Section 10 — Board Exam-Pattern Questions (Class 12 Biology · Chapter 12)
Chapter 12 (Ecosystem) is one of the most dependable scoring chapters in the Class 12 Biology paper. Its parent unit (Ecology and Environment, about 10 marks) draws on it every year — questions spread across short definitions, 2/3-mark distinctions and numericals, and very often a 5-mark question on energy flow, productivity or ecological pyramids.
This section curates 24 high-yield Board exam-pattern questions (modelled on the CBSE style — not year-tagged official questions) in the mark-wise format used in Board papers:
| Marks | Number of Qs | Indicative frequency in papers |
|---|---|---|
| 1 mark | 7 questions (Q1–Q7) | 2–3 / paper |
| 2 marks | 6 questions (Q8–Q13) | 2 / paper |
| 3 marks | 6 questions (Q14–Q19) | 1–2 / paper |
| 5 marks | 5 questions (Q20–Q24) | 1 / paper (high probability) |
| Total | 24 questions | 62 marks of practice |
How to use: cover the answer, attempt it yourself scaled to the marks, then check against the model answer and its keyword notes.
No quiz at the end: pair this set with the Solved Examples section.
What the Examiners Keep Asking
Most questions on Chapter 12 fall into six recurring themes:
1. Structure and function (1- or 2-mark) — definition of ecosystem, abiotic and biotic components, species composition, stratification, the four functions.
2. Productivity (2-, 3- or 5-mark) — production versus productivity, GPP, NPP and the relation , secondary productivity, factors affecting primary productivity, the biosphere and ocean figures.
3. Decomposition (2- or 3-mark) — detritus, the five steps, humus, factors controlling the rate.
4. Energy flow (3- or 5-mark) — PAR, unidirectional flow, the two laws of thermodynamics, producers and consumers, trophic levels, the ten per cent law.
5. Food chains and webs (2- or 3-mark) — grazing versus detritus food chain, saprotrophs, how chains form a web.
6. Ecological pyramids (3- or 5-mark) — pyramids of number, biomass and energy, upright versus inverted, why the energy pyramid is always upright, limitations.
1-Mark Questions (Q1–Q7)
Q1. [1 mark] Write the relationship between gross primary productivity, respiration and net primary productivity.
Answer:
Keyword note: GPP, respiration losses R, and NPP must all appear.
Q2. [1 mark] Name the two main structural features of an ecosystem.
Answer: Species composition and stratification.
Q3. [1 mark] According to the ten per cent law, what fraction of energy is transferred to each higher trophic level from the level below?
Answer: About 10 per cent.
Q4. [1 mark] Which ecological pyramid is always upright and can never be inverted?
Answer: The pyramid of energy.
Q5. [1 mark] (Assertion–Reason)
Assertion (A): The pyramid of biomass in the sea is generally inverted. Reason (R): The biomass of the fishes far exceeds that of the phytoplankton that support them.
Options: (a) Both A and R true and R explains A. (b) Both true but R does not explain A. (c) A true, R false. (d) A false, R true.
Answer: (a) — the sea's pyramid of biomass is inverted precisely because the fish biomass exceeds the small standing crop of phytoplankton, so R correctly explains A.
Q6. [1 mark] Name the dark-coloured, colloidal substance formed during humification that acts as a reservoir of nutrients.
Answer: Humus.
Q7. [1 mark] Except for one ecosystem, the sun is the only source of energy on Earth. Name that exception.
Answer: The deep-sea hydrothermal ecosystem.
2-Mark Questions (Q8–Q13)
Q8. [2 marks] Distinguish between production and productivity, giving the unit of each.
Answer: Production is the amount of biomass or organic matter produced per unit area, expressed as weight () or energy (). Productivity is the rate of biomass production and carries a time unit, expressed as or , so that different ecosystems can be compared.
Keyword note: amount versus rate, and the time unit on productivity, carry the marks.
Q9. [2 marks] Differentiate between the grazing food chain and the detritus food chain on the basis of their starting point and main organisms.
Answer: The grazing food chain (GFC) begins with producers (green plants) and passes through herbivores and carnivores. The detritus food chain (DFC) begins with dead organic matter (detritus) and is made up of decomposers (saprotrophs), mainly fungi and bacteria, which degrade the dead matter.
Q10. [2 marks] What is decomposition? Name any two of its steps.
Answer: Decomposition is the process by which decomposers break down complex organic matter (detritus) into inorganic substances such as carbon dioxide, water and nutrients. Any two steps from fragmentation, leaching, catabolism, humification and mineralisation are acceptable.
Q11. [2 marks] Why is a trophic level said to be a functional level and not a species? Illustrate with an example.
Answer: A trophic level is defined by the source of an organism's nutrition, not by its identity, so the same species can occupy more than one level at the same time. A sparrow is a primary consumer when it eats seeds and a secondary consumer when it eats insects and worms.
Q12. [2 marks] The pyramid of numbers can be inverted but the pyramid of energy cannot. Give the reason for each.
Answer: The pyramid of numbers can be inverted when a single large producer, such as a big tree, supports a huge number of consumers (insects). The pyramid of energy is always upright because some energy is always lost as heat at each transfer, so a higher level can never hold more energy than the one below it.
Q13. [2 marks] State two factors that control the rate of decomposition, and describe how each affects it.
Answer: (i) Chemical composition of the detritus — decomposition is slower if the detritus is rich in lignin and chitin, and quicker if it is rich in nitrogen and sugars. (ii) Climatic factors, mainly temperature and soil moisture — a warm and moist environment favours decomposition, while low temperature and anaerobiosis inhibit it.
3-Mark Questions (Q14–Q19)
Q14. [3 marks] A field has a gross primary productivity of , of which the plants use in respiration. Calculate the net primary productivity and state what NPP represents and its unit.
Answer: Using substitute the values: . The net primary productivity is . NPP is the biomass available for consumption by heterotrophs (herbivores and decomposers).
Keyword note: correct formula, substitution with unit, and the meaning of NPP.
Q15. [3 marks] Producers in a food chain fix of energy. Using the ten per cent law, find the energy available to the herbivores and to the primary carnivores, and state why food chains are short.
Answer: Only about 10 per cent passes upward. Herbivores: . Primary carnivores: . Because the energy shrinks tenfold at every step, too little is left after a few transfers to support another level, so food chains remain short.
Q16. [3 marks] Describe the five steps of decomposition in the order in which they are usually named, and add a note on whether they occur one after another.
Answer: (1) Fragmentation — detritivores such as earthworms break detritus into smaller particles. (2) Leaching — water-soluble inorganic nutrients wash down and precipitate as unavailable salts. (3) Catabolism — bacterial and fungal enzymes degrade detritus into simpler inorganic substances. (4) Humification — dark, colloidal humus accumulates as a nutrient reservoir. (5) Mineralisation — humus is degraded, releasing inorganic nutrients. The first three — fragmentation, leaching and catabolism — operate simultaneously on the detritus; humification and mineralisation occur during decomposition in the soil.
Q17. [3 marks] Explain why energy flow in an ecosystem is unidirectional, relating your answer to the two laws of thermodynamics.
Answer: Energy moves from the sun to producers and then to consumers, and never back — a consumer cannot return energy to the plant it ate. This one-way transfer is consistent with the first law of thermodynamics, since energy is only transformed and transferred, not created or destroyed. Ecosystems also obey the second law: because the universe tends toward increasing disorder (entropy), a constant supply of energy is needed to build and maintain the ordered molecules of life.
Q18. [3 marks] Define an ecological pyramid. What do its base and apex represent, and name the three types.
Answer: An ecological pyramid expresses the feeding relationship between organisms at different trophic levels in terms of number, biomass or energy, in the shape of a pyramid. The base represents the producers (first trophic level) and the apex represents the top-level or tertiary consumer. The three types are the pyramid of number, the pyramid of biomass and the pyramid of energy.
Q19. [3 marks] What are decomposers? Explain how they obtain their nourishment and why they are called saprotrophs.
Answer: Decomposers are heterotrophic organisms, mainly fungi and bacteria, that make up the detritus food chain. They meet their energy and nutrient needs by degrading dead organic matter: they secrete digestive enzymes that break dead and waste material into simple inorganic substances, which they then absorb. Because they live on decaying matter they are called saprotrophs (sapro meaning to decompose).
5-Mark Questions (Q20–Q24)
Q20. [5 marks] Explain primary productivity under the following heads: its definition, the difference between GPP and NPP, secondary productivity, the factors affecting primary productivity, and the productivity of the biosphere.
Answer:
Definition: Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis; productivity is the rate of this production, expressed as or .
GPP and NPP: Gross primary productivity (GPP) is the total rate of production of organic matter in photosynthesis. A considerable part is used by the plants in respiration, and the remainder is the net primary productivity: NPP is the biomass available to heterotrophs.
Secondary productivity: the rate of formation of new organic matter by consumers.
Factors: primary productivity depends on the plant species present, environmental factors, availability of nutrients and the photosynthetic capacity of the plants, so it varies between ecosystems.
Biosphere: the annual NPP of the whole biosphere is about 170 billion tons (dry weight); the oceans give only about 55 billion tons despite covering about 70 per cent of the surface.
Keyword note: the equation, the meaning of NPP, and the factors each carry marks.
Q21. [5 marks] Give an account of energy flow through an ecosystem, covering the sun as the source, PAR, producers and consumers, trophic levels and the ten per cent law.
Answer:
Source and PAR: except for the deep-sea hydrothermal ecosystem, the sun is the only source of energy. Less than 50 per cent of incident solar radiation is photosynthetically active radiation (PAR), and plants capture only about 2 to 10 per cent of the PAR.
Producers and consumers: producers (green plants) fix this energy into food. Animals are consumers (heterotrophs) — primary consumers (herbivores) feed on producers, secondary consumers (primary carnivores) feed on herbivores, and tertiary consumers (secondary carnivores) feed on them.
Trophic levels: producers form the first trophic level, herbivores the second, carnivores the third. Energy flow is unidirectional, from sun to producers to consumers.
Ten per cent law: only about 10 per cent of the energy at one level passes to the next; for example . Because so little energy survives each transfer, the number of trophic levels is restricted and food chains stay short.
Q22. [5 marks] Describe ecological pyramids: the three types, their usual shape, the exceptions in the pyramids of number and biomass, why the pyramid of energy is always upright, and the limitations of pyramids.
Answer:
Three types: pyramids of number, biomass and energy, each with the producers at the base and the top consumer at the apex.
Usual shape: in most ecosystems all three are upright — producers exceed herbivores in number and biomass, and herbivores exceed carnivores; energy at a lower level is always more than at a higher level.
Exceptions: the pyramid of numbers can be inverted (many insects on a single big tree), and the pyramid of biomass in the sea is generally inverted (the biomass of fish exceeds that of the phytoplankton).
Energy pyramid: the pyramid of energy is always upright and never inverted, because some energy is always lost as heat at each transfer, so a higher level can never hold more energy than the one below.
Limitations: pyramids assume a simple food chain and cannot accommodate a food web, they ignore a species that belongs to two or more trophic levels, and they give no place to the decomposers (saprophytes).
Q23. [5 marks] Explain decomposition in detail: the meaning of the term, the raw material, the five steps, the role of humus, and the factors that control the rate.
Answer:
Meaning and raw material: decomposition is the breakdown of complex organic matter into inorganic substances such as carbon dioxide, water and nutrients. The raw material is detritus — dead plant remains (leaves, bark, flowers) and dead animal remains, including faecal matter.
Steps (the first three simultaneous): fragmentation (detritivores like earthworms break detritus into small particles); leaching (soluble nutrients wash down and precipitate as unavailable salts); catabolism (bacterial and fungal enzymes make simpler inorganic substances); humification (dark, colloidal humus accumulates); mineralisation (humus is degraded, releasing inorganic nutrients).
Humus: a dark, amorphous, colloidal substance, highly resistant to microbial action, that decomposes slowly and serves as a reservoir of nutrients.
Controlling factors: decomposition is largely oxygen-requiring; its rate depends on the chemical composition of the detritus (slower with lignin and chitin, quicker with nitrogen and sugars) and on climatic factors (favoured by warm, moist conditions, inhibited by low temperature and anaerobiosis).
Q24. [5 marks] Describe the structure of an ecosystem, taking a pond as an example, and explain how it functions as a self-sustaining unit.
Answer:
Components: an ecosystem has abiotic components (non-living factors such as air, water and soil) and biotic components (producers, consumers and decomposers); their interaction gives each ecosystem a characteristic physical structure. Two structural features are species composition and stratification.
The pond: in a pond, the abiotic component is the water with dissolved substances and the rich bottom soil, regulated by solar input and climate. The autotrophs are phytoplankton, algae and floating, submerged and marginal plants. The consumers are zooplankton and free-swimming and bottom-dwelling animals. The decomposers are fungi, bacteria and flagellates at the bottom.
Functioning: the pond performs all four functions — productivity, decomposition, energy flow and nutrient cycling. Autotrophs convert inorganic material into organic matter using solar energy; heterotrophs consume the autotrophs; decomposers mineralise dead matter and release nutrients back for reuse. Throughout, there is a unidirectional flow of energy to higher trophic levels with continual loss as heat, which is why a constant input of solar energy keeps the pond self-sustaining.
End of Section 10
You have now worked through 24 high-yield Board questions spanning ecosystem structure, productivity, decomposition, energy flow, food chains and webs, and ecological pyramids. If you can answer the five 5-markers (Q20–Q24) from memory with their key points, and can carry out the productivity and ten-per-cent-law numericals cleanly, you have effectively secured this chapter's contribution to your paper.
Final tip: on exam day, draw and label diagrams wherever allowed — the trophic-level energy flow, the three pyramids, the decomposition cycle. Tabulate comparison answers (production versus productivity, GFC versus DFC, upright versus inverted pyramid), and always show numerical working with the formula first.