Q.The flow of energy through the trophic levels of an ecosystem is
Concept understanding — Energy Flow
Energy flow is the transfer of energy between successive trophic levels in an ecosystem, and it is the ecosystem's single most important function. Energy captured by a producer moves on to consumers and then decomposers step by step, losing some as heat at every single transfer, and — critically — this flow runs in one direction only, never reversing. Two thermodynamic laws govern it. The first law of thermodynamics states energy is never created or destroyed, only transformed from one form to another (e.g. light energy converted into the chemical energy of starch during photosynthesis, with the total quantity of energy unchanged). The second law of thermodynamics states that every energy transformation reduces the system's usable (free) energy and is never 100% efficient — as food energy passes between organisms, part is retained as living tissue while a large share is irreversibly dissipated as heat through respiration. This second-law loss is captured precisely by Lindeman's (1942) ten percent law: at each trophic transfer only about 10% of the energy is retained by the receiving level, with the remaining 90% lost to respiration, decomposition and heat. Concretely: of 1000 joules of solar energy trapped by producers, about 100 joules is stored (900 lost); herbivores retain about 10 joules of that (90 lost); carnivores retain about 1 joule (9 lost); and tertiary consumers retain about 0.1 joule (0.9 lost) — a roughly ten-fold shrinkage at each successive step, which is exactly why food chains rarely extend beyond four or five trophic levels and why a pyramid of energy is always upright.
Energy moves through an ecosystem strictly one way, from producers upward, and is never reused once dissipated.
(b) Unidirectional
Step 1. Once energy captured by producers is passed to primary consumers, then to secondary consumers, and so on up the food chain, it can never flow back down to a lower trophic level. Step 2. At each transfer, a large majority of the energy is lost as metabolic heat through respiration, and this lost energy can never be recovered or reused by the ecosystem -- it is permanently dissipated. Step 3. This strictly one-way, non-reusable movement is what distinguishes energy flow from the cycling of mineral nutrients, which move in repeating loops (biogeochemical cycles) and can be used again and again; energy, by contrast, flows through the ecosystem only once, in one direction.
(b) Unidirectional
- Confusing the one-way flow of energy with the cyclic movement of nutrients, and answering 'cyclic'.
- Assuming energy lost as heat can somehow be reabsorbed and reused later in the ecosystem.
- CBSE 2026Set ANNUAL1 markQ.The producers receive 5000 Joules of light energy, then how much energy will be transferred to the carnivores (secondary consumers)?
›Reveal solutionSolution
Applying the 10% law twice (producers→herbivores→carnivores) from 5000 J gives 50 J reaching secondary consumers.
According to Lindeman's ten percent law, only about 10% of the energy present at one trophic level is transferred to and becomes incorporated as biomass at the next trophic level; the rest is lost mainly as heat through respiration and other metabolic processes. Starting with 5000 Joules of light energy captured by the producers: primary consumers (herbivores) would receive about 10% of this, i.e., 5000 × 10/100 = 500 Joules. Secondary consumers (carnivores, i.e., the level asked about) would then receive about 10% of the herbivores' energy, i.e., 500 × 10/100 = 50 Joules. So 50 Joules of energy would ultimately be available to/transferred to the carnivores (secondary consumers).
✓Final answerOnly about 50 Joules of energy will be transferred to the secondary consumers (carnivores), since energy transfer between trophic levels follows the 10% law — only about 10% of the energy of one trophic level is passed on to the next.
- CBSE 2025Set ANNUAL1 markMCQQ.In the given food chain, if plants absorb 1000 Joules of light energy, then how much energy is available for Lion? Plants (Producer) → Deer (Herbivore) → Lion (Carnivore).(a) 10 Joules(b) 100 Joules(c) 1000 Joules(d) 0.1 Joule
›Reveal solutionSolution
Applying the 10% law twice: 1000 J → 100 J → 10 J reaches the lion.
Energy flow through a food chain is unidirectional and follows Lindeman's 10% law: at each trophic transfer, only about 10% of the energy received is passed on to the next level, the rest being lost as heat during respiration, used in metabolism, or left undigested. Starting with 1000 J captured by the producers (plants), the herbivore (deer) receives about 10% of this, i.e. 100 J. The carnivore (lion), one level further, receives about 10% of 100 J, i.e. 10 J. This progressive energy loss is why food chains rarely extend beyond 4–5 trophic levels and why the energy pyramid is always upright.
✓Final answer(a) 10 Joules.
- CBSE 2025Set ANNUAL1 markMCQQ.Solar energy used by green plants for photosynthesis is only :(a) 3 - 10%(b) 2 - 8%(c) 2 - 9%(d) 2 - 10%
›Reveal solutionSolution
Green plants photosynthetically use only about 2-10% of the total solar energy that falls on them.
Working
Sunlight reaching a plant canopy is far from fully converted into chemical energy through photosynthesis. Much of the incident solar radiation is: reflected off leaf surfaces, transmitted straight through the leaf without being absorbed, absorbed but outside the photosynthetically active radiation (PAR) waveband (400-700 nm), or absorbed as PAR but lost as heat rather than being fixed. As a result, in most natural ecosystems, only a small fraction of the total incoming solar energy -- about 2-10% -- is actually captured and fixed by green plants during photosynthesis and made available as chemical (food) energy to enter the food chain. This limited efficiency of primary production is one reason why energy transfer through trophic levels in an ecosystem is comparatively small and why food chains rarely extend beyond four or five trophic levels.
✓Final answerThe correct option is (d): 2 - 10%
- CBSE 2022Set ANNUAL1 markMCQQ.Solar energy used by green plants for only photosynthesis is :(a) 3 - 10%(b) 2 - 8%(c) 2 - 9%(d) 2 - 10%
›Reveal solutionSolution
Green plants use only about 2-10% of incident solar energy for photosynthesis.
Only a portion of the sunlight that falls on a leaf is of a wavelength and intensity that chlorophyll a, chlorophyll b and the accessory pigments can actually absorb and channel into the light reactions of photosynthesis; much of the incident radiation is reflected off the leaf surface, transmitted straight through, or absorbed and simply dissipated as heat rather than being converted into chemical energy. Because of these physical and biochemical losses at every stage -- reflection, transmission, non-photosynthetic absorption and the inherent inefficiency of converting light energy into ATP and NADPH -- ecologists estimate that green plants effectively use only about 2 to 10% of the solar energy incident upon them for photosynthesis, and this modest efficiency is what ultimately caps the Gross Primary Productivity of an ecosystem and the amount of energy available to all subsequent trophic levels.
✓Final answerGreen plants use only about 2-10% of the solar energy incident on them for photosynthesis.
- CBSE 2020Set ANNUAL1 markMCQQ.Solar energy used by green plants for photosynthesis is only :(a) 2 - 9%(b) 2 - 8%(c) 2 - 10%(d) 3 - 10%
›Reveal solutionSolution
Green plants convert only about 2-10% of incident solar radiation into chemical energy via photosynthesis.
A leaf receives the full spectrum of incident sunlight, but only the visible, Photosynthetically Active Radiation (PAR) band can be absorbed by chlorophyll and accessory pigments, and even within PAR much of the energy is lost to reflection, transmission through the leaf, and non-photochemical heat dissipation. As a result, under natural field conditions the overall efficiency with which green plants convert incident solar energy into the chemical energy of photosynthate is only about 2-10%, and this trapped fraction is what constitutes the gross primary productivity (GPP) of an ecosystem, the base on which all subsequent trophic levels depend. This low first-step efficiency, compounded further by respiratory losses and the roughly 10% ecological efficiency of energy transfer between successive trophic levels, explains why food chains are typically short and why higher trophic levels support far less biomass than primary producers.
✓Final answerGreen plants use only 2-10% of the incident solar energy for photosynthesis.
🎓Unlock everything free for 14 days
- ✓Full step-by-step solutions
- ✓Concept-first explanations
- ✓Methods, shortcuts & mistakes
- ✓PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.