10% of the energy is stored as biomass and made available to the next trophic level.
3. Subsequent Trophic Level Transfers (The 10% Law) — At each successive level of consumer (carnivores to top carnivores), only 10% of the energy of the previous level is transferred. Because of this progressive loss of energy, food chains generally contain only 3 to 4 trophic levels.
4. Unidirectional Flow — The flow of energy is strictly unidirectional. Energy captured by producers does not revert back to solar input, nor does energy flow backward from carnivores to herbivores.

Human Activities and Environmental Issues

Human intervention often causes severe imbalances in our ecosystem. The our environment ncert notes highlight two critical environmental phenomena:

1. Biological Magnification: When harmful, non-biodegradable chemical substances (like pesticides and DDT) enter a food chain through soil or water, they cannot be decomposed or excreted. Instead, their concentration increases progressively at each higher trophic level. Since human beings occupy the top position in most food chains, these toxic chemicals accumulate in maximum concentration within our bodies, leading to severe health hazards.

1. Ozone Layer Depletion: Ozone (O3) is a gas formed by three atoms of oxygen. It resides in the upper stratosphere and forms a protective shield, absorbing highly dangerous ultraviolet (UV) radiation from the sun. Exposure to UV rays causes skin cancer, cataracts, and genetic mutations in living organisms. Synthetic chemicals called Chlorofluorocarbons (CFCs), used in refrigeration and fire extinguishers, decompose ozone molecules, causing depletion of this protective layer. International agreements like the Montreal Protocol have successfully limited the production of CFCs.

CBSE Board Exam Tips & Common Mistakes

• Calculating Energy Levels: A very common mistake is applying the 10% rule directly to solar radiation. Remember: plants capture only 1% of solar energy. The subsequent steps between trophic levels (producer -> consumer) follow the 10% rule.
• Unidirectional Energy Flow: When asked to sketch energy flow, always draw single-headed arrows pointing forward. Energy flows strictly in one direction.
• Decomposers Role: Students often miss mentioning that decomposers replenish minerals and organic nutrients back into the soil, keeping the ecosystem self-sustaining. Make sure to emphasize this in your exam answers.

Practice Questions with Solutions

• Q: Calculate the amount of energy available to tigers (tertiary consumers) if the plants (producers) in a ecosystem synthesize 20,000 Joules of energy. A: Step 1: Write down the energy available at the producer level: Plants = 20,000 J. Step 2: Apply the 10% law for the primary consumer level (herbivores): Energy = 10% of 20,000 J = 2,000 J. Step 3: Apply the 10% law for the secondary consumer level (carnivores): Energy = 10% of 2,000 J = 200 J. Step 4: Apply the 10% law for the tertiary consumer level (tigers): Energy = 10% of 200 J = 20 J. Final answer: The energy available to tigers is 20 J.
• Q: Explain why a food chain cannot have more than 4 to 5 trophic levels. A: Step 1: Explain the 10% Law of Energy transfer. At each trophic level, only 10% of the energy is transferred to the next level, while 90% is lost in metabolic activities and heat. Step 2: Describe the progressive depletion. After 4-5 trophic levels, the amount of energy remaining is so negligible that it cannot support the metabolic needs of any further organism. Step 3: Draw a conclusion on the length limitation of food chains. Final answer: Because energy diminishes exponentially at each level, a 5th or 6th trophic level would receive close to zero energy, making it biologically unfeasible.
• Q: Differentiate between biodegradable and non-biodegradable wastes with examples. A: Step 1: Define biodegradable wastes. These are materials that can be broken down into simple, harmless substances by biological agents like bacteria and fungi (e.g., vegetable peels, paper, wood). Step 2: Define non-biodegradable wastes. These are materials that cannot be broken down by biological processes and persist in the environment for a long time, causing pollution (e.g., plastics, glass, DDT). Step 3: Highlight why non-biodegradable waste is harmful due to biomagnification. Final answer: Biodegradable waste is natural and decompose easily, whereas non-biodegradable waste does not decompose and pollutes food chains.
• Q: How is the ozone layer formed in the atmosphere? Explain its importance. A: Step 1: Describe the reaction. High-energy UV radiation splits diatomic oxygen molecules (O2) into free oxygen atoms (O + O). Step 2: Describe the ozone formation step. These free oxygen atoms then react with molecular oxygen (O2) to form ozone (O3): O2 + O -> O3. Step 3: State its physiological importance in shielding UV light. Final answer: Ozone is formed when UV light splits O2 into individual atoms, which combine with molecular oxygen to make O3. It protects living organisms on Earth from carcinogenic ultraviolet radiation.

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