Semi-Conduction and Electrode Processes in Biological Material I. Crustacea and Certain Soft-Bodied Forms

P. S. B. Digby


The presence of semiconducting materials in a biological membrane across which a potential difference is established by ionic diffusion and active transport may be expected to lead to a leakage of charge through the electronically conducting elements. Oxidation and reduction will occur at the two surfaces which will therefore act as electrodes. In experiments with various Crustacea, particularly Palaemonetes varians (Leach) and Carcinus maenas (L.) the normal potential of the living animal is shown to be negative and, whether living or preserved, a negative potential is generated by diffusion when the salinity of the surrounding water is decreased. The presence of semi-conducting properties in the cuticle is shown by the occurrence of electrode processes following the application of potentials higher than those which exist in the normal animal. Thus when the surface is made negative hydrogen is liberated and metallic silver and copper are rapidly deposited from solutions of their salts. In the cuticle of living material at normal potentials or at potentials enhanced by outward diffusion of salt following transfer to reduced salinity an alkaline reaction is produced at the surface. This is shown particularly by the use of the dye nile blue sulphate, which is adsorbed by cathodic surfaces where it then acts as an indicator, alkalinity producing the red free base. Colours similar to those produced at the surface of negatively charged Crustacea appear at metallic cathodes. Alkalinity appears at the surface of living and non-living Crustacea in places where electrode processes are demonstrable at high potentials and comparable effects are not produced by diffusion in soft tissues or in gelatin and other substances in the absence of organic semi-conductors. The appearance of alkalinity at these positions is therefore attributable to cathodic reduction of salt water and not to the segregation of hydrogen and hydroxyl ions by diffusion processes. In distilled water the rate of reduction of ammoniacal silver nitrate on the cuticle is greatly increased by the action of a diffusion potential, showing that this reaction is here at least partly electrolytic in nature and that the surfaces on which deposition occurs are acting as cathodes. Similar phenomena can be seen in Cyanea and other planktonic coelenterates; thus with certain precautions electrolytic hydrogen may be evolved from the surface and in the living and undamaged animal nile blue adsorbed at the surface develops a mauve to purple tint similar to that of undamaged Crustacea, indicating a slightly alkaline reaction. In these forms also the surface is therefore acting partly as a cathode.

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