Almost every living thing on Earth runs on energy that left the Sun a few minutes ago. Plants capture some of that light, store it as chemical energy, and pass it along the food chain. At every step some of it escapes as heat, and none of it ever comes back.
The Sun is the principal source of energy input to biological systems. Producers capture light energy in photosynthesis and store it as chemical energy in organic molecules such as glucose, starch and proteins. Chemical energy passes from organism to organism by feeding, but at every stage some is transferred to the environment, mainly as heat from respiration. Energy flows through an ecosystem in one direction; it is not recycled.
Press Release sunlight and follow the glowing packets of energy leaving the Sun. Most are not captured: they bounce off the grass and fly away as white packets. The green packets are the energy the grass traps as chemical energy. Some green packets go on to the rabbit and a few reach the fox.
Energy is never "used up" or "made". Say it is transferred: from light to chemical energy, then to the environment as heat. And energy is not recycled: nutrients cycle, energy flows through.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the energy flow simulation, what happens to most of the sunlight that reaches the grass?
In the simulation, red packets rise from each organism. What do they represent?
In what form is energy passed from the grass to the rabbit?
The bar chart in the sim shows that each level has about what fraction of the energy of the level below?
Carbon and nitrogen atoms go round and round between living things and their surroundings. Energy behaves differently: it enters as light, and leaves as heat that living things cannot capture again. An ecosystem needs a constant new supply of sunlight.
| Stage | Form of energy | What happens |
|---|---|---|
| Sun to producer | light → chemical | photosynthesis traps a small fraction of the light that falls on the leaves |
| Producer to consumer | chemical | feeding passes on the energy stored in the tissue that is eaten and digested |
| Within every organism | chemical → heat | respiration releases energy for movement, growth and keeping warm; it ends as heat |
| Dead material and waste | chemical | decomposers feed on it; their respiration releases the rest as heat |
Heat lost to the environment cannot be used by plants for photosynthesis. Only light energy can be captured, which is why the flow is one-way.
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What is the principal source of energy input to biological systems?
Which sequence shows the energy changes as energy flows through a food chain?
Why is energy described as flowing through an ecosystem rather than cycling?
Energy stored in the dead leaves of a tree is eventually:
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