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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
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceMay 2021
Most common paperPaper1
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 ["HL"] · TZ24(a)[ 1 ]C4.2.9—Energy release
May 2015Paper1 ["HL"] · TZ214[ 1 ]C4.2.9—Energy release
May 2013Paper1 ["HL"] · TZ229[ 1 ]C4.2.9—Energy release
Practice this objective

Coverage 2013–2021 · 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 marks
How 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

[Maximum number: 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.