C4.2.5—Supply to decomposers
Supply to decomposers explains how energy flow, matter cycling, trophic transfer or carbon movement is represented, measured or limited within ecosystems.
- Syllabus
- First assessment 2025
- Objective
- C4.2.5
- Level
- HL
Exam analysis
Chance of appearing2%of analysed past papers
Latest appearanceMay 2021
Most common paperPaper2
Typical marks1–2
Common command terms
- State
- Explain
- Describe
- Outline
- Suggest
Scoring notes
- Common mistake
- Reversing food-web arrows instead of showing energy transfer direction.
Recent exam appearances
May 2021Paper2 ["HL"] · TZ13(b)(ii)[ 2 ]C4.2.5—Supply to decomposers
November 2014Paper3 ["HL"] · TZ07(c)(ii)[ 1 ]C4.2.5—Supply to decomposers
Practice this objective
Coverage 2014–2021 · Updated 16 Jul 2026
Decomposers return matter from dead material
Decomposers obtain energy from carbon compounds in organic matter derived from faeces, dead parts and whole dead organisms.
Fungi and bacteria secrete enzymes onto detritus, absorb soluble digestion products and oxidize some carbon compounds in respiration. This transfers chemical energy to decomposer metabolism.
Faeces/dead tissue/dead organism → extracellular digestion → soluble carbon compounds absorbed → decomposer respiration and biomass → inorganic nutrients released.
A fungus digests dead leaf tissue externally, absorbs sugars and other products, uses some for respiration and growth, and releases mineral nutrients into soil.
Decomposers transform both matter and energy, but matter can return to nutrient cycles while respiratory energy ultimately leaves as heat.
Supply to decomposers
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through structured response, commonly using State / Explain / Describe.
Command terms
State / Explain / Describe / Outline / Suggest
What earns marks
Build the answer around this relationship: Supply to decomposers must be linked to the correct source, store, transfer or loss process.
Watch for
Reversing food-web arrows instead of showing energy transfer direction.
Representative question
Question 1
Detritus accumulates on coral reefs damaged by ocean acidification.
Suggest two possible impacts of an increase in detritus on the organisms in this 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
- Supply to decomposers must be linked to the correct source, store, transfer or loss process.
- The evidence for supply to decomposers depends on distinguishing energy flow from matter cycling.
- Supply to decomposers is clearer when arrows, units and trophic positions match the biological process.
- Data or diagrams for supply to decomposers need interpretation as ecosystem transfer evidence.