Energy flow

Organisms and their environment · Energy flow, food chains and food webs · note 1 of 3

Spec 19.1.1, 19.1.2Download all Energy flow, food chains and food webs notes (PDF)Download all notes (PDF)4 pages

Energy flowSpec 19.1.1, 19.1.2

In short

The Sun is the principal source of energy input to biological systems. Plants and algae transfer light energy into chemical energy in organic nutrients by photosynthesis. This chemical energy passes to animals by feeding and to decomposers. Energy flows in one direction, not a cycle, and at every stage much of it is transferred to the environment as heat.

The Sun is the principal source of energy input to biological systems. Without light from the Sun there would be almost no energy entering living things.

Energy does not go round in a cycle. It flows through living organisms in one direction and is eventually transferred to the environment.

Light energy from the Sun is absorbed by green plants and algae. They transfer it to chemical energy in organic nutrients (such as glucose) by photosynthesis.
Chemical energy passes to animals when they eat plants, and to other animals when they eat those animals.
Chemical energy passes to decomposers when they feed on dead organisms and waste.
At every stage, organisms transfer energy by respiration. Much of it is eventually transferred to the environment as heat.

Energy flow: light energy from the Sun passes to producers (plants and algae); chemical energy passes to consumers and, in dead organisms and waste, to decomposers; heat energy is transferred to the environment from producers, consumers and decomposers. — Energy flows in one direction through organisms and is not recycled: it ends up as heat in the environment.

Exam tip:

Name the forms of energy: light energy from the Sun, chemical energy in organisms, and heat lost to the environment. Do not say that energy is 'used up' or 'destroyed'.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

Where does the energy in a food chain come from?

The energy in a food chain comes from the Sun, the principal source of energy input to biological systems. Producers such as green plants and algae absorb light energy and transfer it to chemical energy in organic nutrients by photosynthesis. This chemical energy then passes to consumers when they eat, and to decomposers.

Which way do the arrows go in a food chain?

The arrows in a food chain point from the organism that is eaten to the organism that eats it. They show the direction of energy transfer. So grass → rabbit means the rabbit eats the grass, and energy passes from the grass to the rabbit. Every food chain begins with a producer.

What is the difference between a producer and a consumer?

A producer makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. A consumer gets its energy by feeding on other organisms. Primary consumers are herbivores that eat producers, and secondary, tertiary and quaternary consumers are carnivores. Decomposers are different again: they feed on dead or waste organic material.

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