Glycolysis

ATP is generated in glycolysis by a mechanism called substrate-level phosphorylation.

What that means is that ATP is created directly by transfering a phosphate from a phosphorylated sugar to ADP.

The phosphate is attached to the sugar by a high-energy ester linkage
An ester is an alcohol condensed with an acid-in this case the hydroxyl of a sugar with phosphoric acid.

The problem is that beginning material, glucose, is not phosphorylated-how do you make a phosphorylated sugar without consuming a molecule of ATP or its equivalent?

The solution of this problem is glycolysis which manages to produce 4 ATP molecules per glucose while only using up 2 ATP. Net result: production of 2 ATP per glucose.

The first step in glycolysis is 2 phosphorylation reactions (2ATP f 2ADP) and a chemical rearrangement, to create fructose-1,6-bisphosphate.

This six carbon molecule breaks down to two monophosphorylated three carbon units, called PGAL.

We saw PGAL as a major component of the Calvin-Benson cycle.
* Oxidation of PGAL (2) provides the energy to add a second phosphate, while creating a 2 molecules of NADH.

It is the addition of this phosphate which provides the excess ATP yield of the pathway.

The product of this reaction is 1,3-bisphosphoglycerate

By a series of reactions the two phosphates are transfered by substrate level phosphorylation to ATP, giving 4 ATP per glucose molecule (4 ADP to 4 ATP).

The final product of the reaction is pyruvic acid, or pyruvate, a three organic acid. It is the starting material foreach of the three subsequent pathways.

Bottom line: one glucose is converted to two pyruvates with the production of 2 ATP and 2 NADH:

Glucose + 2 NAD(+) + 2 ADP + 2 Pi -----> 2 pyruvate + 2 NADH + 2 ATP + 2 H2O