In ecology, energy flow is the one-way transfer of energy through the living organisms in an ecosystem. Energy flow usually starts with sunlight that is absorbed by plants.
A simple way to understand energy flow is to think of a food chain that starts with radiant energy from the sun. Plants absorb sunlight to make food, small animals eat the plants to gain energy, and larger animals eat the small ones. The energy moves in one direction through these levels, with some released as heat at every transfer. Eventually, decomposers like fungi and bacteria break down dead plants and animals, obtaining energy from the organic matter and releasing nutrients that can be reused by plants.