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Learn5090Paper 2 · TheoryRelationships of organisms with one another and with the environmentFood chains and energy flow
Relationships of organisms with one another and with the environment · Subtopic 2 of 5

Food Chains and Energy Flow

Energy enters ecosystems as sunlight captured by producers (plants) in photosynthesis. It passes along food chains to primary consumers (herbivores), secondary consumers and tertiary consumers (carnivores). Each feeding level is a trophic level. Many chains link into a food web. At each transfer only about 10% of the energy passes on; the rest is lost in respiration, movement, heat, and uneaten or undigested parts. So food chains are short, and pyramids of energy and biomass narrow upwards. Decomposers release energy from dead material.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Construct and interpret food chains and webs.
  • Describe pyramids of numbers, biomass and energy.
  • Explain energy losses and calculate efficiency.
Pause and think

Why are there fewer lions than zebras, and fewer zebras than blades of grass?

Section 2 of 14

Core idea

Start hereFollowing the energy

The energy in a hawk's muscles came from a mouse, which got it from seeds, which captured it from sunlight. At each step, most of it was lost.

The big idea

Energy flows from producers through consumers. About 90% is lost at each step, mainly through respiration, so food chains are short.

1. Energy losses

Where energy goes at each level

Only about 10% passes on.

Worked exampleEfficiency calculation

Producers: 87,000 kJ. Primary consumers: 7000 kJ. Secondary consumers: 600 kJ.

Goal
Efficiency of each transfer.
Strategy
Next ÷ previous × 100.
  1. 700087000×100=8.0%\frac{7000}{87000}\times100=8.0\%
    Producer to primary.
  2. 6007000×100=8.6%\frac{600}{7000}\times100=8.6\%
    Primary to secondary.
AnswerAbout 8% at each step.
✓ CheckClose to the typical 10% ✓.
Key takeawayPyramids Numbers can be inverted; biomass usually pyramid-shaped; energy always pyramid-shaped.
Pause and think

Why are top predators like tigers often endangered?

Summary

  • Producers capture sunlight.
  • ~10% transferred.
  • Pyramids of energy narrow upwards.
Key words
Trophic level
Position in a food chain.
Biomass
Mass of living material.
Food web
Interconnected food chains.
HistoryLindeman's lake. In 1942 Raymond Lindeman measured energy flow through a Minnesota lake and showed that only a small fraction passes from one level to the next, founding ecosystem ecology.
Fun factBlue whale shortcut. Blue whales, the largest animals ever, eat tiny krill, a very short food chain from plankton, which is why such huge animals can exist.
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