Defence mechanisms

In the animal kingdom, survival often depends on how individual species evade or defend against their predators. Interestingly, many animals employed a defence mechanisms to protect themselves from their predators. For example, the Malaysian ants contract their abdomens when threatened, causing the glands in their body to explode, releasing and shooting a sticky secretion at their predators. A studies by Jones et al in 2002 revealed that the ‘toxic glue’ released from the ant contain a mixture of chemicals including phenols, hydrocarbons and terpenoids (Jones et al., 2004).

exploding-ant

source :www.youtube.com

Another example is the skunk, when threatened, the attractive little animal gave off a smelly musk containing a mixture of sulfur-containing compounds call thiols. The smell of the skunk’s spray is difficult to get rid of, but a solution of a mixture of hydrogen peroxide and baking soda might do the trick. Hydrogen peroxide is an oxidising agent which can oxidise the thiols to sulfonic acids.

bombardier-beetles

source: http://www.typesofinsects.com

A deeper look into the defence mechanisms of animals revealed interesting chemistry which can be useful for teaching chemistry concepts. The bombardier beetle is another well-known insect which defend itself by releasing a hot spray of chemicals at their predators when threatened. The chemistry in this mechanism not only involve reactions between organic compounds, but also energy changes during the reaction.  The glands of the beetle contain a reservoir chamber where it stored an aqueous reactant solution of approximately 25% hydrogen peroxide and 10% p-hydroquinones, along with an approximately 10% alkanes as a nonreactive second liquid phase. When threatened, these chemicals flowed from the reservoir chamber into the reaction chamber where the chemical reaction is catalysed by an enzyme (a solution of peroxidase and catalase) reacts to produce p-benzoquinones and liberates oxygen gas, water vapour, and heat, producing a hot, noxious spray at the enemy.(Arndt, Moore, Lee, & Ortiz, 2015)

This is a useful for teachers to show how energy cycles and energy changes in chemical energetics is relevant to nature.

energy cycle

Using Hess Law and constructing an energy cycle, the heat energy released for reaction (a) can be calculated to be approximately -204 kJ mol-1, large enough to bring the mixture to its boiling point.

Reference:

Arndt, E. M., Moore, W., Lee, W., & Ortiz, C. (2015). Mechanistic origins of bombardier beetle (Brachinini) explosion-induced defensive spray pulsation. Science, 348(6234), 563–567. http://doi.org/10.1126/science.1261166

Jones, T. H., Clark, D. A., Edwards, A. A., Davidson, D. W., Spande, T. F., & Snelling, R. R. (2004). The chemistry of exploding ants, Camponotus spp. (cylindricus complex). Journal of Chemical Ecology, 30(8), 1479–1492. http://doi.org/10.1023/B:JOEC.0000042063.01424.28

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