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What steps occur before a taste or odor molecule reaches a receptor cell?

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A pine grosbeak with pink and gray plumage perches on a branch, eating red berries among autumn foliage. — Chemoreception Animals, such as this pine grosbeak, use chemoreception to respond to chemicals that can be tasted and smelled in their environments. Many of these chemicals affect behaviors such as food preference and defense.

Before a taste or odor molecule can reach a receptor cell to initiate a signal, it must undergo several perireceptor events to navigate the environment surrounding the receptor endings.1

The primary steps include:

Solubilization: While water-soluble compounds like sugars and amino acids move freely through the fluid covering receptor cells, many volatile or bitter-tasting compounds are not water-soluble. These molecules must be made soluble to reach the receptors, a process thought to involve binding them to specific soluble proteins found in saliva or nasal mucus.2
Transport and Filtration: These binding proteins act as carriers that move molecules through the mucus or sensillar lymph (in insects) to the receptor cell membrane. These proteins can also function as filters, governing which chemicals reach the membrane and influencing the sensitivity of different receptor cells.2
Capture and Traversal: The molecule must be captured in and successfully traverse the layer of mucus or fluid in which the receptor cell endings are bathed.1

Once these steps are complete, the molecule can interact with the receptor protein to trigger signal transduction.1