Respirometer Rate Calculator
How to Calculate Rate of Oxygen Consumption in A-Level Biology
Calculating the rate of oxygen consumption is a fundamental skill in A-Level Biology, particularly when analyzing data from a respirometer experiment. This calculation helps quantify the rate of aerobic respiration in organisms such as woodlice, maggots, or germinating seeds.
The Respirometer Experiment Logic
In a standard simple respirometer setup, an organism is placed in a sealed chamber with a substance (like Soda Lime or Potassium Hydroxide) that absorbs carbon dioxide. As the organism respires, it consumes oxygen ($\text{O}_2$) and releases carbon dioxide ($\text{CO}_2$).
Because the $\text{CO}_2$ is absorbed by the chemical, the total volume of gas in the chamber decreases. This reduction in pressure causes the colored fluid in the attached capillary tube to move towards the chamber. The volume of oxygen consumed is equal to the volume of the cylinder of liquid that moves.
Key Formulas
To process the raw data from the experiment, you need to perform two main steps:
1. Calculate the Volume of Oxygen ($\text{mm}^3$)
The capillary tube is a cylinder. To find the volume of oxygen consumed, you calculate the volume of the liquid displaced in the tube.
- π (Pi): Approx 3.14159
- r (Radius): Half of the diameter of the capillary tube (Diameter / 2).
- h (Height): The distance moved by the fluid (meniscus) along the scale.
2. Calculate the Rate ($\text{mm}^3 \text{ min}^{-1}$)
Once you have the total volume, divide it by the time taken for that movement to occur.
3. Calculate Mass-Specific Rate ($\text{mm}^3 \text{ min}^{-1} \text{ g}^{-1}$)
To compare the respiration rates of different organisms (e.g., a mouse vs. a maggot), you must standardize the results by mass. This tells you how efficient the respiration is per gram of tissue.
Worked Example
Imagine an experiment using woodlice:
- Distance moved ($h$): 30 mm
- Diameter of tube: 1 mm (Radius $r$ = 0.5 mm)
- Time ($t$): 5 minutes
- Mass ($m$): 2 grams
Step 1 (Volume): $\pi \times 0.5^2 \times 30 = 23.56 \text{ mm}^3$
Step 2 (Rate): $23.56 / 5 = 4.71 \text{ mm}^3 \text{ min}^{-1}$
Step 3 (Mass Specific): $4.71 / 2 = 2.36 \text{ mm}^3 \text{ min}^{-1} \text{ g}^{-1}$
Common A-Level Exam Mistakes
- Forgetting to halve the diameter: Often the question gives diameter, but the formula requires radius ($r$).
- Unit Inconsistency: Ensure distance and diameter are in the same units (usually mm) before calculating volume.
- Ignoring Control Tubes: In real experiments, remember to subtract any movement observed in a control tube (thermobarometer) caused by temperature or pressure changes.