E25 Chemical Substances and Molecular Transport

Topic

Chemoreception—whether it involves the sense of smell, taste, or the chemotaxis of a much simpler organism—relies on detecting a single molecule or a small group of molecules; this is the same physical quantity that other models in this framework identify as the only one a chemoreceptor can detect directly. The process begins with the transport of these molecules from their source to the chemoreceptor. While the free diffusion of these molecules through the air is very slow on its own, air convection transports them much more rapidly, creating odor plumes that an animal actively tracks rather than waiting for free diffusion alone to bring the molecules within range.

The binding kinetics between the receptor and its ligand follow the Hill model: when binding is non-cooperative, the binding curve is hyperbolic with respect to ligand concentration; conversely, when binding is cooperative—as is the case with hemoglobin—the curve becomes sigmoidal, showing a much sharper response near the dissociation constant. Consequently, the human olfactory system exhibits a very low typical dissociation constant for numerous odorant molecules, rendering it extraordinarily sensitive to them. Increased cooperativity in the binding between the receptor and its ligand causes the binding curve—according to the Hill model—to become steeper near the dissociation constant.

The chemoreceptor's detection threshold quantifies the minimum number of molecules required to activate it; in an insect olfactory receptor, this threshold can be extraordinarily low—amounting to just a handful of molecules of a specific pheromone. Consequently, a moth's olfactory system is capable of detecting this sex pheromone at extremely low concentrations in the surrounding air—equivalent to just a few scattered molecules within a considerable volume of that air. The fact that the detection threshold of this chemoreceptor is even lower means that the receptor can detect the odorant molecule at an even more dilute concentration in the surrounding air.

Ultimately, the concentration gradient of this odorant molecule is the signal driving this chemotaxis: a bacterium such as *Escherichia coli* detects an extraordinarily small concentration difference between its two ends, and this spatial gradient directs its movement toward a higher concentration—if the molecule acts as an attractant—or toward a lower concentration—if it acts as a repellent. In a mammal, neurons in the olfactory bulb combine signals from thousands of these individual chemoreceptors to detect the spatial odor gradient—following the same optimal fusion of independent estimators used elsewhere in this framework to reduce the final estimation variance. Because the mammal combines signals from an even larger number of these individual chemoreceptors, this optimal fusion results in an even more precise final detection of the spatial odor gradient. Thus, this model—characterized by the molecular transport and cooperative binding with which it was introduced—uses the Péclet number to compare the convection of these molecules against the free diffusion of those same molecules (also a feature of the model's initial description). When the Péclet number is very high, convective transport dominates over free diffusion; conversely, when the Péclet number is very low, free diffusion dominates. Consequently, a butterfly's antenna operates in an intermediate regime between these two mechanisms, where both convection and free diffusion are significant; the antenna's geometry—shaped to maximize surface area exposed to the air—thereby maximizes molecular capture within this intermediate regime.

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