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C4.2.15—Primary production

Primary production explains how energy flow, matter cycling, trophic transfer or carbon movement is represented, measured or limited within ecosystems.

Syllabus
First assessment 2025
Objective
C4.2.15
Level
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper2
Typical marks1–3

Common command terms

  • State
  • Identify
  • Compare
  • Calculate
  • Distinguish
  • Annotate
  • Describe
  • Explain
  • Evaluate

Scoring notes

Common mistake
Confusing gross production with net production after respiration losses.

Recent exam appearances

May 2023Paper2 ["HL"] · TZ11(g)[ 3 ]C4.2.15—Primary production
May 2023Paper2 ["HL"] · TZ11(e)[ 1 ]C4.2.15—Primary production
May 2023Paper2 ["HL"] · TZ11(d)(ii)[ 2 ]C4.2.15—Primary production
November 2021Paper2 ["HL"] · TZ01(e)[ 3 ]C4.2.15—Primary production
May 2014Paper3 ["HL"] · TZ211(b)[ 1 ]C4.2.15—Primary production
Practice this objective

Coverage 2012–2023 · Updated 16 Jul 2026

Primary production is new producer biomass

Primary production is the accumulation of carbon compounds in autotroph biomass through growth and reproduction.

Report it as mass of carbon per unit area per unit time, commonly g C m⁻² yr⁻¹. A rate requires a defined area and interval rather than only a standing biomass measurement.

Biomes differ in capacity to accumulate biomass because light, temperature, water, nutrients and growing-season length constrain autotroph growth.

Compare ecosystem primary-production values only after checking that carbon mass, area and time units match; the larger rate represents faster producer carbon accumulation.

Primary production is producer biomass accumulation, not total biomass already present. Biomass also accumulates when heterotrophs grow or reproduce, but that is secondary production.

Primary production

Assessment in practice

1–3 marks
How it is assessed

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

Command terms

State / Identify / Compare / Calculate / Distinguish / Annotate / Describe / Explain / Evaluate

What earns marks

Build the answer around this relationship: Primary production must be linked to the correct source, store, transfer or loss process.

Watch for

Confusing gross production with net production after respiration losses.

Representative question

Question 1

[Maximum number: 3]

In each forest, there are two or three trial plots per CO2 treatment. The bar chart shows the allocation of carbon from net primary production to different parts of the trees in these trial plots.

Evaluate the evidence from the bar chart that increases in carbon dioxide cause increases in carbon storage in young, developing forests.

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

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