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C4.2.4—Food chains and food webs

Food chains and food webs explains how energy flow, matter cycling or carbon transfer is represented, measured or limited within ecosystems.

Syllabus
First assessment 2025
Objective
C4.2.4
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2019
Most common paperPaper2
Typical marks2

Common command terms

  • Describe
  • Identify
  • Label
  • Draw
  • Explain
  • Compare
  • Contrast
  • Outline
  • Evaluate

Scoring notes

Common mistake
Saying energy is recycled instead of flowing through and leaving ecosystems as heat.

Recent exam appearances

November 2019Paper3 ["HL"] · TZ014(b)[ 2 ]C4.2.4—Food chains and food webs
May 2019Paper2 ["HL"] · TZ28(a)[ 4 ]C4.2.4—Food chains and food webs
May 2018Paper3 ["HL"] · TZ115(c)[ 2 ]C4.2.4—Food chains and food webs
November 2013Paper2 ["HL"] · TZ03(a)(i)[ 2 ]C4.2.4—Food chains and food webs
November 2012Paper1 ["HL"] · TZ016[ 1 ]C4.2.4—Food chains and food webs
Practice this objective

Coverage 2012–2019 · Updated 16 Jul 2026

Food webs connect many feeding pathways

Food webs connect many feeding pathways.

A food chain shows one route of feeding; a food web joins overlapping routes. The web reveals alternative prey, predators and indirect effects when one population changes.

trace arrow direction; identify trophic position; follow direct and indirect paths.

Removing a predator can increase one prey species and indirectly reduce the plants eaten by that prey.

Arrows show transfer of food/energy from resource to consumer, not who is ‘stronger’.

Food chains and food webs

Assessment in practice

1–4 marks
How it is assessed

This objective is assessed through structured response, commonly using Describe / Identify / Label.

Command terms

Describe / Identify / Label / Draw / Explain / Compare / Contrast / Outline / Evaluate

What earns marks

Build the answer around this relationship: Food chains and food webs must be linked to the correct source, store, transfer or loss process.

Watch for

Saying energy is recycled instead of flowing through and leaving ecosystems as heat.

Representative question

Question 1

[Maximum number: 6]

Describe what is meant by a food chain and a food web.

Energy and Matter

  • Ecosystems are open systems: energy flows through them and leaves as heat, while matter is recycled and may enter or leave.
  • Photoautotrophs capture light; chemoautotrophs oxidize inorganic substances. Both build biomass from inorganic carbon. Heterotrophs obtain organic carbon from other organisms.
  • Food-web arrows show energy and biomass transfer. Energy decreases between trophic levels through respiration, heat, egestion, excretion and uneaten material, limiting chain length.
  • Gross primary production minus producer respiration gives net primary production; secondary production is heterotroph biomass gain.
  • Decomposers obtain energy from detritus and return inorganic nutrients to producers.
  • Carbon-cycle diagrams distinguish stores and fluxes. Photosynthesis removes CO2; respiration, decomposition and combustion release it.
  • A sink absorbs more carbon than it releases; a source does the reverse. The Keeling Curve shows a long-term atmospheric CO2 rise with seasonal oscillation.

Concept essentials

  • Food chains and food webs must be linked to the correct source, store, transfer or loss process.
  • The evidence for food chains and food webs depends on distinguishing energy flow from matter cycling.
  • Food chains and food webs is clearer when arrows, units and trophic positions match the biological process.
  • Data or diagrams for food chains and food webs need interpretation as ecosystem transfer evidence.