In terrestrial vertebrates, including humans, the receptors are located on olfactory receptor cells, which are present in very large numbers (millions) and are clustered within a small area in the back of the nasal cavity, forming an olfactory epithelium.
There are about 1,000 genes in the olfactory gene family, the largest known family of genes. (Although humans possess all 1,000 olfactory receptor genes, making up roughly 3 percent of the entire human genome, only about 350 of these genes encode working olfactory receptors.) Since each gene produces a different odour receptor protein, this contributes to the ability of animals to smell many different compounds. Animals not only can smell many compounds but can also distinguish between them. This requires that different compounds stimulate different receptor cells. Consistent with this, evidence indicates that only one olfactory gene is active in any one olfactory receptor cell. As a consequence, each receptor cell possesses only one type of receptor protein, though it has many thousands of the particular type on the membrane of the exposed cilia of the cell. Since each cell expresses only one type of receptor protein, there must be large numbers of cells expressing each type of receptor protein to increase the likelihood that a particular odour molecule will reach a cell with the appropriate receptor protein. Once the molecule reaches the matching receptor, the cell can respond.
Credit : Britannica
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