John Park (wisejsm@yahoo.com)
Monday, April 4, 2011
Smelling in different environments
In Chapter 8, Neil Shubin starts to talk about the evolution of the nose and how the nose is used for smelling scents. However, there is no specific information about the difference of smelling except that the ones on ground and ones in water have different types of receptors. Go into more depth than Your Inner Fish and talk about difference in receptors and how they have to be different, with different physics of air and water for example.
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ReplyDeleteAs humans, we have the ability to differentiate between hundreds of scents, to our olfactory receptors. When animals on land smell scents, they are either smelling gases in the air or volatile liquids that are evaporating. The olfactory receptors as what enables organisms, such as humans, detect smells. Our olfactory receptors are located in our nasal cavity. The receptor cells can be found in the olfactory epithelium. These receptors are bipolar, and contain nerve fibers that converge in the olfactory bulb. We smell when impulses pass from the olfactory bulbs to the brain, where they are interpreted into scents. This mechanism has qualities similar to the transcription of a eukaryotic gene. Our olfactory neurons contain many receptor genes, and like eukaryotic genes, there is an enhancer that turns these genes on or off when a promoter is present, thus enabling us to smell scents. Similarly, in eukaryotic cells genes, when the enhancer is activated by activators, RNA synthesis proceeds, which would be analogous to “scent synthesis” (Campbell 359-60). The large variety of scents that Shubin mentions is made possible because of the thousands of genes that code for those types of scents. If an organism has more of these genes turned on, then they have a better sense of smell, as we see in dogs.
ReplyDeleteThe difference in the odor receptors of aquatic and terrestrial animals lies in the type of odor receptors they possess; terrestrial animals have mainly Class II odor receptors, while aquatic organisms use class I odor receptors. Class II odor receptors detect water-soluble scents more easily, while the odor receptors of terrestrial animals are better suited for smelling volatile or gaseous scents. This is an example of the theme structure and function. Since Class II odor receptors are bipolar, it makes sense that they are more suited for non-water based scents, thus fitting the conditions that terrestrial animals are often under. The class II odor receptors probably evolved from class I odor receptors, after organisms made the move from water to land, enabling them to smell in their new environment. Being able to smell scents allows animals to not only find food, but also identify their spouses and offspring, which is why they are important and help animals survive and reproduce. Land animals would be at a significant evolutionary disadvantage if they lost the ability to smell once they moved from water to land.
Posessing these olfactory receptor genes also differs from species to species. Many marine cetaceans have lost most of their olfactory receptor genes, while other organsims such as sea lions and sea turtles still possess their olfactory receptor genes. This occurred because the genes helped the sea lions to identify prey and other organisms, thus enabling them to survive, while in marine cetaceans, being able to smell other organisms did not aid their survival a great deal, which is why we see the absence of the gene. (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2390674/)
But regardless of the environment (land or water) the organism lives in, the types of molecules that make up the scents are similar. The sixteen chemicals that cause scent sensations are hydrogen, carbon, silicon, nitrogen, phosphorus, arsenic, antimony, bismuth, oxygen, sulfur, selenium, tellurium, fluorine, chlorine, bromine, and iodine.
Sources:
http://www.unmc.edu/physiology/Mann/mann10.html
Institute of Molecular evolutionary genetics and department of Biology at Pennsylvania State University
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2390674/
Anna Leng (annaissbananas@gmail.com)