C4.2.1—Ecosystems as open systems
Ecosystems as open systems explains how energy flow, matter cycling or carbon transfer is represented, measured or limited within ecosystems.
- Syllabus
- First assessment 2025
- Objective
- C4.2.1
- Level
- SL
Exam analysis
Chance of appearing7%of analysed past papers
Latest appearanceMay 2024
Most common paperPaper1
Typical marks1–3
Common command terms
- Describe
- Identify
- Distinguish
- Outline
- Compare
- Contrast
- State
- Explain
Scoring notes
- Common mistake
- Saying energy is recycled instead of flowing through and leaving ecosystems as heat.
Recent exam appearances
May 2024Paper3 ["SL"] · TZ13(d)[ 3 ]C4.2.1—Ecosystems as open systems
May 2024Paper1 ["SL"] · TZ217[ 1 ]C4.2.1—Ecosystems as open systems
May 2024Paper1 ["SL"] · TZ116[ 1 ]C4.2.1—Ecosystems as open systems
November 2022Paper1 ["SL"] · TZ018[ 1 ]C4.2.1—Ecosystems as open systems
November 2019Paper2 ["SL"] · TZ05(c)[ 7 ]C4.2.1—Ecosystems as open systems
Practice this objective
Coverage 2010–2024 · Updated 16 Jul 2026
An ecosystem is open to matter and energy
An ecosystem is an open system because both energy and matter can cross its boundary.
| System | Energy across boundary | Matter across boundary |
|---|---|---|
| Open ecosystem | Enters, often as light; exits, largely as heat | Enters and exits, while also cycling internally |
| Closed system | Can enter and leave | Does not enter or leave |
A forest receives sunlight, rainfall and atmospheric gases, exports heat and dissolved materials, and cycles carbon and nutrients among organisms, soil and air.
Matter can be recycled because atoms remain available in different compounds; useful energy is not recycled in the same way and ultimately dissipates as heat.
Ecosystems as open systems
Assessment in practice
1–3 marksHow it is assessed
This objective is assessed through structured response, commonly using Describe / Identify / Distinguish.
Command terms
Describe / Identify / Distinguish / Outline / Compare / Contrast / State / Explain
What earns marks
Build the answer around this relationship: Ecosystems as open systems must be linked to the correct source, store, transfer or loss process.
Watch for
Saying energy is recycled instead of flowing through and leaving ecosystems as heat.
Representative question
Question 1
Describe the movement of energy and nutrients in an 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
- Ecosystems as open systems must be linked to the correct source, store, transfer or loss process.
- The evidence for ecosystems as open systems depends on distinguishing energy flow from matter cycling.
- Ecosystems as open systems is clearer when arrows, units and trophic positions match the biological process.
- Data or diagrams for ecosystems as open systems need interpretation as ecosystem transfer evidence.