The Connection Between Glucose and Energy

     The body's principle source of energy is ATP, which is synthesized through a series of reactions. In humans, glucose serves as the most basic input for these reactions, which ultimately produces ATP for other bodily functions. 

     Glucose is a sugar monomer; more complex sugars and carbohydrates can be broken down to this basic form to then be utilized in ATP synthesis. The first pathway involved in this process is a series of 10 reactions known as glycolysis. The steps are as follows:

1. Glucose is phosphorylated (a phosphate group is added from an ATP), to produce glucose-6-phosphate.
2. Glucose-6-phosphate isomerizes (same composition, but rearranged), to produce fructose-6-phosphate.
3. Fructose-6-phosphate is phosphorylated (again expending an ATP), to produce fructose-1,6,-bisphosphate.
4. Fructose-1,6-bisphosphate is cleaved, producing glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
5. Dihydroxyacetone phosphate isomerizes to glyceraldehyde-3-phosphate.
6. Each of the two glyceraldehyde-3-phosphates are oxidized to produce two 1-3,bisphosphateglycerates.
7. Each of the two 1-3,bisphosphateglycerates transfers a phosphate group to an ADP, producing two ATPs and two 3-phosphoglycerates.
8. Each of the two 3-phosphoglycerates isomerizes to two 2-phosphoglycerates.
9. Each of the two 2-phosphoglycerates are dehydrated to produce two phosphoenolpyruvates (PEPs).
10. Each of the two PEPs transfers a phosphate group to an ADP, producing two ATPs and two pyruvates.

*So for every one molecule of glucose, a cycle of glycolysis produces two pyruvates and two ATPs (4 were produced, but 2 were expended, netting 2 ATP).*

     Alternatively, if glucose is in excess or ATP is not immediately necessary, glucose can be stored in the form of glycogen via glycogen synthesis reactions. Once stored, glycogen can be broken back down into glucose for the aforementioned processing, as needed.

     Glycolysis is just the first step in ATP synthesis. Products of this step will enter the citric acid cycle (an 8-step cycle), which in turn produces the necessary inputs for the electron transport chain and oxidative phosphorylation (under aerobic conditions). Through multiple control points and allosteric enzymatic regulations, this complicated process generates the energy we need to go about our lives. Though the multiple steps are complicated, it is the simple sugar monomer, glucose, which serves as the initial fuel for the steps to follow!