Favourable differences in cell structure: the third principle of selectivity

Abstract

It is an everyday observation that the different forms of life differ greatly in their external structures, and this is also the case internally. Plants differ from animals by having photo- synthetic devices, and they also have walls around all cells. Animals differ from plants by having nerves, and muscles. The organization of plant and animal cells into a variety of tissues makes a valuable division of labour possible, and further divisions of labour occur at the sub-cellular level. The many, often conflicting, chemical reactions which take place simultaneously in cells require many isolated compartments constructed from membranes of selective permeability. These membranes comprise up to 80 per cent of the dry weight of animals [1]. The electron microscope has revealed, within cells, many kinds of organelle, each with its own function.