Can we see living structure in a cell?

Colloid chemistry (κολλα: glue, or gelatin) was introduced in 1861 after the discovery of protoplasm, which exhibits gelatin-like properties. Some 80 years later, colloid chemistry (and with it, the concept of protoplasm) was largely abandoned. The membrane (pump) theory, according to which cell water and cell solute like K+ are free as in a dilute KC1 solution, became dominant. Later studies revealed that rejecting the protoplasmic approach to cell physiology was not justified. Evidence against the membrane (pump) theory, on the other hand, has stood the test of time. In a new theory of the living cell called the association-induction (AI) hypothesis, the three major components of the living cell (water, proteins and K+) are closely associated; together they exist in a high- (negative)-energy-low entropy state called the living state. The bulk of cell water is adsorbed as polarized multilayers on some fully extended protein chains, and K+ is adsorbed singly on β- and γ-carboxyl groups carried on aspartic and glutamic residues of cell proteins. Extensive evidence in support of the AI hypothesis is reviewed. From an extension of the basic concepts of the AI hypothesis and the new knowledge on primary structure of the proteins, one begins to understand at long last what distinguishes gelatin from other proteins; in this new light, new definitions of protoplasm and of colloid chemistry have been introduced. With the return of the concept of protoplasm, living structure takes on renewed significance, linking cell anatomy to cell physiology. Finally, evidence is presented showing that electron microscopists have come close to seeing cell structure in its living state.