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Ecosystem

NCERT Class 12 Biology Chapter 12

Ecosystem for Class 12 Biology and NEET: structure and function, productivity, decomposition, energy flow, food chains and ecological pyramids.

5 topics Updated 8 Nov 2026

For: Class 12 and NEET

In the syllabus: NEET Class 12 syllabus

With your textbook

NCERT Biology (Class 12), Chapter 12 “Ecosystem”, reprint 2026-27.

Open the official NCERT chapter (PDF, 2.4 MB, ncert.nic.in)

Checked on 19 Oct 2026.

Learn in this order

  1. Ecosystem: structure and function

    Ecosystem structure and function for Class 12 Biology and NEET: components, species composition, stratification, the four functions and the pond.

    NCERT Section 12.1 11 min read 3 solved examples 9 practice questions

  2. Productivity

    Productivity for Class 12 Biology and NEET: primary productivity, GPP vs NPP, what it depends on, land versus ocean and secondary productivity.

    NCERT Section 12.2 10 min read 3 solved examples 9 practice questions

  3. Decomposition

    Decomposition for Class 12 Biology and NEET: detritus, fragmentation, leaching, catabolism, humification, mineralisation and what controls the rate.

    NCERT Section 12.3 10 min read 3 solved examples 9 practice questions

  4. Energy flow

    Energy flow in ecosystems for Class 12 Biology and NEET: PAR, grazing and detritus food chains, food webs, trophic levels, standing crop, 10 per cent law.

    NCERT Section 12.4 15 min read 3 solved examples 10 practice questions

  5. Ecological pyramids

    Ecological pyramids for Class 12 Biology and NEET: pyramids of number, biomass and energy, upright and inverted pyramids, and their limitations.

    NCERT Section 12.5 14 min read 3 solved examples 9 practice questions

Key formulas in this chapter

Structure and function

  • components: abiotic and biotic
  • biotic: producers, consumers, decomposers
  • structure: species composition, stratification
  • function: productivity, decomposition, energy flow, nutrient cycling
  • gaseous cycle: carbon; sedimentary: phosphorus
  • energy: one way; nutrients: cycled

Productivity

  • NPP = GPP − R
  • productivity: g m yr or kcal m yr
  • biosphere NPP ≈ 170 billion tons a year
  • oceans 55, land 115 billion tons
  • secondary productivity: by consumers

Decomposition

  • detritus → CO + water + nutrients
  • steps: fragmentation, leaching, catabolism
  • in soil: humification, mineralisation
  • slow: lignin, chitin; fast: nitrogen, sugars
  • favoured: warm, moist; inhibited: cold, anaerobic

Energy flow

  • PAR < 50% of sunlight
  • plants capture 2–10% of PAR
  • energy: sun → producers, then consumers
  • next level gets about 10%
  • GFC: grass → goat → man
  • DFC: starts with dead organic matter

Ecological pyramids

  • pyramids: number, biomass, energy
  • energy pyramid: always upright
  • biomass in the sea: generally inverted
  • numbers on one tree: inverted
  • ideal model: 1% of sunlight into NPP
  • each level: about 10% of the one below

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