Food chains, food webs and energy Cambridge IGCSE Co-ordinated Sciences (9–1) revision
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In plain words
All the energy in your dinner came, in the end, from the Sun. Plants trap it; animals get it by eating plants or by eating other animals. A food chain shows who eats whom, and at every step most of the energy is lost, which is why there are far fewer lions than zebras.
Three things to know
- The Sun is the main source of energy for living things. Producers (plants) make their own food by photosynthesis. Consumers eat other organisms: primary consumers (herbivores) eat plants, secondary consumers eat them. Decomposers break down dead material.
- In a food chain the arrows show the direction in which energy flows: grass → rabbit → fox. A food web is several chains linked together. Each feeding level is a trophic level.
- Only about 10% of the energy passes from one trophic level to the next. The rest is lost as heat from respiration, in movement, and in waste and uneaten parts. So food chains are short, and pyramids of biomass get narrower towards the top.
Tips and tricks
- The arrow means "is eaten by": it points from the food to the feeder.
- Energy isn't "used up" between levels. Say where it goes: heat from respiration, movement, faeces, parts not eaten.
It lands in your notebook with its questions as flashcards.
Food chains, food webs and energy: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the main source of energy for a food chain?
Producers trap light energy in photosynthesis.
What is a producer?
Green plants and algae are producers.
What do the arrows in a food chain show?
From the organism eaten to the organism that eats it.
About how much energy passes from one trophic level to the next?
Most is lost, mainly as heat.
What do decomposers do?
They return nutrients to the soil.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 8 marks. Write your answers on paper, then check them.
Food chains, food webs and energy
Cambridge IGCSE Co-ordinated Sciences (9–1) 0973 · 8 marks · papermunch.org
Name ______________________________ Date ______________
In the food chain grass → grasshopper → frog → snake, name the producer, the primary consumer and a secondary consumer.[3]
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Producer: grass. Primary consumer: grasshopper. Secondary consumer: frog.
Explain why food chains rarely have more than four or five trophic levels.[3]
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Energy is lost at each level (as heat from respiration, in movement and in waste), so only a small fraction is passed on. After a few levels there is too little energy to support another.
The producers in a field contain 20 000 kJ of energy. About 10% passes to the primary consumers. Calculate the energy passed on.[2]
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2000 kJ. 20 000 × 0.10.
Answers: Food chains, food webs and energy
- 1. Producer: grass. Primary consumer: grasshopper. Secondary consumer: frog.
- 2. Energy is lost at each level (as heat from respiration, in movement and in waste), so only a small fraction is passed on. After a few levels there is too little energy to support another.
- 3. 2000 kJ. 20 000 × 0.10.