C4.2.9—Energy release
Energy release explains how energy flow, matter cycling, trophic transfer or carbon movement is represented, measured or limited within ecosystems.
- Syllabus
- First assessment 2025
- Objective
- C4.2.9
- Level
- SL
Exam analysis
Chance of appearing1%of analysed past papers
Latest appearanceMay 2021
Most common paperPaper2
Typical marks1
Common command terms
- Describe
- Identify
- State
Scoring notes
- Common mistake
- Giving a generic ecosystem answer without the specific transfer or process for energy release.
Recent exam appearances
May 2021Paper2 ["SL"] · TZ24(a)[ 1 ]C4.2.9—Energy release
Practice this objective
Coverage 2021–2021 · Updated 16 Jul 2026
Respiration releases usable energy from organic molecules
Both autotrophs and heterotrophs release energy by oxidizing carbon compounds in cell respiration.
Electrons and hydrogen are transferred from respiratory substrates through enzyme-controlled pathways; part of the released chemical energy is captured in ATP and the rest becomes heat.
Autotroph: makes carbon compounds, then can respire them. Heterotroph: obtains carbon compounds from other organisms, then can respire them. Both use ATP for cellular work.
A plant leaf respires glucose that the plant synthesized, while an animal respires carbon compounds derived from food; in each, oxidation supports ATP production.
Autotrophs do not only photosynthesize—they also respire. Respiration is cellular oxidation, not gas movement in breathing; photoheterotroph details are not required.
Energy release
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Describe / Identify / State.
Command terms
Describe / Identify / State
What earns marks
Build the answer around this relationship: Energy release must be linked to the correct source, store, transfer or loss process.
Watch for
Giving a generic ecosystem answer without the specific transfer or process for energy release.
Representative question
Question 1
State one process that results in the loss of carbon dioxide from a marine organism such as a crustacean or a jellyfish.
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
- Energy release must be linked to the correct source, store, transfer or loss process.
- The evidence for energy release depends on distinguishing energy flow from matter cycling.
- Energy release is clearer when arrows, units and trophic positions match the biological process.
- Data or diagrams for energy release need interpretation as ecosystem transfer evidence.