Energy transformation in biochemical reactions and pathways

In early work on glycogen phosphorylase, in the early 1940s and 1950s, the activity was assayed by measnring the release of phosphate from glucose 1-phosphate in the reaction 

The assay was chosen since phosphate was relatively easy to measure (the activity is now measured using pnrihed con-pling enzymes (Appendix 3.3). 

That syntheses could be achieved by reversal of degradation processes was considered possible at that time, since, by definition, all chemical reactions are reversible. However, it gradually became apparent that synthetic and degradative processes in biochemistry in vivo were distinct, so that a process could only proceed in one direction. 

The theoretical basis underlying the direction of a physical, chemical or biochemical process is derived from the second law of thermodynamics. 

This law was formulated by Rudolph Clausius, a German physicist It is impossible for a machine, unaided by external agency, to convey heat from one body to another at a higher temperature that is, heat will not flow, of its own 

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