Summary of the First Five Reactions in Glycolysis

The first five reactions of glycolysis are referred to as the energy investment phase or energy input phase.  These reactions initially use ATP.  Reaction 1 uses an ATP molecule to convert glucose to glucose 6-phosphate.  This input of energy is similar to priming a pump.  Some energy must be used to initiate or start the reactions of the glycolytic pathway.Reaction 3 uses a second ATP molecule to convert F6P to F 1,6-BP. 

The first phase converts glucose (a 6 carbon molecule) to two glyceralde 3-phosphate molecules (3 carbon molecules).