What is Glycolysis in Cell Metabolism?

Glycolysis :  is the process that breaks down glucose into pyruvate and energy. Glycolysis does not require the presence of oxygen and is a source of energy in almost all living organisms. Glycolysis breaks down one molecule of glucose into two molecules of pyruvate, the ionized form of pyruvic acid as found in cells. The process consumes two ATP molecules, and produces four ATP molecules and two NADH + H+ molecules. Glycolysis is summarized below:

(1) Glucose is combined with a phosphate group (phosphorylated) from ATP on carbon No. 6 of the glucose molecule, to form glucose 6-phosphate (G6P).

(2) Glucose 6-phosphate forms a 5-carbon ring isomer, fructose 6-phosphate (F6P).

(3) F6P is phosphorylated a second time by another ATP to form fructose 1, 6-bi-phosphate (FBP). "1" and "6" refer to the carbon atoms to which the phosphate groups are attached.

(4) The fructose ring is opened by enzyme activity, and two 3-carbon compounds, dihydroxyacetone phosphate (DAP) and glyceraldehyde 3-phosphate (G3P), are formed. These compounds are isomers, and DAP rearranges itself to form G3P. Notice that the reactions described so far are endergonic, requiring an input of energy. The remaining reactions however, are exergonic, yielding energy.

(5) Two molecules of G3P are oxidized, losing hydrogen atoms and gaining phosphate groups to form 1, 3-diphosphoglycerate (DPG). Two molecules of NAD+ form NADH + H+ in the process.

(6) Two DPG molecules transfer phosphate groups to ADP to form two ATPs and two molecules of 3-phosphoglycerate (3PG).

(7) The phosphate groups of 3PG move to carbon number 2, forming 2-phosphoglycerate (2PG).

(8) The two 2PG molecules lose water, forming two phosphoenolpyruvates (PEPs), high-energy compounds.

(9) The two PEPs transfer phosphates to ADP, forming two ATPs and two molecules of pyruvate.