The glycolytic cycle as it occurs in bacteria such as E. coli . The cycle is drawn using the standard abbreviations of the glycolytic intermediates. The enzymes catalysing the different reactions shown in the figure are: (1) the PEP–glucose phosphotransferase system (PTS); (2) phosphoglucoisomerase (PGI); (3) phosphofructokinase (PFK); (4) aldolase; (5) triose phosphate isomerase (TPI); (6) glyceraldehyde‐3‐P dehydrogenase (G3PDH); (7) phosphoglycerate kinase (PGK); (8) phosphoglycerate mutase (PGM); (9) enolase; (10) pyruvate kinase (PK); and (11) PEP carboxylase. Reactions 1–9 comprise the glycolytic cycle. Double‐headed arrows indicate reversible reactions whereas single‐headed arrows indicate essentially irreversible reactions. Reaction 7 has a large negative ΔG′ 0 and is therefore indicated as being irreversible. However, this reaction can be considered to be reversible by virtue of the endergonic nature of the reactions preceding and following step 7.
An exergonic process is one in which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process.  Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs free energy change is negative (∆ G < 0). "Exergonic" (from the prefix exo-, derived for the Greek word ἔξω exō , "outside" and the suffix -ergonic, derived from the Greek word ἔργον ergon , " work ") means "releasing energy in the form of work". In thermodynamics, work is defined as the energy moving from the system (the internal region) to the surroundings (the external region) during a given process.