Energy flow, food chains and food webs — Cambridge IGCSE Biology
How energy flows from the Sun through producers, consumers and decomposers to the environment, and how food chains and webs show feeding links.
Revision notes
3 short notes, in the order of the specification. Each one 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.
A food chain shows the transfer of energy from one organism to the next, beginning with a producer, and a food web is a network of interconnected food chains. Each arrow points from the organism that is eaten to the one that eats it. Overharvesting and introducing foreign species change food webs and affect other populations.
A producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. A consumer gets its energy by feeding on other organisms: herbivores eat plants and carnivores eat other animals. Consumers are primary, secondary, tertiary or quaternary. Decomposers, such as bacteria and fungi, get energy from dead or waste organic material.
Exam questions
7 exam-style questions (19 marks), each with its mark scheme.
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13 cards: flip them, mark what you knew, and practise the rest.
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Quick check questions
State the principal source of energy input to biological systems.
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The Sun.
What does a food chain show?
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The transfer of energy from one organism to the next, beginning with a producer.
What is a food web?
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A network of interconnected food chains.
Define a consumer.
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An organism that gets its energy by feeding on other organisms.
Define a decomposer.
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An organism that gets its energy from dead or waste organic material.
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.
How does overfishing affect a food web?
Overfishing reduces the population of the harvested fish species. Predators that feed on that fish have less food, so their populations may fall. Species that the fish eats may increase in number, because fewer of them are eaten. These effects can spread indirectly through the food web to other populations.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026