Glycolysis and gluconeogenesis are two glycolytic pathways involving the degradation and synthesis of pyruvate from glucose. While these reactions seem like the opposite of each other, gluconeogenesis actually bypasses three pathways involved in glycolysis. The three irreversible reactions must be bypassed due to their change in free energy. The reactions using the enzymes hexokinase, PFK-1, and pyruvate kinase yield a large negative change in free energy making the reaction impossible to reverse. The changes in free energy for the respective pathways are -33 kJ/mol, -22.2 kJ/mol and -33.2 kJ/mol. When a reaction has a change in free energy that is 0 or close to 0, it is easy to reverse and therefore works in the process of gluconeogenesis. However, if the reverse reaction for glycolysis is necessary, the cell uses another pathways and another enzyme to bypass the irreversible change in free energy. Gluconeogenesis is an anabolic pathway because it creates complex molecules from simple molecules. Prior to coupling, the pathway is endergonic and cannot be reversed.
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