How do odor molecules reach olfactory receptor cells in the nose?

In terrestrial vertebrates, odor molecules reach the olfactory receptor cells primarily through the process of breathing, which draws air into the nasal cavity and over the olfactory epithelium. This airflow can be enhanced by sniffing, which increases the volume of air sampled and creates turbulence, improving the likelihood that odorants will reach the sensory area.12

Once inside the nasal cavity, the process involves several steps:

• Solubilization: The olfactory receptor cells are covered by a layer of mucus. Because many odorants are not water-soluble, they must be made soluble to reach the receptors. This is facilitated by binding to soluble proteins that move freely through the mucus.314
• Dissolution: Microscopic particles carried by the air dissolve into the thin layer of moisture covering the cilia of the olfactory receptor cells.35
• Binding: Once dissolved, these molecules stimulate the receptor cells chemically. Stimulation occurs when an odor molecule fits into a corresponding "pocket" on an olfactory receptor protein, similar to a key fitting into a lock.35

To maintain sensitivity, odor molecules are eventually removed from the environment of the receptor cells by odorant-degrading enzymes present in the mucus.4