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Chapter 7 : The Main Respiratory Pathway, 1920 to 1960
This chapter on main respiratory pathway covers the history of research on both the tricarboxylic acid (TCA) cycle and the cytochrome system. The first account of the TCA cycle came after the elucidation of the part played by cytochromes in respiration, but the account of the research on the cytochromes is here given second because, in terms of physiology, the activity of the cytochromes depends on the TCA cycle. The concept of free energy produced by intracellular oxidations depended on the early findings in thermodynamics, and in the second half of the same century, Moritz Traube did much to develop the idea of intracellular oxidations in relation to respiration and fermentation. Establishing the TCA cycle as the main pathway of carbohydrate oxidation in yeasts had to wait another 20 years. The impermeable nature of the plasma membrane and the lack, in Saccharomyces cerevisiae, of carriers which take succinate and other intermediates of the cycle into the cytosol, were the chief obstacles for realizing that the TCA cycle operates in yeasts, just as it does in plants and animals.