Chapter 8 – Making Scents

1. Briefly explain how we perceive a smell.

Floating in the air are tiny molecules that we register as odor with our noses. We suck in these odor molecules in our nostrils as we breath or sniff. From there, the odor molecules go to an area behind our nose where they are trapped by the mucous lining of our nasal passages. Inside this lining is a patch of tissue containing millions of nerve cells, each with little projections into the mucous membrane. When the molecules in the air bind to the nerve cells, signals are sent to our brain. Our brain then records these signals as a smell.

2. Jawless fish have a very few number of odor genes while mammals have a much larger number.  Why does this make sense and how is it possible?

This occurrence makes sense because mammals are highly specialized smelling animals, which requires a large number of odor genes in order to recognize the odor. A larger number of odor genes is possible because the “extra” genes in mammals are all variations of a theme: they look like copies, albeit modified ones, of the genes in jawless fish. This means that our large number of odor genes arose by many rounds of duplication of the small number of genes.

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5 Comments

  1. Regarding number 2. Relate to the anatomy, biology and ecology of each group. What about those three things makes it more adaptive for mammals to have more odor sensing genes? What selection pressures might have contributed to this observation?

    Reply
  2. I guess smell plays a large part in the learning processes of many mammals. For example, some smells indicate danger or scare predators away (like skunks). Also, would mammals that are more sensitive to smells, like dogs, have more smell genes or simply more olfactory receptors? Or both?

    Reply
  3. I think the answer to your question, max, is here: http://www.aces.edu/pubs/docs/U/UNP-0066/UNP-0066.pdf “In fact, a dog has more than 220 million olfactory receptors in its nose, while humans have only 5 million.”

    Here’s another interesting article I found that discusses MHC linked Odorant Receptors (OR) and their gene regulation in humans: ftp://ftp.sanger.ac.uk/pub4/theses/younger/chapter6.pdf

    Reply
  4. 2012wangv

     /  May 2, 2012

    The idea that smells are just tiny molecules that the brain registers as odors helped me understand how aquatic animals also perceive smell. This was something that confused me for a while. Thanks for the information!

    Reply

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