C4.2.17—Carbon cycle diagrams
Carbon cycle diagrams explains how energy flow, matter cycling, trophic transfer or carbon movement is represented, measured or limited within ecosystems.
- Syllabus
- First assessment 2025
- Objective
- C4.2.17
- Level
- SL
Exam analysis
Chance of appearing15%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–6
Common command terms
- Draw
- Identify
- Explain
- Label
- Discuss
- Outline
- Describe
Scoring notes
- Common mistake
- Reversing food-web arrows instead of showing energy transfer direction.
Recent exam appearances
November 2025Paper1A ["SL"] · TZ128[ 1 ]C4.2.17—Carbon cycle diagrams
November 2025Paper1A ["SL"] · TZ326[ 1 ]C4.2.17—Carbon cycle diagrams
May 2025Paper2 ["SL"] · TZ17(b)[ 4 ]C4.2.17—Carbon cycle diagrams
November 2024Paper1 ["SL"] · TZ219[ 1 ]C4.2.17—Carbon cycle diagrams
May 2023Paper2 ["SL"] · TZ17(b)[ 5 ]C4.2.17—Carbon cycle diagrams
Practice this objective
Coverage 2011–2025 · Updated 16 Jul 2026
Carbon-cycle diagrams track stores and transfers
Carbon-cycle diagrams track stores and transfers.
A carbon cycle represents carbon stores such as atmosphere, biomass, soil and ocean, and transfers such as photosynthesis, respiration, decomposition and combustion.
label store versus flow; direction; rate or stock; time interval.
A forest store can gain carbon through growth while releasing carbon through respiration and decomposition at the same time.
Arrows are transfers, not extra carbon; a diagram should not be read as one-way flow.
Carbon cycle diagrams
Assessment in practice
1–5 marksHow it is assessed
This objective is assessed through experimental design, commonly using Draw / Identify / Explain.
Command terms
Draw / Identify / Explain / Label / Discuss / Outline / Describe
What earns marks
Build the answer around this relationship: Carbon cycle diagrams 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
Living organisms at every trophic level are part of the carbon cycle. Draw a labelled diagram of the carbon cycle to show the processes involved.
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
- Carbon cycle diagrams must be linked to the correct source, store, transfer or loss process.
- The evidence for carbon cycle diagrams depends on distinguishing energy flow from matter cycling.
- Carbon cycle diagrams is clearer when arrows, units and trophic positions match the biological process.
- Data or diagrams for carbon cycle diagrams need interpretation as ecosystem transfer evidence.