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C4.2.22—Recycling of all chemical elements

Recycling of all chemical elements explains how energy flow, matter cycling or carbon transfer is represented, measured or limited within ecosystems.

Syllabus
First assessment 2025
Objective
C4.2.22
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2020
Most common paperPaper3
Typical marks3–6

Common command terms

  • Identify
  • Describe
  • Outline
  • Explain
  • Suggest

Scoring notes

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

Recent exam appearances

November 2020Paper3 ["HL"] · TZ019[ 6 ]C4.2.22—Recycling of chemical elements
November 2019Paper2 ["HL"] · TZ08(c)[ 7 ]C4.2.22—Recycling of all chemical elements
May 2018Paper2 ["HL"] · TZ27(c)[ 3 ]C4.2.22—Recycling of all chemical elements
May 2018Paper3 ["HL"] · TZ118[ 6 ]C4.2.22—Recycling of chemical elements
May 2016Paper3 ["HL"] · TZ014(b)[ 2 ]C4.2.22—Recycling of all chemical elements
Practice this objective

Coverage 2016–2020 · Updated 16 Jul 2026

Chemical elements cycle even when energy does not

Every chemical element required by living organisms is recycled through ecosystems; carbon is only one example.

Atoms move between organisms and abiotic stores through uptake, feeding, excretion and death. Decomposers digest dead organic matter and wastes, returning elements in inorganic forms that producers can use again.

Organic matter → decomposer action → inorganic compounds in soil, water or air → producer uptake → food-web transfer → waste/death → decomposition.

Carbon, nitrogen, phosphorus and other required elements can all pass through biomass and return to abiotic stores; the detailed nitrogen or other nutrient cycles are not required here.

Recycling does not mean immediate or unlimited availability. Elements may be lost across ecosystem boundaries or remain in slow stores, while energy follows a one-way dissipative flow.

Recycling of all chemical elements

Assessment in practice

2–7 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Describe / Outline.

Command terms

Identify / Describe / Outline / Explain / Suggest

What earns marks

Build the answer around this relationship: Recycling of all chemical elements 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: 7]

Explain how carbon is recycled in a terrestrial ecosystem.

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

  • Recycling of all chemical elements must be linked to the correct source, store, transfer or loss process.
  • The evidence for recycling of all chemical elements depends on distinguishing energy flow from matter cycling.
  • Recycling of all chemical elements is clearer when arrows, units and trophic positions match the biological process.
  • Data or diagrams for recycling of all chemical elements need interpretation as ecosystem transfer evidence.