5.2.9 Investigating the Rate of Respiration

Using a Respirometer

Respirometers are devices designed to measure the rate of respiration by tracking changes in gas volume. The setup typically includes:

• A sealed container to hold the organism or tissue under investigation.
• A manometer to detect changes in pressure caused by gas exchange.
• A substance to absorb $CO₂$, such as sodium hydroxide or soda lime. (This ensures any pressure change is due only to oxygen consumption.)

Steps to Investigate Oxygen Uptake

1. Assemble the respirometer:

• Place the organism in the sealed container.
• Ensure the respirometer is airtight to prevent gas leaks.

2. Set up a control:

• Use a similar respirometer setup without a living organism to control for environmental pressure changes.

3. Absorb $CO₂$:

• Place a $CO₂-$ absorbing material in the container.
• This ensures only oxygen consumption affects gas volume.

4. Measure oxygen uptake:

• As the organism respires, it consumes oxygen, reducing the gas volume in the chamber.
• The resulting decrease in pressure causes the manometer fluid to move, and the distance moved is proportional to the oxygen consumed.

5. Record results:

• Measure the movement of the fluid in the manometer over a set time interval.
• Use the equation for the volume of a cylinder to calculate the oxygen uptake: Volume = πr²h (where r is the radius of the capillary tube, and h is the distance moved by the fluid).

Using the Data

• Calculate the rate of respiration by dividing the volume of oxygen consumed by the time taken.

$$\text{Rate of respiration = Volume of oxygen / Time}$$

• Repeat the experiment under varying conditions to determine how temperature, substrate type, or organism type affect the rate of respiration.

Key Variables

• Independent Variable: The factor being tested, e.g., temperature or substrate.
• Dependent Variable: The volume of oxygen consumed or carbon dioxide produced.
• Control Variables: Ensure all other factors are kept constant, such as the same mass of organism, the same environmental pressure, or the same apparatus.

Considerations and Sources of Error

• Ensure the respirometer is airtight.
• Accurately calibrate the manometer.
• Use a sufficient mass or number of organisms to ensure measurable results.
• Avoid excessive heat or low oxygen levels that could harm the organisms.

Alternative Methods

1. Measuring $CO₂$ Production:

• If $CO₂$ is not absorbed, it can be measured instead of oxygen uptake.
• Use a data logger with a gas sensor to record $CO₂$ levels over time.

2. Investigating Yeast Respiration:

• Use a simple setup where yeast respires anaerobically, producing $CO₂$ that can be collected in an inverted measuring cylinder.