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C1.2.6—Variables affecting rate

Respiration rate changes with metabolic demand, temperature, substrate, body size and oxygen availability, and can be measured experimentally, supporting usable cellular work and measurable biological activity.

Syllabus
First assessment 2025
Objective
C1.2.6
Level
SL

Exam analysis

Chance of appearing7%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper3
Typical marks1–2

Common command terms

  • Suggest
  • Explain
  • State
  • Describe
  • Identify
  • Outline
  • Calculate

Recent exam appearances

November 2025Paper1B ["SL"] · TZ33(d)[ 2 ]C1.2.6—Variables affecting rate
November 2025Paper1B ["SL"] · TZ33(c)[ 1 ]C1.2.6—Variables affecting rate
November 2025Paper1B ["SL"] · TZ33(b)[ 1 ]C1.2.6—Variables affecting rate
November 2025Paper1B ["SL"] · TZ33(a)[ 1 ]C1.2.6—Variables affecting rate
November 2024Paper3 ["SL"] · TZ21(e)[ 2 ]C1.2.6—Variables affecting rate
Practice this objective

Coverage 2011–2025 · Updated 15 Jul 2026

Respiration Rate Depends on Limiting Conditions

Cell-respiration rate can be measured as substrate use or product formation per unit time, with results normalized when organisms differ in mass or number.

In a respirometer, soda lime absorbs carbon dioxide, so a pressure or gas-volume decrease reflects oxygen uptake. Keep temperature constant in a water bath and use a control without respiring material to correct for non-biological pressure changes.

Respirationrate=changeinoxygenvolume÷time.Mass−specificrate=changeinoxygenvolume÷(time×organismmass),withunitssuchascm3O2g−1min−1.Respiration rate = change in oxygen volume ÷ time. Mass-specific rate = change in oxygen volume ÷ (time × organism mass), with units such as cm³ O₂ g⁻¹ min⁻¹.

Use the calibrated capillary displacement to find oxygen-volume change, subtract the control change, divide by elapsed time, then divide by sample mass if samples are being compared.

A respirometer measures oxygen uptake only when carbon dioxide is appropriately absorbed and the system is sealed. Temperature, pressure, mass, activity and acclimation time must be controlled.

Variables affecting rate

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Suggest / Explain / State.

Command terms

Suggest / Explain / State / Describe / Identify / Outline / Calculate

What earns marks

Build the answer around this relationship: Respiration rate can be measured from oxygen uptake or carbon dioxide production over time.

Representative question

Question 1

[Maximum number: 3]

Describe how the apparatus measures the oxygen consumption of the mouse.

SL Transfer: Explain Core Respiration

ATP is the immediate energy carrier; hydrolysis powers cell work and phosphorylation reloads ATP. Cell respiration transfers energy from carbon compounds into ATP in controlled steps. Aerobic respiration in humans uses oxygen and mitochondria for high ATP yield, while anaerobic respiration in cytoplasm produces lactate and low ATP. Rate evidence comes from oxygen uptake or carbon dioxide production per unit time. Examples include membrane pumps, macromolecule synthesis, and chromosome movement. Rate depends on metabolic demand, organism size, oxygen, substrate, temperature, and pH.

  • Use ATP as the link between respiration and life processes such as active transport, biosynthesis, movement, and homeostasis.
  • Compare aerobic and anaerobic respiration by oxygen use, location, ATP yield, and product in humans.
  • For rate questions, name the variable, measurement per unit time, and controlled variables.

Concept essentials

  • Respiration rate can be measured from oxygen uptake or carbon dioxide production over time.
  • Soda lime absorbs carbon dioxide so volume change reflects oxygen consumption.
  • Temperature must be controlled because gas volume and respiration rate both depend on it.
  • Higher metabolic demand usually increases oxygen use and ATP production.