Energy Flow
ஆற்றல் ஓட்டம்
Energy does not cycle; it flows. If you think energy returns to the sun from a tertiary consumer, you have already lost four marks. Remember: Ecosystems are open systems for energy, but closed for nutrients.
The idea
Energy flow is strictly unidirectional, starting from solar radiation. Producers (autotrophs) capture less than 50% of PAR. Energy transfer follows the 10% Law of Lindeman, where only 10% of energy is stored as biomass at each successive trophic level. The remainder is lost as metabolic heat, adhering to the laws of thermodynamics.
Why it matters
Energy is required to maintain biological order and counteract the universal tendency toward entropy (disorder).
NEET lens
NEET focuses on the 10% law calculations and the unidirectional nature of energy. Expect questions comparing energy flow with nutrient cycling (which is cyclic). Do not confuse 'standing crop' with 'standing state'. Memorize that PAR is the limiting factor for primary productivity.
Clinical significance
In human physiology, this is analogous to the metabolic cost of ATP synthesis. When mitochondrial efficiency drops, energy is lost as heat rather than work, leading to metabolic disorders. An ecosystem failing to maintain this flow collapses, just as a cell dies without a constant supply of chemical energy.
Introduction
Except for deep-sea hydrothermal ecosystems, the sun is the only source of energy for all ecosystems on Earth. Energy flow through an ecosystem is unidirectional.
Core concept
Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR). Plants and photosynthetic bacteria (autotrophs) fix the sun's radiant energy to make food. Plants capture only 2-10 per cent of the PAR, and this small amount of energy sustains the entire living world. Energy flows from the sun to producers and then to consumers — unidirectional. Ecosystems are not exempt from the second law of thermodynamics: they need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness. All animals depend on plants (directly or indirectly) for their food needs — hence they are called consumers or heterotrophs.
Go deeper
Energy passes along food chains, losing a large fraction as heat at each step, which is why food chains rarely exceed four or five levels.
From Class 10
Sunlight is captured by plants and passed on to animals that eat them — but it never comes back to the sun. Energy flows one way.