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C4.2.7—Energy sources

Energy sources explains how energy flow, matter cycling, trophic transfer or carbon movement is represented, measured or limited within ecosystems.

Syllabus
First assessment 2025
Objective
C4.2.7
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1A
Typical marks1

Common command terms

  • Identify

Scoring notes

Common mistake
Reversing food-web arrows instead of showing energy transfer direction.

Recent exam appearances

May 2025Paper1A ["SL"] · TZ128[ 1 ]C4.2.7—Energy sources
May 2025Paper1A ["SL"] · TZ226[ 1 ]C4.2.7—Energy sources
Practice this objective

Coverage 2025–2025 · Updated 16 Jul 2026

Ecosystems can use light or chemical energy

Ecosystems can use light or chemical energy.

Photoautotrophs capture light; chemoautotrophs obtain energy by oxidising inorganic molecules. Both fix carbon, but their energy inputs and habitats differ.

classify energy source; name oxidation or light capture; link to carbon fixation.

Nitrifying bacteria can use energy from ammonia oxidation in a dark soil while plants use photons at the surface.

Chemosynthesis is not ‘photosynthesis without sunlight’; the electron donor and pathway differ.

Energy sources

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: Energy sources 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

[Maximum number: 1]

Which organisms use oxidation of simple inorganic substances as an energy source?

A

Photoautotrophs

B

Heterotrophs

C

Chemoautotrophs

D

Saprotrophs

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 sources must be linked to the correct source, store, transfer or loss process.
  • The evidence for energy sources depends on distinguishing energy flow from matter cycling.
  • Energy sources is clearer when arrows, units and trophic positions match the biological process.
  • Data or diagrams for energy sources need interpretation as ecosystem transfer evidence.
ConceptIB Biology SL