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C4.2.13—Heat loss

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

Syllabus
First assessment 2025
Objective
C4.2.13
Level
SL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2017
Most common paperPaper1
Typical marks1

Common command terms

  • Identify

Scoring notes

Common mistake
Reversing food-web arrows instead of showing energy transfer direction.

Recent exam appearances

May 2017Paper1 ["SL"] · TZ217[ 1 ]C4.2.13—Heat loss
Practice this objective

Coverage 2017–2017 · Updated 16 Jul 2026

Heat loss limits energy available to the next level

Both autotrophs and heterotrophs release heat when chemical energy is converted during cell respiration and when ATP is used for cell work.

Energy transfers are not 100% efficient. Some substrate energy becomes heat while ATP is produced, and further heat is released when ATP hydrolysis drives synthesis, transport or movement.

Carbon-compound oxidation → ATP plus heat; ATP use → cell work plus heat; heat dissipates to the environment and is unavailable as chemical energy to the next trophic level.

A plant and a mammal both respire and lose heat during ATP production; both also release heat as ATP powers active transport or biosynthesis.

Heat loss is energy transfer, not loss of carbon mass. Ecosystems require continuing external energy input because dissipated heat is not recycled into chemical energy by organisms.

Heat loss

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: Heat loss 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

[Maximum number: 1]

Plankton are major producers in marine ecosystems. Only a small proportion of the energy harvested by plankton is passed to the primary consumers. Which process in phytoplankton results in the largest loss of energy that could otherwise be utilized by consumers?

A

Reproduction

B

Homeostasis

C

Excretion

D

Respiration

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

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