Q.The energy source for an ecosystem is –
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Imagine you are looking at a simple meal: a plate of rice and vegetables. Where did the energy in that food come from? The rice grew in a paddy field, using sunlight. The vegetables grew in the soil, also using sunlight. That sunlight is the original source of nearly all energy on Earth. Now think of a tiger eating a deer, or a cow eating grass. In every case, energy is being passed from one living thing to another. This passing of energy from the sun through plants, then to animals, and eventually back to the environment, is what ecologists call energy flow.
The key idea is that energy does not cycle. Unlike water or carbon, which get reused, energy enters an ecosystem, is used, and then leaves as heat. It is a one-way street. The sun provides a constant supply, but at every step, some energy is lost — mostly as heat from the organism’s own life processes like breathing, moving, and growing. This is why there are fewer tigers than deer, and fewer deer than grass plants. Each level can support only a fraction of the energy that came from the level below.
In your NCERT textbook (Class 12 Biology, Chapter 14), this is explained through the concept of trophic levels — the feeding steps in a food chain. The first trophic level is always the producers (green plants). The second is herbivores (plant-eaters). The third is carnivores (meat-eaters). And so on. The flow of energy from one trophic level to the next is called energy transfer, and it is never 100% efficient. Most of the energy is used for the organism’s own survival and is lost as heat.
Energy flow is unidirectional — it moves from the sun to producers to consumers and finally to decomposers, and then dissipates as heat. It never flows backward. This is a fundamental law of ecology, rooted in the second law of thermodynamics.
Why does this matter for you, even as a humanities or commerce student? Because it explains why food chains are short — usually only 4 or 5 steps. Beyond that, there simply isn’t enough energy left to support another level. It also explains why the number of organisms at each level decreases as you go up. This is often shown as a pyramid of energy, where the base (producers) has the most energy, and the top (apex predators) has the least.
Here are the key points to remember:
- Source: The sun is the primary energy source for almost all ecosystems.
- Direction: Energy flows in one direction — from sun → producers → consumers → decomposers → lost as heat. …
Every ecosystem needs a constant supply of energy, and this energy ultimately comes from one non-living source that green plants can capture. …
The energy source for an ecosystem is the Sun — option (iii). Producers convert solar energy into chemical energy.
Concept
An ecosystem runs on a one-way flow of energy that must be continually supplied. The Sun is the ultimate source of this energy. Producers (green plants) trap a small fraction of the incoming solar radiation (PAR, photosynthetically active radiation) and convert it into the chemical energy of food by photosynthesis. This energy then flows to herbivores, carnivores and decomposers.
Why the other options are wrong
- ATP (i) is the energy currency inside cells, not the external source of an ecosystem. …
Showing the 12 most recent of 48 on this concept.
- CBSE 2026Set 57/1/11 markMCQQ.Assertion (A) : In terrestrial ecosystem much larger fraction of energy flows through grazing food chain. Reason (R) : Grazing food chain may be connected to Detritus food chain at some levels. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.
›Reveal solutionSolution
In terrestrial ecosystems, the detritus food chain — not the grazing food chain — carries the larger share of energy, so the Assertion is false; the Reason, that the two food chains connect at some levels, is true but does not explain a claim that is itself incorrect. The correct choice is (D).
Energy in an ecosystem flows through two linked pathways: the grazing food chain (GFC), which starts with living green plants consumed by herbivores, and the detritus food chain (DFC), which starts with dead organic matter broken down by decomposers and detritivores. Which pathway carries the larger share of energy depends on the type of ecosystem.
Examining the Assertion. The Assertion claims that in terrestrial ecosystems, a much larger fraction of energy flows through the grazing food chain. This is actually the opposite of what NCERT states. In most terrestrial ecosystems — forests and grasslands, for example — only a small fraction of the net primary production is consumed directly by herbivores while still alive; the bulk of plant material (fallen leaves, dead wood, and other litter) is never grazed and instead enters the soil as detritus, where decomposers and detritivores process it. So it is the detritus food chain, not the grazing food chain, that carries the much larger share of energy in terrestrial ecosystems. (The situation is reversed in many aquatic ecosystems, where the grazing food chain often dominates.) Because the Assertion states the reverse of the real relationship, Assertion (A) is false.
NoteThis grazing-vs-detritus split is one of the most commonly confused facts in the Ecosystem chapter — remember it as: terrestrial land systems lean on decomposers (detritus dominant), many aquatic systems lean on herbivory (grazing dominant). …
- CBSE 2026Set 57/3/11 markMCQQ.Assertion (A) : Ecosystems require a constant supply of energy to synthesize the molecules they require. Reason (R) : This is to counteract the universal tendency towards increasing disorderliness as per Second Law of Thermodynamics.
›Reveal solutionSolution
Ecosystems need a continuous energy supply to build and maintain their complex structures, directly counteracting the natural tendency towards increasing disorder described by the Second Law of Thermodynamics.
An ecosystem is a dynamic, self-regulating unit comprising living organisms interacting with each other and their non-living environment. For any ecosystem to function and sustain itself, a fundamental and continuous input of energy is absolutely essential. This energy primarily originates from the sun, captured by producers (like green plants) through photosynthesis, converting light energy into chemical energy stored in organic molecules. This chemical energy then forms the very foundation of all food chains and webs, driving life processes throughout the ecosystem.
As energy flows through different trophic levels – from producers to primary consumers (herbivores), then to secondary consumers (carnivores), and so on – a significant portion of it is inevitably lost. This loss occurs mainly as heat during metabolic activities, respiration, and other life processes that organisms undertake. Typically, only about 10% of the energy from one trophic level is transferred to the next, a principle often referred to as the 10% Law. This constant dissipation of energy means that higher trophic levels have progressively less energy available, which inherently limits the number of organisms they can support.
Given this continuous energy dissipation, ecosystems require a fresh and constant supply of energy to synthesize the complex organic molecules necessary for life. These molecules are the fundamental building blocks of cells, tissues, and entire organisms. Without this ongoing energy input:
- Growth and Development: Organisms would be unable to grow, repair damaged tissues, or develop new structures.
- Reproduction: The energy-intensive process of reproduction would cease, leading to the eventual decline and extinction of populations.
- Maintenance: Basic metabolic functions, such as maintaining body temperature, pumping blood, nerve impulses, and active transport across membranes, all demand a continuous supply of energy.
- Biomass Production: The overall biomass of the ecosystem, representing the total living organic matter, would diminish, leading to a collapse of the system.
ImportantThe sun is the ultimate source of energy for almost all ecosystems on Earth, driving the entire biological machinery from the smallest microbe to the largest whale.
This constant energy requirement is directly linked to one of the most fundamental principles of physics: the Second Law of Thermodynamics. This law states that in any isolated system, the total entropy, or disorder, can only increase over time. In simpler terms, the universe naturally tends towards a state of greater randomness, disorganization, and energy dispersal.
Biological systems, such as individual organisms and entire ecosystems, are characterized by their remarkable order and complexity. Consider the intricate structure of a cell, the highly organized arrangement of tissues within an organism, or the complex interdependencies within a forest community. Maintaining this high level of organization requires constant work, and work, by definition, requires energy. …
- CBSE 2026Set A1 markMCQQ.Which of the following is not the primary consumer of an ecosystem?(a) Zooplankton(b) Cow(c) Grasshopper(d) Wolf
›Reveal solutionSolution
Primary consumers are herbivores; the wolf eats other animals, so it is a carnivore, not a primary consumer.
Primary consumers feed directly on producers (plants). Zooplankton feed on phytoplankton, a cow and a grasshopper feed on plants — all herbivores and therefore primary consumers. A wolf, however, prey …
- CBSE 2026Set A1 markMCQQ.What is secondary consumer called?(a) Herbivores(b) Primary carnivores(c) Secondary carnivores(d) Consumers
›Reveal solutionSolution
Secondary consumers feed on herbivores, so they are the primary carnivores in a food chain.
In a food chain, primary consumers are herbivores that eat producers. The animals that feed on these herbivores are the secondary consumers; because they are the first flesh-eaters in the chain, they are also called primary carn …
- CBSE 2026Set ANNUAL1 markMCQQ.In ecological terminology, what are green plants called?(a) Producer(b) Primary consumer(c) Secondary consumer(d) Tertiary consumer
›Reveal solutionSolution
In an ecosystem's trophic structure, green plants (and other photosynthetic/chemosynthetic organisms) that fix inorganic carbon into food are called producers.
…
- CBSE 2026Set ANNUAL1 markQ.Design a food chain for an aquatic ecosystem.
›Reveal solutionSolution
A food chain shows the linear transfer of energy from producers to successive levels of consumers; in an aquatic ecosystem this begins with microscopic algae (phytoplankton).
In an aquatic ecosystem such as a pond or lake, energy flow can be represented as a food chain where each organism is eaten by the next, transferring energy along the way:
Phytoplankton (producers) → Zooplankton (primary consumers) → Small fish (secondary consumers) → Large fish (tertiary consumers)
- Phytoplankton are microscopic, free-floating photosynthetic algae that form the base of the food chain, fixing solar energy into organic matter.
- Zooplankton are tiny floating/swimming animals (e.g., water fleas, protozoans) that graze on phytoplankton.
- Small fish feed on zooplankton. …
- CBSE 2026Set ANNUAL1 markQ.If 10,000 Joules of energy is available at the producer level, how much energy would typically be available to the tertiary consumer?
›Reveal solutionSolution
Only about 10% of the energy available at one trophic level is transferred to the next (the 10 per cent law), so starting with 10,000 J at the producer level, the tertiary consumer (three trophic-level transfers away) would typically receive only 10 J.
In any food chain, energy flow is unidirectional and a large fraction of energy is lost as heat (through respiration, movement, and other metabolic activities) at each transfer. As a general rule, only about 10% of the energy available at one trophic level is transferred to and made available for the next trophic level; the remaining ~90% is lost.
Applying this successively:
- Producer level: 10,000 J …
- CBSE 2026Set ANNUAL1 markMCQQ.Starting point in the detritus food chain is :(a) Green plants(b) Dead organic matter(c) Phytoplankton(d) Shrubs & bushes
›Reveal solutionSolution
The detritus food chain begins with dead organic matter (detritus), which is broken down by decomposers/detritivores, unlike the grazing food chain which begins with living green plants.
In an ecosystem there are two main types of food chains. The grazing food chain (GFC) starts from living green plants (producers) and passes to grazing herbivores and then carnivores. The detritus food chain (DFC), by contrast, begins with dead organic matter (detritus) — fallen leaves, dead bodie …
- CBSE 2026Set ANNUAL1 markMCQQ.If 20 joules energy is received in producer level, then how many amount of energy in the form of food will get to Peacock in the following food chain? Plant → Rat → Snake → Peacock(a) 0.02 joule(b) 0.2 joule(c) 0.002 joule(d) 0.0002 joule
›Reveal solutionSolution
Applying the 10% law of energy transfer through 3 trophic transfers: 20 J → 2 J → 0.2 J → 0.02 J reaches the Peacock.
According to the 10% Law (Lindeman's law), only about 10% of the energy available at one trophic level is transferred to the next trophic level; the rest is lost as heat/respiration.
Food chain: Plant (producer) → Rat (primary consumer) → Snake (secondary consumer) → Peacock (tertiary consumer)
…
- CBSE 2026Set ANNUAL1 markQ.Write answer in one word/sentence: Food chain begins with dead organic matter is called:
›Reveal solutionSolution
A food chain starting from dead organic matter is the detritus food chain.
Ecosystems have two main types of food chains. The grazing food chain (GFC) starts with living green plants (producers). The detritus food chain (DFC) starts with dead organic matter (detritus), which is consumed by detritivores and decomposers (detritivorous organisms, fungi and bacteria) that …
- CBSE 2026Set ANNUAL1 markMCQQ.The flow of energy through lower to higher trophic levels in an ecosystem is :(a) always unidirectional.(b) sometimes unidirectional and sometimes bidirectional.(c) always bidirectional.(d) cannot be determined with certainty.
›Reveal solutionSolution
Energy flow through an ecosystem is always one-way (unidirectional) from the sun to producers to successive consumers, so option (a) is correct.
In the CBSE/NCERT Ecosystem chapter, energy enters as sunlight, is fixed by producers, and passes to herbivores and then carnivores. At every transfer a large fraction is lost as heat during respiration (only about 10% passes up, the 10% law). Because this lost heat cannot re-enter the living system, energy …
- CBSE 2025Set KH1 markMCQQ.Which one of the following is secondary consumer?(a) Rat(b) Squirrel(c) Lizard(d) Grasshopper
›Reveal solutionSolution
A lizard eats insect herbivores, so it occupies the third trophic level → secondary consumer → option (c).
Concept. In a food chain: producers → primary consumers (herbivores) → secondary consumers (carnivores that eat herbivores) → tertiary consumers.
Applying it. …
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