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Complete Ecosystem Class 12 Biology notes, summary, keywords, MCQs, short and long questions with answers for board exam preparation.

Introduction of the Chapter – Ecosystem Class 12 Biology

The chapter Ecosystem in Class 12 Biology (NCERT) explains the structure and function of ecological systems. An ecosystem is a functional unit of nature where living organisms interact with each other and with their physical environment.

The Ecosystem Class 12 Biology chapter is important for CBSE board exams and competitive exams like NEET. It covers topics such as components of ecosystem, food chain, food web, ecological pyramids, energy flow, productivity, decomposition, and nutrient cycling.

Understanding this chapter helps students learn how nature maintains balance and how human activities affect ecological stability.

Short Notes on Ecosystem (Bullet Points)

An ecosystem is a structural and functional unit of biosphere.
It consists of biotic components (living organisms) and abiotic components (non-living factors).
Types of ecosystems:
- Terrestrial ecosystem
- Aquatic ecosystem (pond, lake, ocean)
Biotic components include:
- Producers
- Consumers
- Decomposers
Energy flow in an ecosystem follows the 10% law.
Food chains are of two types:
- Grazing food chain
- Detritus food chain
Ecological pyramids represent trophic levels.
Decomposition involves fragmentation, leaching, catabolism, humification, and mineralization.
Nutrient cycles include:
- Carbon cycle
- Nitrogen cycle
- Phosphorus cycle

Detailed Summary of Ecosystem Class 12 Biology (1000–1200 Words)

The Ecosystem Class 12 Biology chapter explains how living organisms and physical components interact in a defined area. The term ecosystem was coined by A.G. Tansley. It includes both biotic and abiotic components.

Structure of Ecosystem

The structure of an ecosystem includes:

1. Abiotic Components

These are non-living components such as:

Temperature
Water
Soil
Air
Minerals
Light

These factors influence the distribution and survival of organisms.

2. Biotic Components

Biotic components are living organisms and are divided into:

(a) Producers

Also called autotrophs.
Example: Green plants, phytoplankton.
They prepare food using photosynthesis.

(b) Consumers

Also called heterotrophs.
They depend on producers for food.

Primary consumers (herbivores)
Secondary consumers (carnivores)
Tertiary consumers

(c) Decomposers

Example: Bacteria and fungi.
They break down dead organic matter into simpler substances.

Functions of Ecosystem

The Ecosystem Class 12 Biology chapter highlights four major functions:

Productivity
Decomposition
Energy flow
Nutrient cycling

Productivity

Productivity refers to the rate of biomass production.

Gross Primary Productivity (GPP): Total organic matter produced.
Net Primary Productivity (NPP): GPP minus respiration losses.

NPP is the available biomass for consumers.

Decomposition

Decomposition is the breakdown of dead organic matter.

Steps involved:

Fragmentation
Leaching
Catabolism
Humification
Mineralization

Decomposition depends on temperature, moisture, and oxygen.

Energy Flow in Ecosystem

Energy enters the ecosystem through sunlight. Producers convert solar energy into chemical energy.

Energy flow:
Sun → Producers → Consumers → Decomposers

Key points:

Flow of energy is unidirectional.
Only 10% energy is transferred to the next trophic level (10% law by Lindeman).
Energy decreases at successive trophic levels.

Food Chain and Food Web

A food chain is a linear sequence of organisms where energy is transferred.

Types:

Grazing Food Chain
Detritus Food Chain

A food web is a network of interconnected food chains. It increases ecosystem stability.

Ecological Pyramids

Ecological pyramids represent trophic levels in terms of:

Pyramid of number
Pyramid of biomass
Pyramid of energy

Pyramid of energy is always upright.

Nutrient Cycling

Nutrients move in a cyclic manner between organisms and environment.

Important cycles:

Carbon cycle
Nitrogen cycle
Phosphorus cycle

Nutrient cycling maintains ecological balance.

Flowchart / Mind Map (Text-Based)

Ecosystem
→ Components
→ Abiotic
→ Biotic
→ Producers
→ Consumers
→ Decomposers
→ Functions
→ Productivity
→ Energy Flow
→ Decomposition
→ Nutrient Cycling
→ Food Chain
→ Food Web
→ Ecological Pyramids

Important Keywords with Meanings

Ecosystem: Functional unit of nature.
Biotic Components: Living parts of ecosystem.
Abiotic Components: Non-living factors.
Trophic Level: Feeding level in food chain.
Productivity: Rate of biomass production.
Decomposition: Breakdown of organic matter.
Food Web: Network of food chains.
Ecological Pyramid: Graphical representation of trophic levels.
10% Law: Only 10% energy passes to next level.
Mineralization: Release of inorganic nutrients.

Important Questions & Answers

10 Very Short Answer Questions

Define ecosystem.
An ecosystem is a structural and functional unit where living organisms interact with environment.
Who coined the term ecosystem?
A.G. Tansley.
What is NPP?
Net Primary Productivity is GPP minus respiration.
State 10% law.
Only 10% energy is transferred to next trophic level.
Name two types of food chains.
Grazing and Detritus food chain.
What are decomposers?
Organisms that break down dead matter.
What is humification?
Formation of humus during decomposition.
Define trophic level.
Each feeding step in a food chain.
What is food web?
Interconnected food chains.
Which pyramid is always upright?
Pyramid of energy.

10 Short Answer Questions

Describe structure of ecosystem.
Structure includes abiotic and biotic components. Biotic include producers, consumers and decomposers. Abiotic include soil, water, light, minerals.
Explain energy flow in ecosystem.
Energy enters through sunlight, converted by producers, passed to consumers. Flow is unidirectional and follows 10% law.
Explain decomposition process.
It involves fragmentation, leaching, catabolism, humification and mineralization.
Describe ecological pyramids.
Three types: number, biomass and energy. Energy pyramid is always upright.
Explain carbon cycle.
Carbon moves between atmosphere, plants, animals and soil through photosynthesis and respiration.

10 Long Answer Questions with Answers

1. Describe the structure of an ecosystem.

Answer:

The structure of an ecosystem includes both biotic and abiotic components that interact with each other.

1. Abiotic Components:
These are non-living factors such as:

Temperature
Light
Water
Soil
Air
Minerals

They influence the survival and distribution of organisms.

2. Biotic Components:
These include living organisms divided into:

Producers (Autotrophs): Green plants and phytoplankton that prepare food by photosynthesis.
Consumers (Heterotrophs): Herbivores, carnivores, omnivores.
Decomposers: Bacteria and fungi that break down dead organic matter.

Thus, the structure of an ecosystem forms the basis for energy flow and nutrient cycling.

2. Explain the energy flow in an ecosystem.

Answer:

Energy flow in an ecosystem is unidirectional. The ultimate source of energy is the sun.

Solar energy is captured by producers through photosynthesis.
Producers transfer energy to primary consumers.
Energy passes to secondary and tertiary consumers.
Decomposers obtain energy from dead organisms.

According to the 10% law (Lindeman), only 10% of energy is transferred to the next trophic level. The rest is lost as heat during respiration. Therefore, energy decreases at each successive trophic level.

3. Describe the process of decomposition.

Answer:

Decomposition is the breakdown of dead organic matter into simpler inorganic substances.

The steps include:

Fragmentation: Breakdown of detritus into smaller pieces by detritivores like earthworms.
Leaching: Water dissolves soluble substances and carries them into soil.
Catabolism: Microbial enzymes degrade complex molecules.
Humification: Formation of dark-colored humus.
Mineralization: Release of inorganic nutrients into soil.

Decomposition depends on temperature, moisture, and oxygen.

4. Explain different types of ecological pyramids.

Answer:

Ecological pyramids represent trophic levels in graphical form.

Pyramid of Number:
Shows number of organisms at each trophic level. It can be upright or inverted.
Pyramid of Biomass:
Represents total biomass at each level. It may be inverted in aquatic ecosystems.
Pyramid of Energy:
Shows flow of energy at each level. It is always upright because energy decreases at higher trophic levels.

5. Explain the carbon cycle in detail.

Answer:

The carbon cycle is a biogeochemical cycle that circulates carbon between atmosphere, organisms, and soil.

Plants absorb CO₂ during photosynthesis.
Carbon enters animals through food chains.
Respiration releases CO₂ back to atmosphere.
Decomposition returns carbon to soil.
Combustion of fossil fuels increases atmospheric CO₂.

The carbon cycle maintains balance of carbon in the ecosystem.

6. Describe the nitrogen cycle.

Answer:

The nitrogen cycle involves circulation of nitrogen in nature.

Steps include:

Nitrogen Fixation: Conversion of atmospheric nitrogen into ammonia by bacteria.
Nitrification: Conversion of ammonia into nitrites and nitrates.
Assimilation: Plants absorb nitrates to form proteins.
Ammonification: Decomposers convert organic nitrogen into ammonia.
Denitrification: Conversion of nitrates back to atmospheric nitrogen.

This cycle ensures availability of nitrogen for living organisms.

7. Differentiate between grazing food chain and detritus food chain.

Answer:

Basis	Grazing Food Chain	Detritus Food Chain
Starting Point	Green plants	Dead organic matter
First Consumer	Herbivores	Detritivores
Energy Source	Solar energy	Organic debris
Example	Grass → Deer → Lion	Leaves → Earthworm → Bird

Both food chains are important for ecosystem functioning.

8. Explain productivity in an ecosystem.

Answer:

Productivity refers to the rate of biomass production.

Types:

Gross Primary Productivity (GPP): Total organic matter produced during photosynthesis.
Net Primary Productivity (NPP): GPP minus respiration losses.

NPP represents the actual biomass available for consumers. Productivity depends on light, temperature, water, and nutrients.

9. What is a food web? Explain its significance.

Answer:

A food web is a network of interconnected food chains in an ecosystem.

Significance:

Provides alternative pathways for energy flow.
Increases ecosystem stability.
Reduces dependency on a single food source.
Maintains ecological balance.

Food webs are more realistic representations of feeding relationships.

10. Discuss the importance of ecosystem in maintaining ecological balance.

Answer:

An ecosystem maintains ecological balance through:

Energy flow from producers to consumers.
Nutrient cycling between organisms and environment.
Regulation of population through food chains.
Decomposition and recycling of organic matter.

Healthy ecosystems support biodiversity, climate regulation, soil fertility, and water purification. Disturbance in ecosystem can lead to environmental imbalance.

30 MCQs with Answers

30 Multiple Choice Questions (MCQs) – Ecosystem

1. The term ecosystem was coined by:

a) Charles Darwin
b) A.G. Tansley
c) E.P. Odum
d) Linnaeus
Answer: b) A.G. Tansley

2. Which of the following is an abiotic component?

a) Bacteria
b) Fungi
c) Water
d) Grass
Answer: c) Water

3. Producers in an ecosystem are:

a) Heterotrophs
b) Carnivores
c) Autotrophs
d) Decomposers
Answer: c) Autotrophs

4. Net Primary Productivity (NPP) is:

a) GPP + Respiration
b) GPP – Respiration
c) Respiration – GPP
d) Total biomass
Answer: b) GPP – Respiration

5. The 10% law was proposed by:

a) Tansley
b) Darwin
c) Lindeman
d) Odum
Answer: c) Lindeman

6. Energy flow in an ecosystem is:

a) Cyclic
b) Bidirectional
c) Unidirectional
d) Random
Answer: c) Unidirectional

7. Which pyramid is always upright?

a) Pyramid of number
b) Pyramid of biomass
c) Pyramid of energy
d) All of these
Answer: c) Pyramid of energy

8. Decomposers mainly include:

a) Algae
b) Fungi and bacteria
c) Insects
d) Birds
Answer: b) Fungi and bacteria

9. The first trophic level consists of:

a) Carnivores
b) Herbivores
c) Producers
d) Decomposers
Answer: c) Producers

10. A grazing food chain starts with:

a) Dead organic matter
b) Green plants
c) Carnivores
d) Omnivores
Answer: b) Green plants

11. A detritus food chain begins with:

a) Producers
b) Herbivores
c) Dead organic matter
d) Carnivores
Answer: c) Dead organic matter

12. Humification leads to formation of:

a) Minerals
b) Humus
c) Water
d) Nitrogen
Answer: b) Humus

13. Mineralization results in:

a) Organic compounds
b) Release of inorganic nutrients
c) Formation of humus
d) Fragmentation
Answer: b) Release of inorganic nutrients

14. Which of the following is a secondary consumer?

a) Grass
b) Rabbit
c) Frog
d) Phytoplankton
Answer: c) Frog

15. Primary consumers are:

a) Carnivores
b) Herbivores
c) Omnivores
d) Producers
Answer: b) Herbivores

16. The amount of solar energy captured by plants is:

a) 1%
b) 10%
c) 50%
d) 100%
Answer: a) 1%

17. Which cycle does not have a gaseous phase?

a) Carbon cycle
b) Nitrogen cycle
c) Phosphorus cycle
d) Oxygen cycle
Answer: c) Phosphorus cycle

18. Food web increases:

a) Instability
b) Energy loss
c) Stability of ecosystem
d) Respiration
Answer: c) Stability of ecosystem

19. Which ecosystem has inverted pyramid of biomass?

a) Forest
b) Grassland
c) Pond
d) Desert
Answer: c) Pond

20. The ultimate source of energy in an ecosystem is:

a) Moon
b) Plants
c) Sun
d) Soil
Answer: c) Sun

21. Fragmentation in decomposition is done by:

a) Bacteria
b) Earthworms
c) Fungi
d) Plants
Answer: b) Earthworms

22. Leaching refers to:

a) Breakdown by microbes
b) Formation of humus
c) Removal of soluble substances by water
d) Mineral formation
Answer: c) Removal of soluble substances by water

23. Which process releases CO₂ into atmosphere?

a) Photosynthesis
b) Respiration
c) Nitrogen fixation
d) Mineralization
Answer: b) Respiration

24. Ecological pyramid of numbers may be:

a) Always upright
b) Always inverted
c) Upright or inverted
d) Horizontal
Answer: c) Upright or inverted

25. Gross Primary Productivity includes:

a) Only NPP
b) Total photosynthesis
c) Only respiration
d) Only biomass
Answer: b) Total photosynthesis

26. Which trophic level has maximum energy?

a) Tertiary consumers
b) Secondary consumers
c) Primary consumers
d) Producers
Answer: d) Producers

27. Biogeochemical cycles help in:

a) Energy destruction
b) Nutrient recycling
c) Food digestion
d) Respiration only
Answer: b) Nutrient recycling

28. The study of ecosystem is called:

a) Physiology
b) Genetics
c) Ecology
d) Anatomy
Answer: c) Ecology

29. Which of the following is a tertiary consumer?

a) Grasshopper
b) Snake
c) Hawk
d) Grass
Answer: c) Hawk

30. Loss of energy between trophic levels occurs mainly due to:

a) Photosynthesis
b) Respiration
c) Growth
d) Reproduction
Answer: b) Respiration

Exam Tips and 5 Value-Based Questions

Exam Tips

Revise diagrams of ecological pyramids.
Practice labeling carbon and nitrogen cycle.
Remember definitions precisely.
Focus on 10% law and NPP formula.
Solve previous year MCQs.

10 Value-Based Questions with Answers – Ecosystem Class 12 Biology

These value-based questions from the chapter Ecosystem Class 12 Biology are designed to develop environmental awareness, critical thinking, and moral responsibility along with exam preparation.

1. A factory releases untreated waste into a nearby pond ecosystem. What values are being ignored? What should be done?

Answer:
The factory is ignoring environmental responsibility, social ethics, and sustainable development. Untreated waste harms aquatic life and disturbs the ecosystem balance. Proper waste treatment, strict pollution control, and adherence to environmental laws should be followed.

2. A student plants trees in his locality to reduce pollution. Which ecological concept is he supporting?

Answer:
He is supporting the carbon cycle and ecological balance. Trees increase carbon sequestration, improve oxygen levels, and help maintain ecosystem stability. This reflects environmental awareness and social responsibility.

3. Overfishing is disturbing the marine food chain. What values should society adopt?

Answer:
Society should adopt sustainable use of resources, conservation ethics, and responsibility towards future generations. Overfishing disrupts trophic levels and food webs in the ecosystem.

4. Farmers excessively use chemical fertilizers. How does this affect nutrient cycling and what value is compromised?

Answer:
Excessive fertilizers disturb nitrogen and phosphorus cycles, leading to eutrophication in water bodies. The compromised value is sustainable agriculture and environmental care.

5. A community protects a forest area from illegal cutting. How does this help the ecosystem?

Answer:
Protecting forests maintains biodiversity, stabilizes food chains, and supports energy flow in the ecosystem. It reflects collective responsibility and conservation values.

6. During a picnic, some students throw plastic waste in a forest area. What ecological and moral lesson can be derived?

Answer:
Plastic waste affects decomposers and soil health, disturbing decomposition processes. The lesson is to practice cleanliness, respect nature, and follow eco-friendly habits.

7. Climate change is altering ecosystem productivity. What human values are required to address this issue?

Answer:
Values such as global cooperation, environmental ethics, and sustainable living are essential. Reducing carbon emissions helps maintain ecosystem productivity.

8. Wetlands are being converted into residential areas. How does this affect energy flow and biodiversity?

Answer:
Wetlands support diverse food chains and nutrient cycles. Their destruction disrupts energy flow and reduces biodiversity. The value needed is responsible urban planning and ecological sensitivity.

9. A village uses compost instead of chemical fertilizers. Which ecosystem function is promoted?

Answer:
Composting enhances decomposition and nutrient recycling. It promotes sustainable farming and environmental conservation.

10. Why is it important to educate people about the Ecosystem Class 12 Biology chapter in real life?

Answer:
Understanding ecosystem concepts helps individuals make environmentally responsible decisions. Knowledge about food chains, energy flow, and nutrient cycling promotes conservation, sustainability, and balanced resource use.

Conclusion – Ecosystem

The chapter Ecosystem is one of the most important units in environmental biology. It explains the structure and functioning of natural systems in a simple yet scientific way. From energy flow and ecological pyramids to nutrient cycling and decomposition, this chapter builds a strong foundation for understanding environmental science.

The Ecosystem Class 12 Biology notes, summary, keywords, important questions, and MCQs provided above are designed for board exams and competitive exams. Regular revision of this chapter improves conceptual clarity and scoring ability.

Mastering the Ecosystem Biology chapter helps students understand how nature sustains life and why conservation is essential. Proper preparation using structured notes, diagrams, and MCQs ensures high marks in CBSE board examinations.

10 Assertion–Reason Questions with Answers – Ecosystem

Options:

A. Both Assertion (A) and Reason (R) are true and R is the correct explanation of A.
B. Both A and R are true but R is not the correct explanation of A.
C. A is true but R is false.
D. A is false but R is true.

1.

Assertion (A): Energy flow in an ecosystem is unidirectional.
Reason (R): Energy lost as heat during respiration cannot be reused by plants.

Answer: A
Energy moves from producers to consumers and is lost as heat, so it does not return to the sun.

2.

Assertion (A): The pyramid of energy is always upright.
Reason (R): Energy decreases at each successive trophic level due to the 10% law.

Answer: A
Energy transfer follows the 10% law, so each higher level has less energy.

3.

Assertion (A): A detritus food chain starts with dead organic matter.
Reason (R): Detritivores consume plant biomass directly.

Answer: C
Detritus food chain begins with dead organic matter, but detritivores do not consume living plant biomass directly.

4.

Assertion (A): Net Primary Productivity (NPP) is always less than Gross Primary Productivity (GPP).
Reason (R): Some part of GPP is used by plants in respiration.

Answer: A
Respiration reduces the total energy available, so NPP is less than GPP.

5.

Assertion (A): The pyramid of biomass in a pond ecosystem is inverted.
Reason (R): Biomass of phytoplankton is less than that of zooplankton at a given time.

Answer: A
In aquatic ecosystems, phytoplankton have low standing biomass but high productivity.

6.

Assertion (A): Decomposition is faster in warm and moist environments.
Reason (R): Microbial activity increases with suitable temperature and moisture.

Answer: A
Decomposers function efficiently under warm and moist conditions.

7.

Assertion (A): Food webs increase stability of an ecosystem.
Reason (R): Organisms in a food web depend on a single food source only.

Answer: C
Food webs increase stability, but organisms do not depend on only one food source.

8.

Assertion (A): Phosphorus cycle does not have a gaseous phase.
Reason (R): Phosphorus mainly circulates through rocks and soil.

Answer: A
Phosphorus moves through lithosphere and hydrosphere without a significant gaseous phase.

9.

Assertion (A): Producers occupy the first trophic level.
Reason (R): Producers convert solar energy into chemical energy.

Answer: A
Producers form the base of food chains because they synthesize food.

10.

Assertion (A): Only about 1% of solar energy is captured by plants.
Reason (R): Most solar radiation is reflected, absorbed by atmosphere, or falls on non-photosynthetic surfaces.

Answer: A
Only a small fraction of solar energy is utilized in photosynthesis.

Ecosystem Class 12 Biology Sample Paper (CBSE Pattern)

Chapter: Ecosystem – Class 12 Biology

This Ecosystem Class 12 Biology Sample Paper is designed according to the latest CBSE pattern. It includes MCQs, short answer, long answer, case-based and value-based questions for complete exam preparation.

Section A – MCQs (1 × 10 = 10 Marks)

1. The ultimate source of energy in an ecosystem is:
a) Moon
b) Soil
c) Sun
d) Plants

2. The 10% law of energy transfer was proposed by:
a) Tansley
b) Lindeman
c) Darwin
d) Odum

3. NPP is equal to:
a) GPP + Respiration
b) GPP – Respiration
c) Respiration – GPP
d) Total biomass

4. Pyramid of energy is always:
a) Inverted
b) Horizontal
c) Upright
d) Circular

5. Decomposers include:
a) Herbivores
b) Carnivores
c) Bacteria and fungi
d) Omnivores

6. Which cycle has no gaseous phase?
a) Carbon
b) Nitrogen
c) Phosphorus
d) Oxygen

7. First trophic level consists of:
a) Consumers
b) Producers
c) Decomposers
d) Carnivores

8. Grazing food chain starts with:
a) Dead matter
b) Green plants
c) Carnivores
d) Omnivores

9. Mineralization results in:
a) Formation of humus
b) Release of inorganic nutrients
c) Photosynthesis
d) Energy loss

10. Energy flow in ecosystem is:
a) Cyclic
b) Unidirectional
c) Bidirectional
d) Static

Section B – Very Short Answer Questions (2 × 5 = 10 Marks)

11. Define ecosystem.
12. What is humification?
13. State the 10% law.
14. What is food web?
15. Define Net Primary Productivity (NPP).

Section C – Short Answer Questions (3 × 6 = 18 Marks)

16. Explain the structure of an ecosystem.
17. Differentiate between grazing and detritus food chain.
18. Explain the steps of decomposition.
19. Describe pyramid of biomass with example.
20. Explain nitrogen cycle briefly.
21. What are biogeochemical cycles? Name two.

Section D – Long Answer Questions (5 × 4 = 20 Marks)

22. Explain energy flow in an ecosystem with diagram.
23. Describe ecological pyramids in detail.
24. Explain carbon cycle with diagrammatic representation.
25. Discuss productivity in ecosystem.

Section E – Case-Based Question (5 Marks)

Read the passage carefully and answer the questions:

A pond ecosystem consists of phytoplankton, zooplankton, small fishes, large fishes, and decomposers. The biomass of phytoplankton is less at a given time but supports a larger biomass of zooplankton.

(a) Why is the biomass pyramid inverted in pond ecosystem?
(b) Identify the first trophic level.
(c) What type of food chain operates here?
(d) Why is energy pyramid always upright?
(e) Name the organisms responsible for decomposition.

SOLUTION

Section A – MCQs Solutions

1. Sun (Option c)
The sun is the ultimate source of energy. Producers capture solar energy during photosynthesis.

2. Lindeman (Option b)
Lindeman proposed the 10% law of energy transfer between trophic levels.

3. GPP – Respiration (Option b)
Net Primary Productivity = Gross Primary Productivity – Respiratory losses.

4. Upright (Option c)
Energy pyramid is always upright because energy decreases at each trophic level.

5. Bacteria and fungi (Option c)
They act as decomposers by breaking down organic matter.

6. Phosphorus (Option c)
The phosphorus cycle does not have a significant gaseous phase.

7. Producers (Option b)
Producers occupy the first trophic level.

8. Green plants (Option b)
Grazing food chain begins with producers.

9. Release of inorganic nutrients (Option b)
Mineralization releases nutrients into the soil.

10. Unidirectional (Option b)
Energy flow moves in one direction and is not recycled.

Section B – Very Short Answer Solutions

11. Define ecosystem.
An ecosystem is a structural and functional unit of nature where living organisms interact with one another and with the abiotic environment.

12. What is humification?
Humification is the formation of dark-colored, resistant organic matter called humus during decomposition.

13. State the 10% law.
Only 10% of energy is transferred from one trophic level to the next higher level.

14. What is food web?
A food web is a network of interconnected food chains that increases ecosystem stability.

15. Define Net Primary Productivity (NPP).
NPP is the amount of biomass available to consumers after subtracting respiratory losses from GPP.

Section C – Short Answer Solutions

16. Structure of Ecosystem

The structure includes:

Abiotic components: Temperature, water, soil, light, minerals.
Biotic components:
- Producers (plants)
- Consumers (herbivores, carnivores)
- Decomposers (bacteria, fungi)

These components interact to maintain ecosystem functioning.

17. Difference Between Grazing and Detritus Food Chain

Grazing Food Chain	Detritus Food Chain
Starts with green plants	Starts with dead organic matter
First consumer is herbivore	First consumer is detritivore
Example: Grass → Deer → Lion	Leaves → Earthworm → Bird

18. Steps of Decomposition

Fragmentation – Detritivores break down organic matter.
Leaching – Soluble nutrients dissolve in water.
Catabolism – Microbes break complex molecules.
Humification – Formation of humus.
Mineralization – Release of inorganic nutrients.

19. Pyramid of Biomass

In terrestrial ecosystems, pyramid of biomass is upright.
In aquatic ecosystems like ponds, it is inverted because phytoplankton have lower standing biomass but higher productivity.

20. Nitrogen Cycle

Steps include:

Nitrogen fixation
Nitrification
Assimilation
Ammonification
Denitrification

This cycle ensures nitrogen availability to plants.

21. Biogeochemical Cycles

These are nutrient cycles between biotic and abiotic components.
Examples: Carbon cycle, Nitrogen cycle.

Section D – Long Answer Solutions

22. Energy Flow in Ecosystem

Energy enters ecosystem through sunlight. Producers convert solar energy into chemical energy via photosynthesis. Energy flows to primary, secondary, and tertiary consumers.

Key Points:

Flow is unidirectional.
Follows 10% law.
Energy is lost as heat at each trophic level.
Pyramid of energy is always upright.

23. Ecological Pyramids

Three types:

Pyramid of Number – May be upright or inverted.
Pyramid of Biomass – Upright in forest; inverted in pond.
Pyramid of Energy – Always upright.

They represent trophic structure of ecosystem.

24. Carbon Cycle

Carbon moves through:

Photosynthesis
Respiration
Decomposition
Combustion

Plants absorb CO₂. Animals obtain carbon through food. Respiration and burning fossil fuels release CO₂ back to atmosphere.

25. Productivity in Ecosystem

Productivity refers to rate of biomass production.

Types:

GPP – Total photosynthetic production.
NPP – GPP minus respiration losses.

NPP represents available energy for consumers.

Section E – Case Study Solutions

(a) Biomass pyramid is inverted because phytoplankton reproduce rapidly and have high turnover rate.

(b) First trophic level: Phytoplankton (Producers).

(c) Grazing food chain operates in pond ecosystem.

(d) Energy pyramid is always upright because energy decreases at successive trophic levels.

(e) Decomposers such as bacteria and fungi break down dead matter.

ADDITIONAL SAMPLE PAPER

Additional Sample Paper (SAP – 2)

Ecosystem Class 12 Biology – Practice Paper with Solutions

This Additional Sample Paper (SAP) for Ecosystem Class 12 Biology is prepared according to CBSE board pattern. It includes MCQs, Assertion–Reason, Short Answer, Long Answer, and Case-Based Questions for complete revision.

Section A – MCQs (1 × 10 = 10 Marks)

1. Which of the following represents correct food chain?
a) Grass → Lion → Deer
b) Grass → Deer → Lion
c) Lion → Grass → Deer
d) Deer → Grass → Lion

2. The process of conversion of ammonia into nitrites is called:
a) Nitrogen fixation
b) Nitrification
c) Denitrification
d) Ammonification

3. In a forest ecosystem, pyramid of biomass is generally:
a) Inverted
b) Upright
c) Horizontal
d) Circular

4. Maximum energy is present at which trophic level?
a) Secondary consumers
b) Tertiary consumers
c) Producers
d) Decomposers

5. Humus is formed during:
a) Fragmentation
b) Leaching
c) Humification
d) Mineralization

6. Which of the following is a primary consumer?
a) Grass
b) Rabbit
c) Snake
d) Hawk

7. Which ecosystem shows inverted pyramid of biomass?
a) Desert
b) Forest
c) Pond
d) Grassland

8. Net Primary Productivity is the energy available to:
a) Producers only
b) Decomposers only
c) Consumers
d) Atmosphere

9. Energy flow in ecosystem follows which law?
a) Hardy-Weinberg law
b) 10% law
c) Mendel’s law
d) Law of segregation

10. Which is not an abiotic factor?
a) Water
b) Soil
c) Fungi
d) Temperature

Section B – Assertion–Reason (1 × 5 = 5 Marks)

Choose the correct option:
A. Both A and R are true and R explains A.
B. Both A and R are true but R does not explain A.
C. A is true but R is false.
D. A is false but R is true.

11.
Assertion: Energy pyramid is always upright.
Reason: Energy decreases at each trophic level.

12.
Assertion: Food web increases ecosystem stability.
Reason: It provides alternate feeding pathways.

13.
Assertion: Phosphorus cycle has no gaseous phase.
Reason: Phosphorus circulates mainly through rocks and soil.

14.
Assertion: NPP is always greater than GPP.
Reason: Respiration reduces available energy.

15.
Assertion: Decomposition is slower in dry conditions.
Reason: Microbial activity decreases in absence of moisture.

Section C – Short Answer Questions (3 × 6 = 18 Marks)

16. Define ecosystem and mention its types.
17. Explain the process of mineralization.
18. Write differences between GPP and NPP.
19. Explain carbon cycle briefly.
20. What is ecological succession? (Brief idea linked to ecosystem development)
21. Explain importance of decomposers.

Section D – Long Answer Questions (5 × 4 = 20 Marks)

22. Describe structure and function of ecosystem.
23. Explain ecological pyramids with examples.
24. Describe nitrogen cycle with diagrammatic explanation.
25. Explain energy flow with 10% law and trophic levels.

Section E – Case Study (5 Marks)

A grassland ecosystem consists of grasses, grasshoppers, frogs, snakes and hawks. Decomposers are also present in the soil.

Answer the following:

(a) Identify the first trophic level.
(b) Construct a food chain.
(c) Which organism is the tertiary consumer?
(d) Why is energy less at hawk level?
(e) Name the process by which nutrients return to soil.

SOLUTION

Additional Sample Paper (SAP–2) – Detailed Solutions

Ecosystem Class 12 Biology – Complete Answer Guide

These are step-by-step solutions to the Additional Sample Paper (SAP) for the chapter Ecosystem Class 12 Biology. Answers are written in CBSE board-exam format with proper explanation.

Section A – MCQs Solutions

1. Grass → Deer → Lion (Option b)
Energy flows from producers (grass) to herbivores (deer) and then to carnivores (lion).

2. Nitrification (Option b)
Nitrification is the conversion of ammonia into nitrites and nitrates by nitrifying bacteria.

3. Upright (Option b)
In forest ecosystem, producers have maximum biomass; hence pyramid of biomass is upright.

4. Producers (Option c)
Producers capture solar energy, so maximum energy is present at first trophic level.

5. Humification (Option c)
Humus formation occurs during humification stage of decomposition.

6. Rabbit (Option b)
Rabbit feeds on plants, so it is a primary consumer.

7. Pond (Option c)
In pond ecosystem, biomass pyramid is inverted due to low standing biomass of phytoplankton.

8. Consumers (Option c)
NPP represents biomass available to consumers.

9. 10% law (Option b)
Energy transfer between trophic levels follows Lindeman’s 10% law.

10. Fungi (Option c)
Fungi are living organisms (biotic component), not abiotic.

Section B – Assertion–Reason Solutions

11. Answer: A
Energy decreases at each trophic level; therefore, pyramid of energy is always upright.

12. Answer: A
Food web provides alternate feeding pathways, increasing stability.

13. Answer: A
Phosphorus mainly circulates through rocks and soil; hence no gaseous phase.

14. Answer: D
Assertion is false (NPP is less than GPP).
Reason is true because respiration reduces available energy.

15. Answer: A
Moisture increases microbial activity; hence decomposition is slower in dry conditions.

Section C – Short Answer Solutions

16. Define Ecosystem and Types

An ecosystem is a structural and functional unit of nature where biotic and abiotic components interact.

Types:

Terrestrial ecosystem (forest, desert, grassland)
Aquatic ecosystem (pond, lake, ocean)

17. Mineralization

Mineralization is the process in which decomposers convert organic nutrients into inorganic forms (like nitrates, phosphates) that plants can absorb.

18. Difference Between GPP and NPP

GPP	NPP
Total photosynthetic production	GPP – Respiration
Includes energy used in respiration	Energy available to consumers

19. Carbon Cycle

Plants absorb CO₂ in photosynthesis.
Animals obtain carbon through food.
Respiration releases CO₂ back.
Decomposition and combustion add CO₂ to atmosphere.

It maintains carbon balance in ecosystem.

20. Ecological Succession

Ecological succession is the gradual and orderly change in species composition of an area over time, leading to a stable climax community.

21. Importance of Decomposers

Recycle nutrients
Maintain soil fertility
Break down organic waste
Complete biogeochemical cycles

Without decomposers, ecosystem would collapse due to nutrient accumulation.

Section D – Long Answer Solutions

22. Structure and Function of Ecosystem

Structure:

Abiotic components: Water, soil, light, temperature.
Biotic components: Producers, consumers, decomposers.

Functions:

Productivity
Energy flow
Nutrient cycling
Decomposition

These functions maintain ecological balance.

23. Ecological Pyramids

Three types:

Pyramid of Number – May be upright or inverted.
Pyramid of Biomass – Upright in forest; inverted in pond.
Pyramid of Energy – Always upright.

Energy pyramid is always upright because energy decreases at each trophic level.

24. Nitrogen Cycle

Steps:

Nitrogen fixation – Atmospheric N₂ converted to ammonia.
Nitrification – Ammonia → Nitrites → Nitrates.
Assimilation – Plants absorb nitrates.
Ammonification – Organic nitrogen → Ammonia.
Denitrification – Nitrates → N₂ gas.

Ensures nitrogen availability to plants.

25. Energy Flow with 10% Law

Energy enters ecosystem through sunlight.

Sun → Producers → Primary Consumers → Secondary Consumers → Tertiary Consumers

Only 10% energy is transferred to next level.
90% is lost as heat during respiration.
Flow is unidirectional.

Thus, energy decreases at higher trophic levels.

Section E – Case Study Solutions

(a) Grasses (Producers)

(b) Grass → Grasshopper → Frog → Snake → Hawk

(c) Hawk (Tertiary consumer)

(d) Due to 10% law, energy decreases at each trophic level.

(e) Decomposition and mineralization return nutrients to soil.