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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
HL

Ecosystems can use light or chemical energy

Photoautotrophs use light as their external energy source; chemoautotrophs use energy released by oxidation reactions.

Type Energy source Required example Carbon source
Photoautotroph Absorbed light Green plant or alga Inorganic carbon such as CO₂
Chemoautotroph Oxidation of inorganic substances Iron-oxidizing bacterium Inorganic carbon such as CO₂

Oxidation transfers electrons and releases energy that chemoautotrophs can couple to ATP production and carbon fixation even where light is unavailable.

Chemosynthesis is not photosynthesis without light: the external energy source is an oxidation reaction. Both groups are autotrophs because they synthesize carbon compounds from inorganic carbon.

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.