Describe and carry out investigations using simple respirometers to determine the effect of temperature on the rate of respiration.
1. What is Respiration?
Respiration is the set of metabolic reactions that release energy from organic molecules (mainly glucose) and make it available for cellular processes. In plants and animals it occurs in two stages:
Glycolysis – occurs in the cytoplasm, breaks glucose into two molecules of pyruvate.
Cellular respiration – includes the Krebs cycle and oxidative phosphorylation in mitochondria.
The overall chemical equation for aerobic respiration is:
The rate at which organisms respire is an indicator of metabolic activity and is influenced by environmental factors. Understanding these influences helps to explain:
Plant growth and productivity.
Animal performance and adaptation.
Effects of climate change on ecosystems.
3. Factors Affecting the Rate of Respiration
Key factors include:
Temperature – enzymes have an optimum temperature.
Substrate availability – amount of glucose or other fuels.
pH – affects enzyme structure.
4. Simple Respirometer
A simple respirometer measures the volume of gas produced (or consumed) by a biological sample over time. The basic components are:
A sealed chamber (e.g., a graduated syringe or a glass tube).
A water‑filled trough to maintain constant pressure.
A one‑way valve to prevent back‑flow of gas.
A thermometer to monitor temperature.
Suggested diagram: Sketch of a simple water‑filled respirometer showing the chamber, valve, and water trough.
5. Experimental Procedure
Prepare the respirometer and ensure all joints are airtight.
Place a known mass of the biological sample (e.g., 5 g of peeled potato slices) into the chamber.
Fill the trough with water at the desired experimental temperature (e.g., 10 °C, 20 °C, 30 °C, 40 °C).
Close the valve and record the initial gas volume (or the position of the syringe plunger).
Start the timer and record the gas volume at regular intervals (e.g., every 2 min) for a total of 20 min.
Repeat the experiment for each temperature, performing at least three trials per temperature.
Maintain constant oxygen availability by ensuring the water trough is open to the atmosphere.
6. Data Collection
Record the volume of gas produced (in mL) at each time point. An example data table is shown below.
Temperature (°C)
Trial
Time (min)
Gas volume (mL)
Rate of respiration (mL min⁻¹)
10
1
0
0
-
5
2
10
4
15
6
20
8
20
1
0
0
-
5
5
10
10
15
15
20
20
30
1
0
0
-
5
12
10
24
15
36
20
48
40
1
0
0
-
5
18
10
36
15
54
20
72
7. Calculations
Rate of respiration is calculated as the slope of the gas‑volume vs. time graph or by the formula:
\$\text{Rate} = \frac{\Delta V}{\Delta t}\$
where \$\Delta V\$ is the change in gas volume (mL) and \$\Delta t\$ is the time interval (min).
8. Expected Results and Interpretation
Typical observations:
Rate increases with temperature up to an optimum (usually around 30–35 °C for plant tissue).
Beyond the optimum, the rate declines sharply due to enzyme denaturation.
Plotting temperature (°C) on the x‑axis against the average rate of respiration (mL min⁻¹) on the y‑axis yields a bell‑shaped curve, illustrating the temperature dependence of enzymatic activity.
9. Safety and Ethical Considerations
Handle hot water with care to avoid burns.
Dispose of biological material according to school laboratory waste guidelines.
Ensure the respirometer is not over‑pressurised; vent if necessary.
10. Extensions and Further Investigations
Investigate the effect of different substrates (e.g., glucose solution vs. starch).
Compare aerobic and anaerobic respiration by excluding oxygen.
Use a digital gas sensor to measure \$CO_2\$ production directly.
11. Summary
By using a simple respirometer, students can quantitatively explore how temperature influences the rate of respiration. The experiment reinforces key concepts such as enzyme kinetics, the role of \$O_2\$ in aerobic metabolism, and the practical skills of data collection, analysis, and scientific reporting.