Glycolysis is the breakdown of glucose to pyruvic acid, and thereare nine intermeddaite products formed, and each one catalyzed by anenzyme. Glycolysis has two key functions: It generate some ATP from the freeenergy available from the rearrangement of the atoms in monosacharides(particularly glucose). It also partially breaks down glucose and provides astarting point for the complete oxidation of glucose by another pathway tocarbon dioxide and water with the generation of much ATP. Glycolysis is aperfect example of a “coupled reaction”, involving exergonic and energonicreactions.
Exergonic reactions release energy?the bond energy of the product orproducts is lower than that of the reactants. Endergonic reactions requireenergy input?the energy of the products is higher than that of the reactants. An exergonic reaction can drive endergonic ( for food breakdowns and movement) -this is how they perfrom coupled reactions. ndergonic is when chemical reactionswith a negative standard free energy change.
These reactions dont proceedspontaneously in the direction concentartions of all reactants and products. Since the sign of a standrad free-energy change is negative, the conversion ofglucose 1-phosphate to glucose 6-phosphate is an exergonic process. An initialreaction for glycolysis ( a molecule of ATP donates its phosphate group to theglucose): Coupled reaction occurs in glycolysis when it tries to convert ( forexample) glucose-6-phosphate into a nearly identical compoundfructose-6-phosphate: The positive G shows that it is an “uphill”,endergonic reaction, one that couldnt have happend spontaneously. This wasbecause the coupled reaction ( they shared a common intermeddiate molecule),glucose-6-phsphate, the product of step 1 and the reactant of step 2 – canproceed as a single reaction. The -4.
0 kcal/mole broken down in step 1 iscombined with the +0. 4 kcal/mole taken in by step 2 to yield a net percentchange of -3. 6 kcal/mole. These two together, are strongly exergonic – so thereaction proceeds. The glycolytic pathway is an arrangement of these kinds ofcoupled reactions, where exergonic steps push or pull endergonic steps, with thefavorable net free energy change of the steps taken together, allowing theseries of reactions to go on.