Wednesday, March 30, 2011
Sense of Smell
Shubin describes the sense of smell as a "lock and key". Are there any other "lock and key" relationships that we have observed earlier in the year? What similarities and differences exist between smell and this relationship? Describe the evolution of smell in fish, amphibians, reptiles, and mammals.
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Earlier in the year, we learned about the "lok and key" fit of enzyme-substrate complexes, where the enzymes will only react with substrates that fit their active sites. The book uses the example of sucrase only bonding sucrose (Campbell, 153).
ReplyDeleteSimilarly, our sense of smell also fits this model. Shubin describes nerve cell receptor as the key, which opens the lock which is the odor molecule. We are able to smell so many different odors because there is are so many genes for smell; 3 percent as stated on page 144.
A recent finding report that in birds, their scent receptors evolved within the bird species, and did not evolve from a single common ancestor, suggesting that that genes are unique to the animal, not the result of variation in from an early ancestor. (http://www.biologynews.net/archives/2010/04/06/darwins_finch_and_the_evolution_of_smell.html)
Anna Leng (annaissbananas@yahoo.com)
Smell is the sensation received from nasal nerve cell receptors that are stimulated by odor molecules inhaled by the nasal cavity. The surrounding environment contains billions of odor particles, which are carried through the nostrils of the human. The purpose of the nose is to “smell, moisten, and filter the air you breathe” (Oracle ThinkQuest Education Foundation). The mucous lining traps the odor molecules to bind to certain receptors on nerve cells (Shubin, 141). This is where the lock and key relationship occurs; the receptor being the key, and the odor molecule being the lock Shubin, 141). This lock and key relationship in determining smells have keys or receptors, in this case, that are specific to certain odor molecules. An odor can have many molecules whose individual smells, together, make up the odor (Shubin, 141). The nerve sends signals to the brain that deciphers these signals, finally, as smells.
ReplyDeleteEarlier in the year, we had discussed the lock and key model in relation to enzymes; the lock and key is one substrate-enzyme binding method. The substrate, compared to the key, binds to the active site (key hole) of the enzymes or lock. With this theory, the shape of the active site of the enzyme is specific meaning that its shape will only accept a certain substrate, and incorrectly-shaped substrates will not fit and bind into the active site to create a reaction (Campbell, 153). Other relationships that we had discussed in regards to a lock and key model included hormonal reactions. In this case, the hormone or neurotransmitter acts as the “key”, which binds to specific receptor molecules or the lock in the cell membrane of the target cells. By activating the receptor of the target cell, a response is directly stimulated by the hormone or neurotransmitter (Campbell, 979).
There are no similarities and differences between smell and the lock and key relationship because smell is simply a large concept or process that includes a lock and key model to function.
Just as these fish must move water over their gills by swimming or moving certain muscles, they must also move this water over their nares. The sense of smell of fish most likely evolved as their swimming ability and muscle evolved, which would include the advancement of the structure of their fins, tails, and gills over time. Amphibians, however, detect smell by sensing chemical changes in the air that are detected form their nostrils but also from collection of odor molecules on their eyes and skin. The smell organ of frogs, in particular, located in the mouth is called the Jacobsen organ that is used to detect food. (“Smell”, The Frog). In reptiles, snakes and lizards, in particular, smell with their forked tongues. The paired sensory organ in the roof of these animals’ mouths analyze the particles received by the tongue as it is flicked, so often, out of their mouths to test the air for nearby food, a threat, or a mate (“Lizards and Snakes”, Snakes Harmful and Harmless). The tongue in most reptiles have a dual role in sensing smell and taste. In fish and reptiles, the olfactory bulb is large in relation to their entire brain. The cerebrum of reptiles is larger than fish and was the first to have developed the neocortex. Their cerebrum had evolved later than that of fish because this “neocortex” or “new cortex” expanded the outer layer of the cerebrum to process more information including vision, taste, and touch while the cerebrum of fish is primarily dedicated to olfaction (“Comparative Anatomy: The Vertebrate Brain”, The Human Evolution Coloring Book). The evolution of smell in humans caused the nose to be “[lifted away] from the noxious ground environment as they adopted to a bipedal posture” (“The Filtering Apparatus of the Nasal Cavity”, PLoS Biology). The snout of the mammal was reduced in complexity as they evolved while maintaining the amount of odorized air reaching the olfactory epithelium (“The Filtering Apparatus of the Nasal Cavity”, PLos Biology).
Sonia Doshi (soniadoshi7@gmail.com)
One additional example of "lock and key" relationships not covered by Anna or Sonia is that of antigen receptors with epitopes in the immune system. Both B cells and T cells have receptors consisting of a set of heavy chains and a set of light chains, which together form a unique receptor for a specific molecular arrangement of an antigen's epitope, the small, accessible portion of the antigen on its membrane (Campbell 937).
ReplyDeleteThis is similar to the "lock and key" functionality of olfactory receptors in that each configuration of receptor matches exactly one molecule. In the immune system, this target molecule is, hopefully, the epitope of an antigen to be identified and destroyed; in the sense of smell, this target molecule is a specific "smell," a molecular arrangement that is perceived as such (http://student.ccbcmd.edu/courses/bio141/lecguide/unit4/innate/innate.html).
The specificity of the immune system's antigen receptors on B cells is a selective advantage because, if an organism's immune system has a B cell with a receptor for the epitope of a harmful pathogen, that organism's B cell will be able to undergo clonal selection to proliferate, secrete antibodies specific for that antigen, and more effectively destroy it. Smell's specificity is similarly an advantage in that organisms with a greater array of smell receptors have a more accurate sense of smell and can consequently detect odors more effectively. This is useful for scavenging, a behavior many mammals, like dogs, use, as well as other behaviors, like detecting predators or prey (MSU Department of Biology).
In this way, more developed senses of smell have developed with varying priorities for each type of organism - for humans, for instance, the value of smell is less than it is for dogs because our different ecological roles (niches) place different selective pressures on different organisms.
- Vincent Fiorentini
(vincent@panatechcomputer.com)