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The direct approach to a study of the dynamics of the essential intermediates in life processes is afforded by sensitive optical techniques that accurately measure and record the absorbancy of the iron proteins (cytochromes) in living cells, tissues, and particles derived therefrom. This paper outlines the nature of physical phenomena measurable in the biological systems and emphasizes current thinking on the nature of electron transfer between the proteins which involves oxidation and reduction of their iron-containing active centers with the simultaneous conservation of energy required for driving essential biological processes. The methods for these measurements are reviewed and spectrophotometric techniques at room and liquid nitrogen temperatures, and new developments such as microspectrophotometry of the cytochromes in portions of the living cell are emphasized. Data evaluation and representation of electron transfer processes and metabolic control sequences by analog and digital computers are described and particular reference is made to the operation of metabolic controls in ascites tumor cells.