concerns about electron transfer at olfactory receptors, together with our extensive experimental data, argue against the plausibility of the vibration theory." In commenting on this work, Vosshall writes "In PNAS, Block et al.... shift the "shape vs. vibration" debate from olfactory psychophysics to the biophysics of the ORs themselves. The authors mount a sophisticated multidisciplinary attack on the central tenets of the vibration theory using synthetic organic chemistry, heterologous expression of olfactory receptors, and theoretical considerations to find no evidence to support the vibration theory of smell." While Turin comments that Block used "cells in a dish rather than within whole organisms" and that "expressing an olfactory receptor in human embryonic kidney cells doesn't adequately reconstitute the complex nature of olfaction...", Vosshall responds "Embryonic kidney cells are not identical to the cells in the nose ... but if you are looking at receptors, it's the best system in the world." In a Letter to the Editor of PNAS, Turin et al. raise concerns about Block et al. and Block et al. respond. Recently, Saberi and Allaei have suggested that a functional relationship exists between molecular volume and the olfactory neural response. The molecular volume is an important factor, but it is not the only factor that determines the response of ONRs. The binding affinity of an odorant-receptor pair is affected by their relative sizes. The maximum affinity can be attained when the molecular volume of an odorant matches the volume of the binding pocket. A recent study describes the responses of primary olfactory neurons in tissue culture to isotopes and finds that a small fraction of the population (<1%) clearly discriminates between isotopes, some even giving an all-or-or -none response to H or D isotopomers of octanal. The authors attribute this to differences in hydrophobicity between normal and deuterated odorants. See also Odotope theory Docking theory of olfaction Quantum biology References Olfactory system Quantum biology Theories