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.

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 t

• Why do you see things upside down before your brain turns it around?

Theories on chemoreception attempt to explain how we perceive odours and how we can tell one odour from another. At present our understanding of chemoreception is not well advanced and, in the absence of substantial, hard scientific evidence, theories are based on ideas of how the process might work. Another difficulty is that the evidence we do have relates to the observation of the whole process, from the evolution of an odour from its source to the consciousness of that odour in the brain.

• Why do different people like different tastes and smells?

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