Rate of Respiration as a Function of the Size of Organisms

Leah Engleberg, Ellen Griffith, Timmy Duggan, Debra Wilson

CU Boulder, Fall 2001

Our group was curious to know if the size of an organism affects the rate of respiration. We assumed that larger organisms would require more oxygen, produce more CO2, and would thus have a higher respiration rate. To test these possibilities, we derived a hypotheses: if larger organisms are more metabolically active, then they will produce more CO2. This would result in a higher respiration rate in large organisms vs. smaller organisms of the same species.

First, we gathered and weighed small and large pillbugs (5 each) and small and large crickets (5 each). We used a carbon dioxide probe to measure the change in CO2. We included an experimental control by running the experiment without any organisms in the chamber in order to see if any change in CO2 was due to factors other than crickets or pill bugs. We then tested the respiration rate of each set of organisms. The experiment was repeated to ensure that the results were accurate and valid.

We found that large crickets respired at an average rate of 146.0 ppm/min/gram, whereas smaller crickets respired at 316.3 ppm/min/g. In pill bugs, the rate was 90.6 ppm/min/g in large organisms and 413.3 ppm/min/g in small organisms.

Our results caused us to reject out original hypothesis because the respiration rate was actually much higher in smaller organisms than in larger organisms. To further support our conclusions, t-tests were run that proved there was indeed a significant difference between the respiration rates of large and small organisms. These results showed us that in fact larger organisms metabolize slower than small organisms, possibly because small organisms are still growing at a fast rate and would therefore produce excess carbon dioxide.