Academic journal article The Science Teacher

Measuring Metabolism: Examining the Effects of Temperature on the Metabolic Rates of Beetles

Academic journal article The Science Teacher

Measuring Metabolism: Examining the Effects of Temperature on the Metabolic Rates of Beetles

Article excerpt

Several decades ago, physiologists devised a simple way to indirectly measure metabolic rate--the rate at which an organism burns calories--in small animals such as hamsters. These experiments involved a metabolic chamber made by connecting a sealed glass chamber to tubing and a 50 ml syringe. The bottom of the chamber was covered with soda lime to absorb expired carbon dioxide (C[O.sub.2]), allowing for measurement of oxygen ([O.sub.2]) consumption through the displacement of the syringe plunger (Lyman 1948).

Because of the known relationship between [O.sub.2] consumption, C[O.sub.2] production, and heat production during cellular respiration, physiologists can calculate metabolic rate based on [O.sub.2] consumption. Scientists have found measurements of C[O.sub.2] production to be a reliable source for calculating metabolic rate, particularly when the measured volumes are small (Rogowitz and Chappell 2000).

Cellular respiration refers to the process of converting the chemical energy of organic molecules into a form that organisms can immediately use. Glucose may be oxidized completely if enough [O.sub.2] is available. The process is summarized by the following equation:

[O.sub.6][H.sub.12][O.sub.6] + 6[O.sub.2](g) [right arrow] 6 [H.sub.2]O + 6 C[O.sub.2](g) + heat energy

Many organisms, including plants and animals, oxidize glucose for energy. The rate of this conversion can be measured by examining [O.sub.2] consumption. Given that the average measure of metabolic rate in animals is 4.8 kilocalories (kcal) per liter of [O.sub.2] (Schmidt-Neilsen 1997), scientists can use this value as a conversion factor for the rate of [O.sub.2] consumption (L or ml/hour) to metabolic rate (kcal/hour).

These indirect measures of metabolic rate have been replicated in high school biology classes, but one limitation of this experiment is that the chamber is prone to leaks that affect data collection and the final calculations. We modified the experiment by using readily available electronic sensors to measure beetles' C[O.sub.2] production in different temperature conditions. (Teachers can also replicate this experiment with [O.sub.2] sensors or a combination of [O.sub.2] and C[O.sub.2] sensors.) Students investigated the effects of temperature on metabolic rate.

This lesson was taught at a school whose students are primarily economically disadvantaged and can be modified for those with special needs. The investigation aligns with the Next Generation Science Standards (NGSS Lead States 2013; see box, p. 59).

The beetle investigation

During the initial 45-minute class period, each group of three students was given a C[O.sub.2] sensor, data collection device, closed chamber, one to three live beetles (see sidebar), tongs, ice bath, warm water bath, and data sheets. (Alternatively, this experiment can be performed using the classic apparatus previously described.) Safety note: Students must wear goggles during the investigation and use tongs when preparing and handling the 32[degrees]C warm water bath. Remind students to handle the beetles with care, keeping the insects away from clothing so as not to injure them, and to wash hands before and after touching the beetles.

Each group was assigned a specific temperature condition (cold/4[degrees]C; room temperature/22[degrees]C; warm water bath/32[degrees]C). First, students placed the beetle(s) in the chamber and then inserted the gas sensor (Figure 1). The beetles were allowed to acclimate for about 5-10 minutes and then began a 10-minute run, during which students measured C[O.sub.2] levels in the chamber. Students also monitored the beetles' activity and recorded the behaviors at different time intervals. Temperature probes were used to monitor the water bath temperature to ensure it did not rise above 35[degrees]C, which kept the beetles safe.

The rate of C[O.sub.2] production can be determined manually or on the data collection device from the slope of a linear regression (C[O. …

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