In Chapter 12 of Class 12 Biology - Ecosystem, students learn how living organisms and their physical surroundings work together as a functional unit. It also introduces concepts like nutrient cycles, ecological pyramids, and productivity to help students understand how balance is maintained in nature.
To streamline exam preparation, ALLEN provides high-quality NCERT Solutions with focus on clarifying the distinction between GPP (Gross Primary Productivity) and NPP (Net Primary Productivity), while providing step-by-step breakdowns of the decomposition process. Students can develop a deep, intuitive understanding of ecological balance, ensuring they are fully prepared for descriptive board questions and competitive medical entrance exams.
This chapter explains how energy and nutrients move through natural systems in a simple and structured way. The NCERT Solutions for Class 12 Biology Chapter 12 Ecosystem help students revise key ecological processes and practise textbook questions easily. Download the free PDF now to study anytime and prepare confidently for exams.
The chapter focuses on understanding how ecosystems are organised and maintained. Some of the key lessons covered in this chapter are given below:
1. Fill in the blanks.
(a) Plants are called as_________because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is_________type.
(c) In aquatic ecosystems, the limiting factor for the productivity is_________.
(d) Common detritivores in our ecosystem are_________.
(e) The major reservoir of carbon on earth is_________.
Ans:
(a) Producers
(b) Inverted
(c) Light
(d) Earthworms
(e) Oceans
2. Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers
Ans: (a) Decomposers
3. The second trophic level in a lake is
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes
Ans: (b) Zooplankton
4. Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above
Ans: (d) None of the above
5. What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?
(a) 100%
(b) 50 %
(c) 1-5%
(d) 2-10%
Ans: (c) 1-5%
6. Distinguish between
(a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) Upright and inverted pyramid
(d) Food chain and Food web
(e) Litter and detritus
(f) Primary and secondary productivity
Ans: (a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) Upright and inverted pyramid
(d) Food chain and Food web
(e) Litter and detritus
(f) Primary and secondary productivity
7. Describe the components of an ecosystem.
Ans: An ecosystem consists of living (biotic) and non-living (abiotic) components that interact with each other to maintain the flow of energy and cycling of nutrients.
Abiotic Components (Non-living): Abiotic components are the physical and chemical factors that influence living organisms. Some of them are given below:
Biotic Components (Living): These include all organisms present in the ecosystem and are divided into three groups:
8. Define ecological pyramids and describe with examples, pyramids of number and biomass.
Ans: A graphical representations that show the trophic structure of an ecosystem is called an ecological pyramid. They illustrate the relationship between producers, consumers, and top-level organisms in terms of number of organisms, biomass (total living matter) and energy flow. These pyramids help us understand how food and energy move through different levels of a food chain.
Pyramid of Numbers:
Definition: The pyramid of numbers represents the number of individual organisms at each trophic level in an ecosystem.
Structure: In the pyramid of numbers, the base of the pyramid is occupied by the producers (Plants), the middle level by the primary and secondary consumers and the top constitutes the top carnivores.
Example:
(a) Upright pyramid of numbers
Grassland Ecosystem
Many grasses → Few deer → Fewer lions
(b) Inverted pyramid of numbers
Forest Ecosystem
Few trees → Many insects → More parasites
Pyramid of Biomass:
Definition: The pyramid of biomass shows the total mass of living organisms at each trophic level, measured as dry weight per unit area.
Example:
(a) Upright Pyramid of Biomass
Example: Forest and grassland ecosystems
(b) Inverted Pyramid of Biomass
Example: Aquatic ecosystem (pond or sea)
9. What is primary productivity? Give brief description of factors that affect primary productivity.
Ans: Primary productivity is the rate at which green plants, algae, and photosynthetic microorganisms produce organic matter (biomass) from inorganic substances like carbon dioxide and water using solar energy through the process of photosynthesis.
In other words, the amount of food or energy made by producers in an ecosystem per unit area per unit time is called the primary productivity. Its unit is g/m2/year or kcal/m2/year
Factors affecting primary productivity:
10. Define decomposition and describe the processes and products of decomposition.
ANs: Decomposition is the natural process by which dead plants and animals (detritus) are broken down into simpler inorganic substances like carbon dioxide, water, and mineral nutrients by the action of microorganisms (bacteria and fungi) and detritivores.
The process of decomposition is:
(a) Fragmentation - In this step, Large pieces of detritus are broken into smaller particles by detritivores like earthworms, termites and insects.
(b)Leaching- Here, the water dissolves the soluble nutrients from dead matter. These seep into the soil and become available for plant roots.
(c) Catabolism - Catabolism is the stage where microorganisms like bacteria and fungi release enzymes that break complex organic compounds into simpler substances.
(d) Humification - In this stage, a dark colored amorphous substance called humus is formed in the soil. It is rich in nutrients and helps improve soil fertility and water retention.
(e) Mineralisation - In this stage, the humus and simple compounds are further broken down by the microbes into inorganic nutrients like nitrates, phosphates and potassium. These minerals after broken down are absorbed by the plants.
Products of decomposition:
The main products following the processof decomposition are:
11. Give an account of energy flow in an ecosystem
Ans: Energy flow in an ecosystem refers to the movement of energy from the sun to producers and then through different levels of consumers and finally to decomposers.
This flow of energy is one-directional and non-cyclic, meaning energy cannot be reused once it is lost as heat.
Process
Sun as the Ultimate Source: The sun provides the primary source of energy for all living organisms in the ecosystem.
Absorption by Producers: Green plants trap solar energy and convert it into chemical energy by photosynthesis.
Transfer Through Trophic Levels: Energy passes from producers to primary, secondary, and tertiary consumers through food chains and food webs.
10% Law of Energy Transfer: Only about 10% of energy at one trophic level is transferred to the next level; the remaining energy is lost as heat and metabolic activities.
Role of Decomposers: Decomposers break down dead plants and animals and recycle nutrients back to the soil, while the remaining energy is released as heat.
Energy Pyramid: The energy pyramid is always upright, showing a gradual decrease in energy at higher trophic levels.
(Session 2026 - 27)