Glucose is a major source of bodily energy, as it is used to produce ATP when it is oxidized by glycolysis and the citric acid cycle. The pentose phosphate pathway uses glucose to make reducing equivalents for biosynthetic reactions and ribose for nucleotide biosynthesis. Plants synthesize glucose using photosynthesis, a process that is critical for all life. Glycolysis is also coordinately regulated with gluconeogenesis. In glycolysis, one molecule of glucose is converted to two molecules of pyruvate.
Glycolysis is an anaerobic pathway that occurs in all organisms.
The reactions of glycolysis that require ATP are the phosphorylation of glucose and the phosphorylation of fructose 6-phosphate. The reactions that produce ATP are the transfer of a phosphate from 1,3-bisphosphoglycerate to ADP and the phosphorylation of ADP by phosphoenolpyruvate. Gluconeogenesis has the effect of reversing glycolysis, but some of the steps differ. Gluconeogenesis converts pyruvate to glucose and glycolysis converts glucose to pyruvate. The key regulated enzymes in glycolysis are shown in red and the key regulated enzymes in gluconeogenesis are shown in blue.