Glycolysis can be slpit up into two sections: Glycolysis 1 and Glycolysis 2.
Glycolysis 1 is a series of endothermic reactions. In order to kick start Glycolysis 1, an activation energy is required.
The energy source is in the from of ATP. this comes from the first reaction of glycolysis 1 called substrate level phosphorylation.
During this reaction an enzyme transfers a Pi ( inorganic phosphate) from one substrate to another. this process can either
remove a phosphate from ATP or add a phosphate to ADP. An ATP molecule is used to add a phosphate to glucose making glucose-6-phosphate.
another Molecule of ATP is used to phosphorylize fructose-6-phosphate to fructose-1,6-diphosphate. This molecule is than split
into two molecules of PGAL (glyceraldehyde-3-phosphate). These PGAL function as the primary reactants for glycolysis 2.
Glycolysis 2 consists of exothermic reactions that help to furnish the needed energy for the cell. after Glycolysis 1,
each PGAL is oxidized. the electrons are taken up by NAD+ that is than reduced. An ion of H+ is joined to NAD to form NADH.
the oxidized form of PGAL is now able to take on another free phosphate ion from the cytosol to form PGAP (1,3 Diphosphoglycerate).
Two PGAP molecules are formed for each glucose molecule that enters into glycolysis.
After the formation of PGAP, two molecules of ADP, with the help of enzymes remove a phosphate group from PGAP, to form
PGA ( 3-phosphoglycerate). Here, substrate level phosphorylation produces two molecules of ATP, one for each molecule
of PGAP. At this stage, since two molecules of ATP were synthesized and two molecules were used to catalyze
glycolysis, the net output thus far is zero ATP.
The two molecules of PGA are oxidized releasing two water molecules and PEP (phosphoenolpyruvate). finally another substrate
level phosphorylation occurs. two molecules of ADP with the help of enzymes remove the remaining phosphates from PEP
to form to molecules of ATP and the final product of glycolysis 1 and 2, pyruvate.
The overall process of glycolysis includes glycolysis 1 and 2. with the final production of two more ATP molecules,
the net output of ATP is 2. The energy banked in these ATP, as well as the pyruvate, can now be used as ingredients
in the rest of aerobic respiration.