CBSE · Class 12 · Biology · Chapter 13

Chapter 13:
Ecosystem

Complete chapter resources for CBSE Class 12 Biology — energy flow, food chains and webs, ecological pyramids, nutrient cycling, and ecosystem productivity, with sample questions and board exam tips.

4Topics
4–6Board marks
8Sample questions
3PYQ included
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Key Concepts — Chapter 13
  • 10% Energy Law: Only ~10% of energy transfers to the next trophic level (Lindeman, 1942)
  • GPP − R = NPP: Net Primary Productivity = Gross PP minus Respiration loss
  • Trophic levels: Producers (T1) → Herbivores (T2) → Primary carnivores (T3) → …
  • Standing crop: Mass of living organisms per unit area at a given time
  • Detritus food chain: Dead organic matter → Detritivores → Decomposers
  • Nutrient cycling: Gaseous cycle (C, N) vs. Sedimentary cycle (P, S)
Chapter Overview

What this chapter covers

An ecosystem is a functional unit of nature in which living organisms interact with one another and with the physical environment. Chapter 13 of NCERT Class 12 Biology introduces the structural and functional aspects of ecosystems — how energy flows through trophic levels, how matter cycles between biotic and abiotic components, and how productivity is measured. The chapter covers both the grazing food chain (starting from green plants) and the detritus food chain (starting from dead organic matter), and explains why food webs are more ecologically realistic than isolated food chains.

A central concept is energy flow, governed by Lindeman's 10 per cent law: at each trophic level only about 10% of the energy received from the level below is stored and passed on; the rest is lost as heat during respiration. This explains why ecological pyramids of energy are always upright and why the number of viable trophic levels in any ecosystem is limited. The chapter also distinguishes between Gross Primary Productivity (GPP) — total photosynthetic fixation — and Net Primary Productivity (NPP = GPP − R), the biomass actually available for consumers.

The second half of the chapter deals with nutrient cycling (biogeochemical cycles). Students learn how carbon moves between the atmosphere, living organisms, and the lithosphere through photosynthesis, respiration, decomposition, and combustion. The phosphorus cycle, which lacks a significant gaseous phase, is contrasted with the carbon and nitrogen cycles. Board questions on this chapter regularly test energy calculations using the 10% law, interpretation of ecological pyramids, and the steps of decomposition — making it a focused, high-yield chapter for examination preparation.

Topics

What's inside Chapter 13

As per NCERT Class 12 Biology (CBSE syllabus)

Topic 1
Ecosystem Structure & Function
Components of an ecosystem (biotic and abiotic). Producers, consumers, decomposers. Grazing food chain vs. detritus food chain. Food webs and their stability advantages.
Topic 2
Energy Flow & Productivity
Lindeman's 10% law of energy transfer. Gross Primary Productivity (GPP), Net Primary Productivity (NPP), and Secondary Productivity. Measurement of productivity. Standing crop and standing state.
Topic 3
Decomposition & Ecological Pyramids
Steps of decomposition: fragmentation, leaching, catabolism, humification, mineralisation. Factors affecting decomposition. Ecological pyramids of number, biomass, and energy — upright vs. inverted forms.
Topic 4
Nutrient Cycling (Biogeochemical Cycles)
Carbon cycle: photosynthesis, respiration, decomposition, combustion. Phosphorus cycle: weathering, uptake, decomposition — no atmospheric reservoir. Distinction between gaseous and sedimentary cycles.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Ch 12 · Biodiversity & Conservation
Species interactions and community ecology underpin ecosystem function
Ch 11 · Organisms & Populations
Population dynamics, growth models, and interspecific interactions
Chapter 13 Ecosystem
Leads to
Ch 16 · Environmental Issues
Pollution, biomagnification, and greenhouse effect build on nutrient/energy cycles
Competitive Exams (NEET)
Ecology section — energy flow, pyramids, and cycling are perennial NEET topics
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 12 Biology board papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Assertion–Reason 1 1–2 10% law, pyramid type identification, GPP vs. NPP distinction
Very Short Answer 2 1 Define detritus, name steps of decomposition, or calculate energy at a trophic level
Short Answer 3 1 Explain the carbon cycle with a labelled diagram; distinguish food chain from food web
Long Answer 5 0–1 Energy flow diagram with trophic levels; types of ecological pyramids with examples
Total (approximate) 4–6 3–4 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 12 Biology Chapter 13. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
According to Lindeman's 10 per cent law, if producers in an ecosystem fix 10,000 kcal of energy, how much energy (in kcal) would be available to secondary consumers? (a) 1,000 (b) 100 (c) 10 (d) 1,000,000
Q2 Easy 2 marks Short Answer
Differentiate between Gross Primary Productivity (GPP) and Net Primary Productivity (NPP). Give the relationship between them.
Q3 Medium 2 marks Short Answer
Why is the pyramid of energy always upright in any ecosystem? Can the pyramids of number and biomass ever be inverted? Justify with an example for each.
Q4 Medium 3 marks Short Answer
List the sequential steps involved in the process of decomposition of detritus. Name the end products of decomposition and state their role in the ecosystem.
Q5 Medium 3 marks Short Answer
Draw a simplified diagram of the carbon cycle showing at least four processes. Briefly explain how human activities have disturbed the balance of the carbon cycle.
Q6 Hard 4 marks Long Answer
Explain the structure and function of a pond ecosystem. In your answer, identify the abiotic components and describe the roles of producers, primary consumers, secondary consumers, and decomposers.
Q7 Hard 5 marks Long Answer
With the help of a labelled diagram, describe the flow of energy through the different trophic levels in a grassland ecosystem. What percentage of solar energy fixed by plants is actually available to the tertiary consumers? Why does the length of a food chain rarely exceed four to five trophic levels?
Q8 Hard 5 marks Case-Based
In a forest ecosystem, the following food chain was observed: Leaves → Caterpillars → Small birds → Hawks The producers (leaves) contain 1,00,000 J of energy. (i) Calculate the energy available at each trophic level using the 10% law. (ii) What is the role of decomposers in this ecosystem? (iii) How does a food web differ from this food chain, and why is a food web more stable?
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Previous Year Questions

From CBSE board examinations

Actual questions from past Class 12 Biology board papers — Ecosystem chapter.

Board 20223 marks
Name the biotic components of an ecosystem. Describe the role of decomposers in an ecosystem. (CBSE All India 2022)
Board 20232 marks
Differentiate between the upright and inverted pyramids of biomass. Give one example of each. (CBSE Delhi 2023)
Board 20205 marks
Draw a diagrammatic representation of the carbon cycle in nature. Explain the role of microorganisms in the carbon cycle. How does the burning of fossil fuels affect the carbon cycle? (CBSE 2020)
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FAQ

Questions teachers ask

How many marks does Ecosystem carry in the CBSE Class 12 Biology board exam? +
Ecosystem typically contributes 4–6 marks in the CBSE Class 12 Biology board exam. Questions appear across different sections — one MCQ or assertion–reason (1 mark), one short-answer (2–3 marks), and occasionally part of a long-answer question (5 marks) when paired with Biodiversity and Conservation.
What is the 10 per cent law of energy transfer and why is it important for board exams? +
Lindeman's 10 per cent law states that only about 10% of the energy available at one trophic level is transferred to the next. It explains why food chains are typically limited to 3–5 trophic levels. Board questions frequently ask students to calculate the energy available at a specific trophic level when the energy at the first trophic level is given — for example, if producers have 10,000 J, primary consumers have 1,000 J, secondary consumers 100 J, and so on.
What is the difference between a food chain and a food web? +
A food chain is a linear sequence showing the flow of energy from one organism to the next (e.g., Grass → Grasshopper → Frog → Snake → Eagle). A food web is a complex network of interconnected food chains within an ecosystem. A food web is more realistic and stable because the removal of one organism does not cause the collapse of the entire system. CBSE board questions often ask students to draw a food chain from a given food web or explain why food webs provide greater stability.
Which nutrient cycles are most important for CBSE board exams? +
The carbon cycle and phosphorus cycle are the most frequently tested in CBSE Class 12 board exams. For the carbon cycle, students must know the processes of photosynthesis, respiration, decomposition, and combustion. For the phosphorus cycle, the key point is that phosphorus has no significant atmospheric reservoir — it cycles only through lithospheric and aquatic compartments. The nitrogen cycle, while more detailed, also appears in questions about nitrification, denitrification, and nitrogen fixation.
How do I generate a custom question paper for Ecosystem using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 12 Biology, select Chapter 13 (Ecosystem), set your preferred question-type mix (MCQ, short answer, long answer) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.