How can one kind of cell enable us to discriminate among so many different odors?
Humans can discriminate between hundreds, perhaps thousands, of different odorant molecules, each with its own structure. How can one kind of cell provide for this? • The mammalian genome contains a family of about 1000 related but separate genes encoding different odor receptors. (No more than 40% of these are functional in humans — the rest are pseudogenes — which may help to explain why dogs are better at detecting odors than we are.) • The olfactory epithelium of rats (which is more convenient to study than that of humans) expresses several hundred genes not expressed in other tissues. • Each gene encodes a transmembrane protein that resembles — but is not identical to — the others. • Each protein contains 7 regions of hydrophobic alpha helix that allow the molecule to pass back and forth 7 times through the plasma membrane. • In some cases, the portion of the molecule exposed outside the cell may be responsible for binding the odorant molecule. • However, many odorant molecules ar
