Glucose is the primary reactant for glycolysis.

It produces two pyruvate molecules and two adenosine triphosphate (ATP) molecules. To accomplish this process, 11 different enzymes are used.

A few eukaryotes (yeast and mature human red blood cells) and many prokaryotes (some bacteria) can survive on the energy produced by glycolysis alone.

Glycolysis has two main phases:

  • Preparatory phase: the endothermic activation phase, which uses ATP.
  • Pay off phase: the exothermic phase, which produces ATP molecules and pyruvate.

Glycolysis pathway diagram

Preparatory phase of glycolysis

In order for glycolysis to begin, activation energy, from an ATP molecule, must be provided.

The first reaction of glycolysis - substrate-level phosphorylation.

One ATP is used to phosphorylate glucose to form glucose-6-phosphate.

Glucose-6-phosphate molecule is then rearranged to form fructose-6-phosphate.

At this point, another ATP molecule must phosphorylate the fructose-6-phosphate, producing fructose-1,6-diphosphate.

In turn, this molecule is split into two molecules of glyceraldehyde-3-phosphate (PGAL).

These PGALs act as the reactants for glycolysis II.

Pay off phase of glycolysis

Glycolysis II is a sequence of exothermic reactions that provides energy for the cell.

Following glycolysis I, each glyceraldehyde-3-phosphate (PGAL) is oxidized.

The oxidized form of PGAL is now able to attract a free phosphate ion in the cytosol, forming 1,3-biphosphoglycerate (PGAP).

Following the formation of PGAP, two ADP molecules each remove one phosphate group from each PGAP to form 3-phosphoglycerate (PGA).

Next, the two PGA molecules are each oxidized, forming two water molecules and two phosphoenolpyruvate (PEP) molecules.

Finally, two ADP molecules each remove the remaining phosphate group from each PEP molecule.

The result is the production of two ATP molecules and two pyruvate molecules.

The entire glycolysis process, including glycolysis I and glycolysis II, produces a net gain of two ATP molecules.

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