Final answer:
ADP is phosphorylated to ATP in a coupled reaction involving the exergonic process of ATP hydrolysis and the endergonic phosphorylation of glucose. This process, known as substrate-level phosphorylation, is a key mechanism of energy coupling that efficiently synthesizes ATP during glycolysis. The free energy change of the coupled reactions yields a net output of energy.
Step-by-step explanation:
The conversion of ADP to ATP during glycolysis involves coupled reactions, where the exergonic reaction of ATP hydrolysis provides the free energy required to drive the endergonic phosphorylation of glucose and the eventual formation of ATP. During this process, substrate-level phosphorylation occurs, which includes transferring a phosphate from a high-energy phosphate compound to ADP to make ATP. Such a high-energy phosphate compound is 1,3-bisphosphoglycerate (1,3 diPG) generated in glycolysis from glyceraldehyde-3-phosphate (GAP), coupling the oxidation of GAP with ATP synthesis.
Furthermore, the energy required for the phosphorylation of ADP to form ATP can also come from the breakdown of glycogen into glucose-1-phosphate (G-1-P), which then gets converted into glucose-6-phosphate (G-6-P). The overall reaction in glycolysis shows that for every glucose molecule, two ATPs are consumed at the beginning, and four ATPs are produced, leading to a net gain of two ATP molecules. The energy coupling in cells ensures this ATP regeneration, harnessing the energy released by ATP hydrolysis to perform cellular work.
To summarize the energetics, ATP hydrolysis releases about 7 Kcal/mol, while glucose phosphorylation requires an energy input of +3 Kcal/mol, resulting in overall free energy change of -4 Kcal/mol for the coupled reactions under standard conditions.