Some Protein Properties of Plant Protoplasm

IN attempting to explain the reactions and behaviour of living organisms, the experimental biologist is often tempted to fall back on the causal consideration of the observed phenomena in terms of the single living cell. While it is true that our physiological knowledge of this unit is almost negligible, there is at any rate a widely accepted belief that its properties may often be expressed in terms of two of its most important chemically active substances, the various proteins and their derivatives or the fatty bodies. So far as proteins are concerned, this assumption is justified by the classical analyses of Reinke and Rodewald,' which show that in the plasmodium of Fuligo varians, about 60 per cent, of the dry weight of protoplasm is composed of protein. It is therefore legitimate to consider the hypothesis that certain properties of normal plant cells appear to be most simply expressed in terms of their protein reactions. Recent work on the proteins, particularly that of Loeb, has shown that hydrogen ion concentration is the most important factor affecting their physical and chemical behaviour. At a particular hydrogen-ion concentration, the iso-electric point, most of the protein properties are at a minimum, the particles are electrically uncharged, and chemically least active. At this point, the protein is most easily precipitated or coagulated by heat. Both its volume and osmotic pressure are least, and it will absorb least water from the surrounding medium. Further, its viscosity is also at a minimum. If then the hydrogen-ion concentration is altered, the protein absorbs water, swells, and the viscosity of the solution increases.