Respirometers and Practical Investigations

Respirometry is the measurement of an organism's rate of respiration. Since aerobic respiration consumes oxygen and produces carbon dioxide, the rate of respiration can be estimated by measuring oxygen uptake or CO₂ production. A respirometer is the apparatus used for this purpose. Understanding how to use, interpret, and evaluate respirometer experiments is an important practical skill at A-Level.

Key Definition: A respirometer is a device used to measure the rate of respiration of an organism by detecting changes in gas volume (usually O₂ consumption).

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Principles of Respirometry

Why Measure O₂ Uptake?

• In aerobic respiration, oxygen is consumed as the terminal electron acceptor in the electron transport chain.
• Measuring the rate of O₂ consumption provides a reliable estimate of the rate of aerobic respiration.

The Problem of CO₂ Production

• Respiration also produces CO₂, which would increase the gas volume and mask the decrease in volume caused by O₂ consumption.
• To solve this, a chemical CO₂ absorbent is placed inside the respirometer — typically:
  • Soda lime (NaOH/Ca(OH)₂ mixture), or
  • Potassium hydroxide (KOH) solution.
• The absorbent removes CO₂ from the air inside the respirometer as it is produced, so any change in gas volume is due solely to O₂ consumption.

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Simple Respirometer Design

A typical simple respirometer consists of:

1. A sealed glass tube or flask containing the living organisms (e.g., germinating seeds, woodlice, maggots, or a small invertebrate).
2. A layer of CO₂ absorbent (soda lime or KOH solution on cotton wool) placed near the organisms but not in direct contact.
3. A capillary tube (manometer tube) connected to the sealed chamber, containing a coloured fluid (e.g., oil or coloured water) that forms a meniscus or bubble. As O₂ is consumed, the gas volume decreases, and the fluid moves along the tube towards the organisms.
4. A scale alongside the capillary tube to measure the distance moved by the fluid over a set time period.
5. A syringe (connected to the apparatus) that allows the fluid to be reset to its starting position between readings.

How It Works

1. The organisms respire aerobically, consuming O₂ and producing CO₂.
2. The CO₂ is absorbed by the soda lime (or KOH).
3. With CO₂ removed and O₂ consumed, the total gas volume inside the sealed chamber decreases.
4. This decrease causes the coloured fluid in the capillary tube to move towards the organisms.
5. The rate of movement of the fluid (distance/time) is proportional to the rate of O₂ consumption (and therefore the rate of respiration).

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Control Experiment

A control tube is set up identically to the experimental tube but with dead organisms (e.g., boiled seeds) or glass beads of equivalent volume instead of living organisms. This control:

• Accounts for any changes in gas volume due to temperature or pressure fluctuations (not caused by respiration).
• Ensures that any movement of the fluid in the experimental tube is due solely to the respiration of the living organisms.
• The difference in fluid movement between the experimental and control tubes gives the true rate of O₂ consumption.

Exam Tip: Always describe the control in respirometer experiments. Without it, you cannot be confident that changes in gas volume are due to respiration rather than environmental factors.

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Calculating Rate of Respiration

From Fluid Movement

• Measure the distance moved by the fluid in the capillary tube over a known time interval.

• If the internal diameter (and hence cross-sectional area) of the capillary tube is known, the volume of O₂ consumed can be calculated:

  Volume = π × r² × distance moved

  where r = radius of the capillary tube.

• The rate of O₂ consumption = volume of O₂ consumed ÷ time.

Units

• Rate may be expressed as:
  • mm³ min⁻¹ (volume per unit time).
  • mm³ min⁻¹ g⁻¹ (volume per unit time per unit mass — to allow fair comparison between organisms of different sizes).

Exam Tip: When comparing respiration rates between different organisms, always express the rate per unit mass to account for differences in body size.

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Calculating Respiratory Quotient (RQ)

As covered in the previous lesson, the RQ is: