C4.2.8—Heterotrophs
Heterotrophs obtain organic carbon compounds from other organisms or detritus, using digestion and respiration to build biomass and release usable energy.
- Syllabus
- First assessment 2025
- Objective
- C4.2.8
- Level
- HL
Exam analysis
Chance of appearing4%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1–5
Common command terms
- Identify
- Describe
- State
- Compare
- Contrast
Scoring notes
- Common mistake
- Reversing food-web arrows instead of showing energy transfer direction.
Recent exam appearances
May 2025Paper2 ["HL"] · TZ310(c)[ 5 ]C4.2.8—Heterotrophs
May 2023Paper2 ["HL"] · TZ24(a)(i)[ 1 ]C4.2.8—Heterotrophs
May 2012Paper1 ["HL"] · TZ215[ 1 ]C4.2.8—Heterotrophs
May 2012Paper1 ["HL"] · TZ115[ 1 ]C4.2.8—Heterotrophs
Practice this objective
Coverage 2012–2025 · Updated 16 Jul 2026
Heterotrophs obtain organic carbon from other organisms
Heterotrophs obtain carbon compounds from other organisms and use them to synthesize the carbon compounds they require.
Complex molecules such as proteins and nucleic acids are too large to assimilate directly. They are digested internally in animals or externally by organisms such as fungi, producing smaller molecules that can be absorbed.
Other organism/detritus → internal or external digestion → small molecules absorbed → assimilation into new, organism-specific proteins, nucleic acids and other carbon compounds.
An animal digests dietary protein to amino acids and then assembles those amino acids into its own enzymes; a fungus performs digestion outside its body before absorption.
Assimilation is not simply absorption: it is incorporation and reconstruction into the organism's own compounds. Heterotrophs still respire and recycle nutrients.
Heterotrophs exam focus
Assessment in practice
1 marksHow it is assessed
This objective is assessed through essay response, commonly using Identify / Describe / State.
Command terms
Identify / Describe / State / Compare / Contrast
What earns marks
Build the answer around this relationship: Heterotrophs 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
Compare and contrast how different types of heterotrophs obtain the energy that they need to produce ATP.
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
- Heterotrophs must be linked to the correct source, store, transfer or loss process.
- The evidence for heterotrophs depends on distinguishing energy flow from matter cycling.
- Heterotrophs is clearer when arrows, units and trophic positions match the biological process.
- Data or diagrams for heterotrophs need interpretation as ecosystem transfer evidence.