Much of biochemistry finds its roots in the study of metabolism. Substantial progress came when it was discovered that important metabolic processes, such as fermentation of glucose to lactate by erythrocytes or to ethanol by yeast, consisted of series of apparently independent chemical reactions. The independence of these reactions was established by purifying protein fractions that were each capable of uniquely cata lysing one of the reactions. Likewise membrane-dependent biological free-energy transduction was reduced to the action of two proton pumps, i.e. one linked to the electron-transfer chain in the mitochondrial or bacterial inner membrane, the other coupled to ATP hydrolysis (Mitchell, 1979). Accordingly, biochemistry and molecular biology assimilated the paradigm that each of the physiological processes of the living cell could be understood in terms of a series of indepndent biochemical reactions (Fig. I). Although some processes are relactant to be reduced in this manner this is usually believed to be a matter of practice rather than of principle.
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