C4.2.3—Chemical energy flow
Chemical energy flow explains how energy flow, matter cycling, trophic transfer or carbon movement is represented, measured or limited within ecosystems.
- Syllabus
- First assessment 2025
- Objective
- C4.2.3
- Level
- SL
Exam analysis
Chance of appearing6%of analysed past papers
Latest appearanceMay 2024
Most common paperPaper2
Typical marks3–5
Common command terms
- Outline
- Explain
- Describe
- Identify
- Distinguish
Scoring notes
- Common mistake
- Skipping the conversion of light energy into chemical energy by producers.
Recent exam appearances
May 2024Paper2 ["SL"] · TZ25(b)[ 4 ]C4.2.3—Chemical energy flow
November 2022Paper2 ["SL"] · TZ06(a)[ 4 ]C4.2.3—Chemical energy flow
May 2022Paper2 ["SL"] · TZ27(a)[ 5 ]C4.2.3—Chemical energy flow
May 2022Paper1 ["SL"] · TZ218[ 1 ]C4.2.3—Chemical energy flow
May 2021Paper2 ["SL"] · TZ24(c)[ 3 ]C4.2.3—Chemical energy flow
Practice this objective
Coverage 2012–2024 · Updated 16 Jul 2026
Chemical energy moves through feeding
Chemical energy moves through feeding.
Organic molecules contain chemical potential energy. Feeding transfers that energy between organisms, while respiration releases some for work and dissipates much as heat.
source molecule; consumer; respiration; useful work versus heat.
Carbon compounds pass from plant to caterpillar to bird; at every step some energy supports metabolism and some leaves as heat.
Energy flow is not identical to carbon flow: carbon atoms may remain while usable energy declines.
Chemical energy flow
Assessment in practice
2–4 marksHow it is assessed
This objective is assessed through structured response, commonly using Outline / Explain / Describe.
Command terms
Outline / Explain / Describe / Identify / Distinguish
What earns marks
Build the answer around this relationship: Chemical energy flow must be linked to the correct source, store, transfer or loss process.
Watch for
Skipping the conversion of light energy into chemical energy by producers.
Representative question
Question 1
Describe how populations in communities rely on each other for supplies of energy.
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
- Chemical energy flow must be linked to the correct source, store, transfer or loss process.
- The evidence for chemical energy flow depends on distinguishing energy flow from matter cycling.
- Chemical energy flow is clearer when arrows, units and trophic positions match the biological process.
- Data or diagrams for chemical energy flow need interpretation as ecosystem transfer evidence.