Here's how it works:

* ATP Used: Glycolysis starts by using 2 ATP molecules to "prime" the glucose molecule, making it more reactive. This step is called the investment phase.

* ATP Produced: As the process continues, the 6-carbon glucose molecule is broken down into two 3-carbon pyruvate molecules. This breakdown releases energy that is used to produce 4 ATP molecules. This is the payoff phase.

Net gain: While 4 ATP are produced, 2 ATP were initially used, resulting in a net gain of 2 ATP molecules per glucose molecule.

Important Note: The 4 ATP molecules produced in glycolysis are generated through substrate-level phosphorylation. This means that ATP is produced directly by the transfer of a phosphate group from a substrate (a molecule involved in the reaction) to ADP. This differs from the way ATP is produced in the electron transport chain, which uses a process called oxidative phosphorylation.