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THE ROLE OF MODELS IN THEORETICAL BIOLO..Y 181

adenosine triphosphate (ATP) systems have been used as specific models of muscular contraction by a number of authors, such as Takauji (1961).

At the tissue level, Easty and Mercer (1962) have reported on “arti-ficial tissues” formed by aggregates of red cells formed under conditions of controlled ionic environment. Others describe physical analogs of celi membranes (Tobias et al. 1962; Stoeckenius, 1962) which show selective ion-concentrating effects and otherwise resemble natural membranes.

' Okada (1961) discusses a model of a nerve synapses based on ion ex-change resins. The pertinent similarity criteria for such models would include dimensional constants and relational criteria.

Modeling is applicable at the cellular level as such. Recent work in exobiology reveals many attempts to create primitive cells from mixtures of proteins, nucleic acids, and lipids. Yarious biochemical relational criteria may apply to such models.

Many reports deal specifically with “molecular models” of one type or another. Beckett and Youssef (1963) have analyzed the behavior of specially treated surfaces as models for the stereospecific behavior of enzymes, while Lemberg and Newton (1961) have used modified heme (similar to hemoglobin) compounds as models of energy-transporting enzymes. A “histochemieal model system” has been described in a report of Hardonk and van Duijn (1964); it deals with enzyme kinetics in mixtures of cellulose or proteins with nucleic acids. The action of pla-cental transport has been studied using “model amino acids” in a paper of Feldman and Christensen (1962). Naturally arising variants of certain enzymes, due to mutations in a bacterium, have been regarded as molecular models of each other by Kapular and Bernstein (1963).

E. Agronomio and Ecological Analogs

Physical modeling is becoming increasingly important in agronomy and biochemical engineering, which deals with, for example, large-scale production of antibiotics, wines, or algae, and controlled photosynthesis. The results of Chemical engineering physical modeling analysis is applicable to such problems immediately, but there has been only limited work in this direction (Webb, 1964). The digestive tract of mammals can be understood as a Chemical processing and mass transfer device. Bowden and Church (1962) describe a model of the rumen (a specialized stornach pouch) of cows which has been used to study biochemical digestion.

A considerable number of reports deal with physical models pertaining to crop growth and certain ecological phenomena. These include quan-