Glycolysis occurs in all organisms that can perform cellular respiration which is the ability of the cells of living organisms to create ATP. Glycolysis is defined as a series of reactions that breakdown glucose by splitting it into two 3-Carbon molecules called pyruvate. This process takes place in the cells cytosol and is broken down into two phases: the Energy Requiring Phase and the Energy Producing Phase.
The Energy Requiring Phase begins with a glucose molecule. Two phosphate groups attach to the glucose molecule causing it to become unstable and ultimately splitting the glucose molecule in half. This splitting of glucose requires energy; thus, two ATP molecules are used in this process. We now have two phosphate bearing 3-Carbon sugars. These two halves are different from each other, one half is favorable (glyceraldehyde 3 phosphate) and the other is unfavorable (DHAP). The favorable half is immediately used in the following phase while the unfavorable half is later used; both halves ultimately finish the pathway.
SUMMARY OF ENERGY REQUIRING PHASE:
Beginning in the cytosol of a cell, one glucose molecule enters and two phosphate groups attach to it making the glucose molecule split in half. Each half becomes a phosphate bearing 3-Carbon sugar (two ATP molecules are used in this process). Of the two halves, only one can be used for the following step while the other half will later be used in the same pathway.
The Energy Releasing Phase is where the newly formed 3-Carbon sugar is converted into another 3-Carbon molecule called pyruvate through a series of reactions. With these reactions, two ATP molecules and one NADH molecule are formed. Since there are two halves of glucose this phase takes place twice which creates four ATP molecules and two NADH molecules overall. (One half of glucose creates two ATP and one NADH molecule*). Each reaction that occurs in glycolysis is done so by enzymes in the cell. The most important enzyme is Phosphofructokinase. This enzyme catalyzes the formation of the unstable two-phosphate sugar molecule.
SUMMARY OF ENERGY RELEASING PHASE:
The favorable half is now converted into another 3-Carbon molecule called pyruvate through a series of reactions; this process creates two ATP molecules and one NADH molecule. Since this phase is done twice, four ATP molecules and two NADH molecules are ultimately created.
Overall
Glycolysis converts one 6-Carbon molecule of glucose into two 3-Carbon molecules of pyruvate. The net products of this process are two ATP molecules and two NADH molecules. (Two ATP molecules are spent to form an unstable sugar within two phosphate groups, therefore we only have two ATP produced overall even though four ATP molecules were created). When the glucose molecule becomes unstable it splits into two halves that are isomers of each other. This allows for ATP and NADH molecules to be created through a series of reactions.
**Isomer: a compound with the same formula as another compound but has a different arrangement of atoms.
