Cambridge O Level Biology · Syllabus 5090 · Relationships of Organisms with One Another and with the Environment
Energy Flow
What is Energy Flow?
The one-way transfer of energy through an ecosystem: light energy is absorbed by producers and transferred into chemical energy by photosynthesis, passes to consumers by feeding, and is transferred to the environment as thermal energy at every step, mainly through respiration. Because energy leaves the ecosystem at each transfer and is never returned to the producers, energy flows through an ecosystem rather than cycling within it.
This definition is part of the Relationships of Organisms with One Another and with the Environment chapter in Cambridge O Level Biology.
Energy Flow in context
Ecosystems run on one input and two very different accounting systems. Light energy from the Sun enters through photosynthesis, passes along food chains by feeding, and is transferred to the environment as thermal energy at every step — so energy flows through an ecosystem and is never recycled. The atoms carried with it are different: carbon and nitrogen move from air to producer to consumer to decomposer and back to the air, so matter cycles and is used again. Everything else in this topic — short food chains, ecological pyramids, the carbon and nitrogen cycles, eutrophication, conservation — is a consequence of those two sentences.
Common mistakes with Energy Flow
- “Energy is recycled by decomposers.” Why it is wrong: decomposers respire like any organism, so the energy they release is transferred to the environment as thermal energy. Nothing returns it to the producers. Exam-safe: decomposers recycle matter, releasing inorganic nutrients. Energy flows through the ecosystem and is not recycled.
Questions students ask about Energy Flow
Why does energy flow but matter cycle?
Energy enters as light, is transferred along the chain by feeding, and is transferred to the environment as thermal energy at every level — and nothing returns it to the producers, so it makes a single one-way pass. Matter is different: the same carbon and nitrogen atoms are returned to the environment by respiration, excretion, death and decomposition, and producers take them up again. Matter is reused; energy is not.