THE ROLE OF ADSORPTION AND MOLECULAR MORPHOLOGY IN OLFACTION: THE CALCULATION OF OLFACTORY THRESHOLDS

Abstract

1. It is proposed that a stimulus is initiated in an olfactory receptor cell only when a critical number (p) of odorant molecules is concentrated within one small area of the cell membrane. An equation is derived which relates the olfactory threshold for humans to this number p and to the adsorption constant for molecules passing from air to the oil/water interface. Olfactory thresholds are calculated for a range of odorants on the assumption that 1/p is a function of the molecular cross-section area of an odorant. The calculated thresholds agree with the observed values; that predicted for glycerol exceeds the saturation concentration in the air so that this substance is odourless. 2. The equation suggests that olfactory thresholds should increase as the temperature is raised, as has been found experimentally. The results suggest that the olfactory cell membrane is lipoid in nature; the calculated "active surface area" of each olfactory cell is less than the observed total value. The effectiveness of compounds as odorants for Phormia and dogs, as well as for humans, depends on the concentration adsorbed on the membrane and upon the shape and size of the odorant molecules. Contact chemoreception in Phormia, however, is dependent only upon the appropriate adsorption constant and not upon molecular morphology.