COLLOIDAL STRUCTURE AND ITS BIOLOGICAL SIGNIFICANCE1

Abstract

SummaryTo sum up the evidence which we have considered here on the close relation between colloidal structure and biological function, we find that we pass over a series of possibilities. At the one end we get highly active tissues such as primitive connective tissue, growing tissues and generative cells. In these there is a high percentage of water, the percentage decreasing with age; the proteins are present as colloid sols, probably uni‐molecular, each molecule consisting of a back‐bone which is not highly hydrated, carrying at periodic intervals side chains or limbs which are of varied character, often lengthy and of complex chemical structure, highly polar, heavily hydrated and undoubtedly playing an important rôle in the metabolic activities of the cells. Next we get biologically inactive tissues such as mesenteries, tendons and skin, which are built up mainly of connective tissue fibres which have a skeletal but not a metabolic function. Here we find a low percentage of water and the protein molecules are arranged in a compact and orderly manner to form fibres; the backbone of the molecule is still the same, it is very little if at all hydrated and carries at periodic intervals the side chains or limbs, but these are now for the most part shorter and of a much simpler molecular structure, less polar, probably less hydrated and playing little if any part in any metabolic cycle. Finally, we get tissues such as the keratinous layer of the epidermis in animals, cellulose cell walls in plants, external fibres, such as wool and cotton hairs, internal fibres such as bast fibres. The biological function of these tissues is purely mechanical; chemical activity is not desired. Here the importance of the backbone of the colloid is paramount; the chemical potentialities of the limbs are of little significance and we find in these tissues, as might have been anticipated, compact, stable ring structures which leave very little space for the entry of water or any other disturbing molecule. In the animal world, these resistant fibres are protein; in the plant world carbohydrate. For biological stability either material is equally effective.