Living organisms continuously convert stored energy from organic molecules into usable forms like adenosine triphosphate (ATP), a process known as cellular respiration. This complex chemical pathway involves breaking down organic substances, often sugars, to release the energy needed for all life functions, including movement, growth, and cellular maintenance. A respirometer is a scientific instrument specifically designed to accurately quantify the rate at which this energy conversion takes place. By measuring the changes in the gaseous environment surrounding a specimen, the device provides a direct, measurable output of its overall metabolic activity.
Defining the Measurement of Respiration
Respiration is measured based on the continuous exchange of gases within an organism. Aerobic respiration, the most widespread form of this process, requires the consumption of atmospheric oxygen ($\text{O}_2$) as the final electron acceptor. Simultaneously, the process generates carbon dioxide ($\text{CO}_2$) as a gaseous waste product, which is then expelled by the specimen.
The specific type of organic fuel being metabolized determines the overall respiratory quotient (RQ), which is the ratio of $\text{CO}_2$ produced to $\text{O}_2$ consumed. When a specimen metabolizes carbohydrates, the RQ is approximately one, meaning the volume of $\text{O}_2$ consumed closely equals the volume of $\text{CO}_2$ produced. However, when fats or proteins are metabolized, the ratio is generally less than one, resulting in a net decrease in the total volume of gas within a closed system.
Respirometers exploit this change in gas volume to calculate the rate of cellular activity. The rate of $\text{O}_2$ uptake is considered the most reliable proxy for the metabolic rate, as it directly reflects the pace of energy production.
The Mechanism of Pressure Change
To isolate oxygen consumption, the respirometer uses a sealed system containing the organism and a chemical reagent. A standard setup places the specimen inside a chamber along with a powerful carbon dioxide absorbent, such as potassium hydroxide ($\text{KOH}$) or soda lime. This absorbent chemically reacts with the $\text{CO}_2$ produced by the specimen, removing it from the gas phase.
Since the absorbent removes the produced $\text{CO}_2$, and $\text{O}_2$ is continuously consumed, a net reduction occurs in the gas volume inside the sealed chamber. This volume reduction causes a corresponding drop in the system’s internal pressure. If the $\text{CO}_2$ were not absorbed, the pressure change would be negligible or zero when metabolizing carbohydrates, making the measurement difficult.
The respirometer quantifies this pressure drop using a manometer or a calibrated capillary tube. The sealed chamber connects to the manometer, which contains a fluid, often a colored oil or water. As the internal pressure drops due to gas consumption, the higher atmospheric pressure outside pushes the fluid column toward the chamber.
The distance the fluid moves along the calibrated tube over time provides a precise measurement of the volume of $\text{O}_2$ consumed. This displacement is converted to a rate of respiration, providing a quantitative assessment of the specimen’s metabolic activity. To ensure accuracy, a control tube, called a thermobarometer, is run in parallel without a specimen to account for fluctuations caused by changes in ambient temperature or atmospheric pressure.
Primary Uses in Biological Study
Respirometry data is used across biological research and education. Researchers frequently use the technique to determine the basal metabolic rates of small organisms, such as insects, amphibians, or germinating seeds. This information is foundational for understanding the energy requirements necessary for survival, growth, and reproduction in different species.
Respirometers are deployed to study the impact of extrinsic factors on living systems. By observing changes in oxygen consumption rates, scientists quantify how metabolism shifts in response to changes in ambient temperature, humidity, or oxygen availability. These experiments are fundamental to ecological physiology, revealing how organisms adapt to environmental stress.
The instruments are used in pharmacological and toxicological studies to assess the effects of specific chemicals or pollutants on cellular function. A measurable change in the respiratory rate indicates the degree of metabolic disruption caused by a test substance, providing insights into its biological mechanism of action. Respirometry is also a standard teaching tool in biology laboratories for demonstrating concepts of cellular function and metabolism.