For example, the reaction of fermentation in an organism producing lactate as the sole product of glucose metabolism can be written as:
The reaction of glycolysis in the cytoplasm can be written as:
The difference between these two reactions is:
If we sum the free energy changes for the fermentation reaction and ATP synthesis (-47 kcal.mol-1 + 14.6 kcal. mol-1) we get -32.4 kcal. mol-1, the free energy change from the glycolysis reaction.
We can see that the change in the system represented by the fermentation reaction, with a -DG, is coupled to a change in the surroundings (the change in a separate system represented by the phosphorylation of ADP to ATP), with +DG.
When two systems are coupled in this way, it is often convenient to treat them as a single system. In this example, the new system is the reaction represented by the glycolysis equation, with a -DG equal to the sum of values for the two processes contributing.
From this example, it will be apparent that we can, from a thermodynamic perspective, treat metabolic processes in several ways. We can treat individual reactions as separate systems, or treat a set of coupled reactions (including the complete set representing the metabolism of the organism as a whole) as a single system. The choice is one of convenience, and the important points are that the system should be carefully defined, the reaction equation balanced in conformity with the Law of conservation of mass, and the energy equation balanced in accordance with the First Law of thermodynamics, and the properties of variables of state.