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Chemoreception (smell, taste, chemotaxis) is based on the detection of single molecules or small groups of molecules. The process begins with the transport of the molecules from the source to the receptor: in air (D 10 m²/s) free diffusion is slow (hours to cross 1 meter), but convection (Pe = uL/D >> 1) transports the molecules much more quickly, forming odor plumes that the animals actively track.

The receptor-ligand binding kinetics follow the Hill model: = [L]^n/(K_d^n + [L]^n). When n = 1 (non-cooperative, Langmuir) the curve is hyperbolic; for n > 1 (cooperative, as in hemoglobin) the curve is sigmoidal with a steeper response near K_d. The human olfactory system has typical K_d of 10 10 M, making it extraordinarily sensitive.

The detection threshold N_min C_threshold · V · N_A quantifies the minimum number of molecules needed to activate the receptor: in insect olfactory receptors, N_min can be as low as 110 pheromone molecules. The moth's olfactory system can detect sex pheromone at concentrations of 10¹ M, equivalent to 6 molecules per cm³ of air.

The concentration gradient C/L_sensor is the signal for chemotaxis: E. coli bacteria detect C/C 0.001 between their extremes (L 2 m), which requires detecting concentration differences of 1 in 1000 between the head and the tail. This spatial gradient directs movement toward higher concentrations (attractants) or lower concentrations (repellents). In mammals, neurons in the olfactory bulb combine signals from thousands of receptors to detect spatial gradients of odor.

The Péclet number Pe = uL/D compares advection with diffusion: Pe >> 1 means that convective transport dominates (the air/water current carries the odor); Pe << 1 means that diffusion dominates. Butterfly antennas (L 1 mm, u 0.1 m/s, D 10 m²/s) have Pe 10, in the regime in which both mechanisms matter and the antenna geometry (pectinated for greater surface area) maximizes molecular capture.

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