Chemical Education Today
Letters Ant Velocity Hypotheses Rigatuso et al. published a fascinating application of Arrhenius’ Law to ant travel velocity (1). They measured ant velocity at various temperatures, and their Arrhenius plot showed a break at 16 °C, with Ea = 40 kJ/mol above 16 °C, and Ea = 85 kJ/mol below 16 °C. They speculate that 16 °C may represent the freezing point for the “trail pheromone” that these ants secrete and follow. Below 16 °C solid pheromone would have a lower vapor pressure and be harder to follow, leading to slower motion. There are several problems with this suggestion. First, as pointed out by Rigatuso et al., pheromone secretions are mixtures of several components; hence they are unlikely to have a single, distinct freezing point. Second, pheromones are generally highly volatile liquids at room temperature; it is unlikely that they would freeze at 16 °C, and if they did, it is unlikely that they could be secreted from the Dufour’s gland. I present here an alternative hypothesis to explain the 16 °C transition temperature in the ant velocity Arrhenius plot. First, consider that locomotion is not a simple chemical reaction. Instead, it is the end result of a series of distinct phases. A partial list of these phases includes: (a) pheromone binding to and activation of receptor, (b) signal transduction and initiation of sensory nerve impulse, (c) processing of sensory nerve impulse leading to locomotion “decision”, (d) initiation and transmission of motor nerve impulse, and finally, (e) muscle contraction causing locomotion. Each of these five phases could in turn be broken down into several distinct steps. For example, muscle contraction involves, among other things, (i) ATP hydrolysis on the myo-
sin head, (ii) a power stroke driven by phosphate release, and (iii) myosin–actin crossbridge cycling driven by ATP–ADP exchange. Also, (iv) the ATP that drives muscle contraction must be supplied by the mitochondrion, which carries out oxidative phosphorylation. The activation energy for any complex reaction will be determined by its rate-determining step. A break point or transition temperature in an Arrhenius plot is often interpreted as a change in rate-determining step. Although in principle any of the five phases (a–e) enumerated above could be rate determining for locomotion, due to the high speed of signal transduction and nerve transmission, it seems likely that phase e, muscle contraction, is the slowest phase. Of the four steps in muscle contraction (i–iv), it is interesting to note that mitochondria have a well-known transition temperature at 15–18 °C. Below this transition temperature, mitochondrial membrane characteristics change dramatically, causing most mitochondrial metabolic reactions (including ATP synthesis) to occur much more slowly. My guess is that this influence of temperature on mitochondrial ATP synthesis causes the activation energy for ant locomotion to nearly double as temperature falls below 16 °C. Literature Cited 1. Rigatuso, R.; Bertoluzzo, S. M. R.; Quattrin, F. E.; Bertoluzzo, M. G. J. Chem. Educ. 2000, 77, 183–185. Todd P. Silverstein Chemistry Department, Willamette University Salem, OR 97301
[email protected] JChemEd.chem.wisc.edu • Vol. 80 No. 11 November 2003 • Journal of Chemical Education
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