The sense of smell
2. The chemistry of smell
Introduction
Smell and taste are our chemical senses. Both allow us to be aware of substances in our environment and rely on our ability to detect these substances by the chemical nature of their molecules. Our other senses allow us to be aware of the physical characteristics of our environment - its temperature, the presence of radiation (e.g. visible) and the texture of materials with which we are in contact.

In this chapter we will focus on the chemistry of olfaction - our sense of smell.

2.1 Chemoreception
An outline
Our sense of smell is stimulated only by gaseous molecules. These may come directly from the air we breathe or may come from volatile substances, released in our mouth, from the food we are eating.

The molecules we perceive as smells are called odorants. Odorant molecules stimulate sensory nerve cells (neurons) at the top of the nasal cavity and these respond by sending impulses to the brain (section 1.1). The sensory nerve cells involved are called receptor cells, their surfaces have regions on them called receptor sites. These detect the odorant molecules and the process triggers a sequence of changes in the cell that eventually generate an electrical signal (figure 2.1).

Receptors
Scientists believe that the receptor sites are protein molecules, embedded in the nerve cell membrane, but protruding from it towards both the inside and outside of the cell. The backbone of the protein zigzags through the membrane seven times so that one end is inside and the other outside the cell. The sections of the molecule that pass through the membrane have the a-helix secondary structure. These sections of helix come together to form a cylindrical shape. Scientists believe that the odorant binding sites lie inside this cylinder (figure 2.1).
Figure 2.1. Roll your cursor over the highlighted steps to follow the changes on the diagram.
Stimulating ion flow
Step 1. The structure of the receptor protein changes when it meets the odorant molecule.

Step 2. The associated G protein activates the enzyme adenylate cyclase

Step 3. the enzyme catalyses the formation of messenger molecules - cyclic adenosine monophosphate (cAMP) - from ATP in the cell.

Step 4. cAMP opens up the Na+ ion channels; Na+ ions pass through the membrane.

Question 6

Nasal mucus contains enzymes that catalyse the breakdown of odorant molecules. Suggest why these are necessary.