Measurement of respiration rate

CIE A-Level Biology· Unit 13: Energy and Respiration· 10 min read

1. Simple Manometric Respirometry★★☆☆☆⏱ 4 min

📘 Definition

Simple manometric respirometer

A closed apparatus that measures changes in gas volume to calculate the rate of oxygen consumption during aerobic respiration

Example:

Typically used for small organisms like germinating seeds or terrestrial invertebrates

In this setup, organisms are placed in a sealed chamber connected to a capillary tube containing a liquid bubble. A vial of potassium hydroxide (KOH) or soda lime is added to the chamber to absorb all carbon dioxide released during respiration. Any movement of the bubble towards the chamber is caused by oxygen consumption by the organisms.

📐 Worked Example

A student measures respiration in 10 germinating pea seeds. The bubble in the capillary tube moves 15 mm towards the chamber over 6 minutes. The capillary diameter is 1.0 mm. Calculate the rate of oxygen consumption in mm³ min⁻¹.

1
Calculate the radius and cross-sectional area of the capillary tube:
2
$r = \frac{d}{2} = 0.5 \text{ mm}, A = \pi r^2 = \pi \times 0.5^2 = 0.785 \text{ mm}^2$
3
Calculate total volume of oxygen consumed:
4
$\text{Volume} = \text{Area} \times \text{Distance} = 0.785 \times 15 = 11.775 \text{ mm}^3$
5
Calculate rate per minute:
6
$\text{Rate} = \frac{11.775}{6} = 1.96 \text{ mm}^3 \text{min}^{-1}$

Exam tip:

CIE examiners always require units for rate calculations; missing units will lose you marks.

2. Oxygen Probe Respirometry (Aquatic Organisms)★★★☆☆⏱ 5 min

📘 Definition

Oxygen probe respirometry

A digital method that measures changes in dissolved oxygen concentration over time to calculate respiration rate in aquatic organisms or cell suspensions

Example:

Used for fish, algae, or aquatic invertebrates

This method directly measures dissolved oxygen concentration in water, so no absorbent for carbon dioxide is required. Rate is calculated by measuring the change in oxygen concentration over a known time period, scaled to the mass of organism and time.

📐 Worked Example

A researcher measures respiration in 25 g of freshwater shrimp in 1 dm³ of water. Initial oxygen concentration is 7.8 mg dm⁻³. After 15 minutes, concentration is 4.8 mg dm⁻³. Calculate respiration rate in mg O₂ g⁻¹ hour⁻¹.

1
Calculate total change in oxygen:
2
$\Delta O_2 = 7.8 - 4.8 = 3.0 \text{ mg dm}^{-3} \times 1 \text{ dm}^3 = 3.0 \text{ mg}$
3
Convert time to hours: 15 minutes = 0.25 hours
4
Calculate rate per gram of tissue:
5
$\text{Rate} = \frac{3.0}{25 \times 0.25} = 0.48 \text{ mg O}_2 \text{ g}^{-1} \text{hour}^{-1}$

3. Variables, Errors and Controls★★★☆☆⏱ 3 min

CIE frequently asks to identify controlled variables and sources of error in respiration experiments. All experiments must control variables that could affect respiration rate or gas measurements.

Temperature: Maintain with a water bath, as temperature affects enzyme activity and gas solubility
Mass of organism: Use dry mass to compare between groups, as water does not respire
Volume of CO₂ absorbent: Use excess absorbent to ensure all CO₂ is absorbed
Atmospheric pressure: Seal the apparatus correctly to prevent pressure changes affecting results

✓ Quick check

Test your understanding of key setup requirements:

Which of the following is the correct function of KOH in a simple respirometer?
- Absorbs excess oxygen
- Absorbs carbon dioxide produced by respiration
- Maintains constant temperature
- Kills unwanted microorganisms
Reveal answer
Absorbs carbon dioxide produced by respiration —
If CO₂ is not absorbed, the volume of CO₂ produced will cancel the volume change from oxygen consumed, so no net movement of the bubble will be measured.

4. Common Pitfalls

Wrong move:

Starting measurements immediately after setting up the apparatus

Why:

The apparatus has not equilibrated to the temperature of the water bath, so temperature-related gas volume changes will skew results

Correct move:

Leave the apparatus to equilibrate for 5-10 minutes with the tap open before starting timing and measurements

Wrong move:

Reporting rate as distance moved per minute instead of volume

Why:

Capillary tubes have different diameters, so distance alone is not a standard measure of gas volume

Correct move:

Always calculate volume from the cross-sectional area of the capillary before calculating rate

Wrong move:

Using wet mass of organisms when comparing respiration rates between groups

Why:

Water content of tissue does not respire, so comparisons between different samples are invalid

Correct move:

Use dry mass of tissue to standardise respiration rate calculations for comparisons

Wrong move:

Not running a control with dead organisms

Why:

Small changes in atmospheric pressure or temperature can cause bubble movement unrelated to respiration

Correct move:

Run a control with killed organisms of the same total mass and subtract the background movement from your experimental results

5. Quick Reference Cheatsheet

Component	Function	Key Formula
Simple respirometer	Measures O₂ uptake via gas volume change	$Rate = \frac{π r ^{2} \times distance}{time}$
KOH / Soda lime	Absorbs CO₂ to isolate O₂ uptake	N/A
Oxygen probe	Measures dissolved O₂ change	$Rate = \frac{Δ [ O _{2} ] \times volume}{mass \times time}$
Control with dead organisms	Subtract non-respiration gas changes	$Corrected rate = measured rate - control rate$

6. Frequently Asked

Why do we use KOH in a simple respirometer?

Potassium hydroxide absorbs all carbon dioxide produced during aerobic respiration. Without this absorption, the volume of CO₂ produced would cancel out the volume change from oxygen consumed, so no net change would be measured. KOH ensures any volume change is only due to oxygen uptake.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
Calculation of respiration rate
2021 · 5
Planning respirometry experiment
2023 · 3
Practical respiration measurement

What's Next

Mastering measurement of respiration rate is a core practical skill for CIE A-Level Biology, frequently assessed in Paper 3 (practical) and Paper 5 (planning/data analysis). This sub-topic builds on your understanding of aerobic respiration pathways and gas exchange, and forms the foundation for calculating respiratory quotient and investigating how different factors like temperature, substrate, and oxygen concentration affect respiration rate. The experimental design skills you learn here will also help you answer other practical planning questions across the A-Level syllabus.