# Measurement of respiration rate

> CIE A-Level Biology · 9700 A2
> Source: https://www.owlsprep.com/study/cie-9700-u13-measurement-of-respiration-rate/

This sub-topic covers experimental methods for measuring aerobic respiration rate, calculation of results, error analysis, and exam expectations for CIE A-Level Biology practical and theory questions.

**Prerequisites:** [Aerobic respiration overview](https://www.owlsprep.com/study/cie-9700-u13-aerobic-respiration/); [Gas exchange in living organisms](https://www.owlsprep.com/study/cie-9700-u11-gas-exchange/)

## Learning objectives

- Describe common experimental methods to measure aerobic respiration rate
- Calculate respiration rate from experimental data
- Identify sources of error and controlled variables in respirometry experiments
- Explain the role of carbon dioxide absorbents in respirometry

## Simple Manometric Respirometry

**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.

## Oxygen Probe Respirometry (Aquatic Organisms)

**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. $$\text{Rate} = \frac{3.0}{25 \times 0.25} = 0.48 \text{ mg O}_2 \text{ g}^{-1} \text{hour}^{-1}$$

> **tip**
>
> Always equilibrate the oxygen probe to the water temperature before starting measurements. Oxygen solubility decreases as temperature increases, so uncalibrated temperature will lead to inaccurate results.

## Variables, Errors and Controls

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

**Check your understanding**

Test your understanding of key setup requirements:

1. 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

   *Why:* 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.

## Common pitfalls

- **Wrong:** Starting measurements immediately after setting up the apparatus
  - Why it fails: The apparatus has not equilibrated to the temperature of the water bath, so temperature-related gas volume changes will skew results
  - Correct: Leave the apparatus to equilibrate for 5-10 minutes with the tap open before starting timing and measurements
- **Wrong:** Reporting rate as distance moved per minute instead of volume
  - Why it fails: Capillary tubes have different diameters, so distance alone is not a standard measure of gas volume
  - Correct: Always calculate volume from the cross-sectional area of the capillary before calculating rate
- **Wrong:** Using wet mass of organisms when comparing respiration rates between groups
  - Why it fails: Water content of tissue does not respire, so comparisons between different samples are invalid
  - Correct: Use dry mass of tissue to standardise respiration rate calculations for comparisons
- **Wrong:** Not running a control with dead organisms
  - Why it fails: Small changes in atmospheric pressure or temperature can cause bubble movement unrelated to respiration
  - Correct: Run a control with killed organisms of the same total mass and subtract the background movement from your experimental results

## Cheatsheet

| Component | Function | Key Formula |
| --- | --- | --- |
| Simple respirometer | Measures O₂ uptake via gas volume change | $\text{Rate} = \frac{\pi r^2 \times \text{distance}}{\text{time}}$ |
| KOH / Soda lime | Absorbs CO₂ to isolate O₂ uptake | N/A |
| Oxygen probe | Measures dissolved O₂ change | $\text{Rate} = \frac{\Delta [O_2] \times \text{volume}}{\text{mass} \times \text{time}}$ |
| Control with dead organisms | Subtract non-respiration gas changes | $\text{Corrected rate} = \text{measured rate} - \text{control rate}$ |

## 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.

- [Photosynthesis](https://www.owlsprep.com/study/cie-9700-u14-overview/)
- [Photosynthetic pigments](https://www.owlsprep.com/study/cie-9700-u14-photosynthetic-pigments/)
- [Light-dependent reactions](https://www.owlsprep.com/study/cie-9700-u14-light-dependent-reactions/)

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