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C4.2.6—Autotrophs as self-feeders

Autotrophs as self-feeders explains how energy flow, matter cycling, trophic transfer or carbon movement is represented, measured or limited within ecosystems.

Syllabus
First assessment 2025
Objective
C4.2.6
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceMay 2017
Most common paperPaper1
Typical marks1

Common command terms

  • Identify
  • Distinguish
  • State
  • Define

Scoring notes

Common mistake
Confusing detritivores that ingest material with saprotrophs that digest externally.

Recent exam appearances

May 2017Paper3 ["HL"] · TZ215(b)[ 1 ]C4.2.6—Autotrophs as self-feeders
November 2016Paper1 ["HL"] · TZ014[ 1 ]C4.2.6—Autotrophs as self-feeders
Practice this objective

Coverage 2016–2017 · Updated 16 Jul 2026

Autotrophs build organic matter from inorganic carbon

Autotrophs use an external energy source to synthesize carbon compounds from simple inorganic substances.

External energy is required both to fix inorganic carbon such as CO₂ into organic intermediates and to drive anabolic reactions that assemble carbohydrates, lipids, proteins and nucleic acids.

External light or chemical energy → carbon fixation → small organic compounds → energy-requiring anabolic synthesis → autotroph biomass.

A plant uses light energy to fix CO₂ into carbon compounds and then combines carbon skeletons with mineral nutrients to build macromolecules for new tissue.

Autotroph refers to carbon-compound synthesis, not independence from external energy, water or mineral elements.

Autotrophs as self-feeders

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Distinguish / State.

Command terms

Identify / Distinguish / State / Define

What earns marks

Build the answer around this relationship: Autotrophs as self-feeders must be linked to the correct source, store, transfer or loss process.

Watch for

Confusing detritivores that ingest material with saprotrophs that digest externally.

Representative question

Question 1

[Maximum number: 1]

Define the term autotroph.

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

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