CBSE Class 12 Biology Long Answer Questions – Ecosystem (NCERT Based)

Unit V: Ecology and Environment | Chapter 14: Ecosystem

CBSE Class 12 Biology Chapter 14 Long Answer Questions – Ecosystem (NCERT)

Course & Examination Details

• Course: CBSE Class 12 Biology
• Unit: Unit V – Ecology and Environment
• Chapter: Chapter 14 – Ecosystem
• Board: Central Board of Secondary Education (CBSE)
• Syllabus Base: Strictly as per NCERT
• Exam Relevance: CBSE Class 12 Board Examinations (5-mark questions)

Long Answer Type Questions (120–150 Words Each)

(Prepared strictly according to NCERT syllabus and CBSE evaluation standards)

Section A: Ecosystem – Structure and Function (Q1–Q5)

Q1. Define an ecosystem and explain its structural components.

Ans:
An ecosystem is a functional unit of nature in which living organisms interact with one another and with the physical environment. It consists of two major structural components: abiotic and biotic. Abiotic components include inorganic substances like carbon dioxide, water, nitrogen, and minerals; organic substances such as carbohydrates and proteins; and climatic factors like light, temperature, and rainfall. Biotic components include producers, consumers, and decomposers. Producers are autotrophs that synthesize food using sunlight. Consumers depend on producers directly or indirectly for food, while decomposers break down dead organic matter into simpler substances. These components are interlinked through energy flow and nutrient cycling, forming an integrated and self-sustaining system.

Q2. Explain the functional aspects of an ecosystem.

Ans:
The functional aspects of an ecosystem describe how its components operate and interact. These include productivity, decomposition, energy flow, and nutrient cycling. Productivity refers to the rate of biomass production, mainly through photosynthesis. Decomposition involves the breakdown of dead organic matter into inorganic nutrients by decomposers. Energy flow is unidirectional, beginning with solar energy captured by producers and transferred to consumers through food chains. Nutrient cycling ensures the continuous circulation of essential elements like carbon, nitrogen, and phosphorus between biotic and abiotic components. Together, these functions maintain ecosystem stability, regulate population dynamics, and ensure the continuity of life processes.

Q3. Distinguish between natural and artificial ecosystems with examples.

Ans:
Natural ecosystems develop naturally without human intervention and are self-regulating and self-sustaining. Examples include forests, grasslands, ponds, and oceans. They show high biodiversity and stable nutrient cycles. Artificial ecosystems are created and maintained by humans for specific purposes. Examples include crop fields, gardens, aquariums, and dams. These ecosystems have lower biodiversity and require continuous external inputs such as fertilizers, irrigation, and human care. Unlike natural ecosystems, artificial ecosystems lack self-regulation and depend heavily on human management for survival and productivity.

Q4. Why is an ecosystem considered an open system?

Ans:
An ecosystem is considered an open system because it continuously exchanges energy and matter with its surroundings. Energy enters the ecosystem in the form of sunlight and is transferred through trophic levels before being lost as heat. Matter, such as nutrients and water, enters and leaves the ecosystem through various biogeochemical cycles. This exchange is essential for maintaining life processes, productivity, and ecological balance. Without continuous input of solar energy and recycling of nutrients, an ecosystem cannot function sustainably.

Q5. Explain the role of decomposers in maintaining ecosystem balance.

Ans:
Decomposers play a crucial role in maintaining ecosystem balance by breaking down dead plants and animals into simpler inorganic substances. Microorganisms such as bacteria and fungi carry out this process. Decomposition releases essential nutrients like nitrogen, phosphorus, and potassium back into the soil and water, making them available for reuse by producers. This recycling prevents accumulation of organic waste and ensures nutrient continuity. Without decomposers, ecosystems would become nutrient-deficient, disrupting productivity and energy flow.

Section B: Productivity (Q6–Q9)

Q6. Explain primary productivity and its types.

Ans:
Primary productivity refers to the rate at which producers synthesize organic matter from inorganic substances using sunlight. It forms the basis of energy flow in an ecosystem. There are two types of primary productivity: Gross Primary Productivity (GPP) and Net Primary Productivity (NPP). GPP is the total amount of organic matter produced through photosynthesis. A portion of this energy is used by plants for respiration. NPP is the remaining biomass after subtracting respiratory losses from GPP and represents the energy available to consumers. NPP is crucial as it determines the productivity and carrying capacity of ecosystems.

Q7. What factors influence primary productivity in ecosystems?

Ans:
Primary productivity is influenced by several factors including light intensity, temperature, water availability, nutrient supply, and the type of vegetation. Tropical regions generally show higher productivity due to abundant sunlight and rainfall. Soil fertility and photosynthetic efficiency of plants also play significant roles. Seasonal variations and climatic conditions can enhance or limit productivity. Human activities such as deforestation and pollution can reduce primary productivity by disturbing these factors.

Q8. Define secondary productivity and explain its significance.

Ans:
Secondary productivity is the rate at which consumers convert the organic matter they consume into new biomass. It depends directly on primary productivity, as consumers rely on producers for energy. Secondary productivity determines population growth and biomass accumulation at higher trophic levels. It also influences food chain length and ecosystem stability. Efficient secondary productivity supports greater biodiversity and sustains predator populations.

Q9. Why is Net Primary Productivity considered more important than Gross Primary Productivity?

Ans:
Net Primary Productivity is considered more important because it represents the actual amount of energy available to consumers. While GPP indicates total photosynthesis, NPP accounts for energy lost through plant respiration. NPP determines ecosystem carrying capacity, supports herbivores and higher trophic levels, and reflects ecosystem health and sustainability.

Section C: Decomposition (Q10–Q13)

Q10. Describe the process of decomposition in an ecosystem.

Ans:
Decomposition is the breakdown of dead organic matter into simpler inorganic substances by decomposers. It occurs in several steps: fragmentation of detritus by detritivores, leaching of soluble substances, enzymatic degradation (catabolism) by microbes, humification forming humus, and mineralisation releasing inorganic nutrients. Decomposition recycles nutrients, maintains soil fertility, and supports primary productivity. Climate and chemical composition of detritus influence the rate of decomposition.

Q11. Explain the importance of humification and mineralisation.

Ans:
Humification results in the formation of humus, a dark, stable organic substance that improves soil structure, fertility, and water-holding capacity. Mineralisation is the final step of decomposition where inorganic nutrients are released into the soil. Together, these processes ensure nutrient availability for plants and maintain ecosystem productivity.

Q12. How do climatic factors affect decomposition?

Ans:
Warm and moist conditions favor rapid microbial activity, leading to faster decomposition. In contrast, cold or dry conditions slow microbial growth and enzymatic action, reducing decomposition rates. Thus, climate plays a major role in nutrient recycling.

Q13. Why is decomposition slower in lignin-rich plant matter?

Ans:
Lignin-rich plant matter is resistant to microbial degradation. Its complex chemical structure slows enzymatic breakdown, resulting in slower decomposition.

Section D: Energy Flow and Ecological Pyramids (Q14–Q18)

Q14. Explain energy flow in an ecosystem.

Ans:
Energy flow in an ecosystem is unidirectional. Solar energy is captured by producers through photosynthesis and transferred to consumers through food chains. At each trophic level, most energy is lost as heat during metabolic activities. Only about 10% is passed to the next level. Energy cannot be recycled, limiting the number of trophic levels.

Q15. State and explain the ten percent law.

Ans:
The ten percent law states that only about 10% of energy at one trophic level is transferred to the next. The rest is lost as heat. This explains why food chains are short and energy pyramids are always upright.

Q16. Describe different types of ecological pyramids.

Ans:
Ecological pyramids include pyramid of number, biomass, and energy. Pyramid of number shows organism count, pyramid of biomass shows total biomass, and pyramid of energy shows energy flow. Energy pyramid is always upright.

Q17. Why is pyramid of energy always upright?

Ans:
Energy decreases at each trophic level due to heat loss, making the energy pyramid always upright.

Q18. Differentiate between food chain and food web.

Ans:
Food chain shows a single pathway of energy transfer, while food web shows interconnected food chains, providing ecosystem stability.

Section E: Nutrient Cycles, Succession & Ecosystem Services (Q19–Q25)

Q19. Explain the carbon cycle.

Ans:
Carbon circulates through atmosphere, biosphere, hydrosphere, and lithosphere. Photosynthesis fixes carbon dioxide into organic matter. Respiration, decomposition, and combustion release carbon dioxide back. This cycle maintains carbon balance.

Q20. Describe the nitrogen cycle.

Ans:
Nitrogen cycle includes nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. It converts atmospheric nitrogen into usable forms essential for proteins and nucleic acids.

Q21. Why is phosphorus cycle called sedimentary cycle?

Ans:
Phosphorus mainly circulates through rocks, soil, and water and lacks a gaseous phase, making it a sedimentary cycle.

Q22. Define ecological succession and its types.

Ans:
Ecological succession is the gradual change in species composition over time. Primary succession occurs on barren land, while secondary succession occurs where life existed earlier.

Q23. Explain the stages of ecological succession.

Ans:
Succession involves nudation, invasion, competition, reaction, and stabilization, leading to a climax community.

Q24. What are ecosystem services? Explain their importance.

Ans:
Ecosystem services are benefits humans obtain from ecosystems, including provisioning, regulating, cultural, and supporting services. They are essential for survival and economic stability.

Q25. Why is conservation of ecosystems necessary?

Ans:
Ecosystem conservation ensures biodiversity protection, sustainable resource use, climate regulation, and continued ecosystem services for future generations.

CBSE Board Exam Tip

These 5-mark long answer questions are ideal for board exams, focusing on definitions, processes, and well-structured explanations strictly based on NCERT Chapter 14 – Ecosystem.