4 Smell and Taste

4.1 Human Smell and Taste on the Molecular Scale

All sensors, biological or technological, contain several elements: the sensing element

that senses the signal, the transducer that transfers the signal, and an amplification

and/or analysis/reporting element that increases the signal and/or analyzes it. Smell and

taste are humans’ chemical sensors. The sensing elements for chemicals are G-protein-

coupled receptors (GPCR) [1, 2] (see Section 1.1, Figure 1.13). For taste, the taste molecules

directly diffuse into the taste pore that contains the GPCR (Figure 4.1) [3–6]. The trans-

fer of the odor molecules to the sensor site is more complicated, since they are a gas

and need to be captured first. An odorant-binding protein (ODP) binds the molecule and

transports it to the GPCR on the surface of the olfactory bulb (Figure 4.2). Different re-

ceptors are concentrated in different areas of the bulb [7]. Sometimes, a surfactant is

needed to aid the odor molecule in diffusing to the ODP, so that it can then bind and be

transported to the receptor. These surfactants are part of the constant secretion in your

nose.

Once the odorant or taste molecule is bound to the GPCR, the receptor is activated

and a signal transduction pathway is started; for taste, it is the phospholipase pathway

(Figure 1.4), and for smell it is the adenylate cyclase pathway (Figure 1.3). As with all

signal transduction pathways, the signal is amplified in the process.

The amplified signals eventually open sodium and calcium channels that depolarize

the cell membranes of these specialized nerve cells, thus triggering an action potential.

The action potential is processed in the brain via the gustatory afferent nerve in case of

taste and the olfactory bulb and olfactory cortex in the case of smell.

Though we have described how the signals travel from molecule to molecule, we

have not discussed yet how a few different receptors can lead to the detection and iden-

tification of a large number of complex smells and tastes. Some of the details of these

processes are not yet known, but principally speaking this happens via each molecule

binding to each receptor but with different strengths, and the combination of signals for

one molecule results in a more complex sensation [8, 9]. Detection and identification is

also combined with memory. Humans avoid chemicals toxic to our body and seek out

“good chemicals”, i. e., food and clean water, using smell and taste. The pathways in the

brain for this memory have been identified [10].

Let us summarize what happened here: odorants and taste molecules bind to

G-protein-coupled receptors in a combinatorial manner in the nose or tongue that are

part of specialized neurons. When bound, these molecules activate the receptor and

start a signal cascade, which then results in an action potential, and is then sent to

the processing part of the brain. Is it possible to use the molecules and methods of the

human chemical senses and make an artificial, molecular-sized chemical sensor with

similar functions?

https://doi.org/10.1515/9783110779196-004