Hydrophobicity of biosurfaces as shown by chemoreceptive thresholds in Tetrahymena, Physarum and Nitella

Responses (chemotaxis and changes in membrane potential) of Tetrahymena, Physarum, and Nitella against aqueous solution of homologous series of n-alcohols, n-aldehydes and n-fatty acids were studied for clarifying the hydrophobic character of chemoreceptive membranes. Results were: (1) All organisms studied responded to homologous compounds examined when the concentration of these chemicals exceeded their respective threshold, Cth, and the response, R, were expressed approximately as R=alpha log (C/Cth) for C greater than Cth. (2) Increase of the length of hydrocarbon chain in homologues decreased Cth. Plots of log Cth against the number of carbon atoms, n, in n-alcohols, n-aldehydes and n-fatty acids showed linear relationships as represented by long Cth=-An+B. A and B are positive constants for respective functional end groups of the chemicals and biological membranes used. The above empirical equation was interpreted in terms of the partition equilibrium of methylene groups between bulk solution and membrane phase. Parameter A was shown to be a measure of hydrophobicity of the membrane, and B represented the sensitivity of chemoreception of the membrane. (3) Thresholds, Cth, for various hydrophobic reagents were compared with those of human olfactory reception, T. Plots of log T against log Cth fell on straight lines for respective organisms with different slopes which were proportional to parameter A.