The sweet, energetic glucose molecule is the energy source providing power for your living cells. Glucose from the food you eat is slowly broken down into two pyruvate molecules, extracting energy in a sequence of enzyme reactions called glycolysis. The glycolysis pathway requires energy to begin delivered by chemical fuel ATP. The primary source of ATP in your cells are mitochondria organelles. Deep inside mitochondria, chemical reactions of aerobic respiration, consume oxygen from the air you breathe to generate vast amounts of ATP. Sitting on this abundant supply of ATP is the first enzyme of the glycolysis pathway. The reaction of the first enzyme transfers a phosphate group from ATP to glucose, creating glucose six phosphate. The second enzyme rearranges glucose six phosphate into its isomer, fructose six phosphate. The third enzyme uses a second molecule of ATP, creating fructose one-sixth is phosphate. The six carbon sugar is now ready to be broken apart. The fourth enzyme cuts the molecule in half, creating two three carbon sugars, each with a single phosphate attached. The sugar products are isomers of each other, but only Lisser aldehyde three phosphate is ready to continue with glycolysis. The sugar isomer, dihydroxy acetone phosphate, must first be converted through isomerization before continuing to the sixth enzyme. The sixth enzyme adds a second phosphate to glycolysis aldehyde three phosphate, creating one three bizfospo glycerate, while two electrons are transferred to NAD+. Halfway through glycolysis, the cell has consumed two ATP molecules breaking down glucose and is now ready to capture energy in return. The seventh enzyme transfers a phosphate group from one three bizfospo glycerate to ADP, creating chemical fuel ATP. The eighth enzyme rearranges the phosphate from the end of the molecule to the middle. The ninth enzyme catalyzes dehydration, increasing the potential energy in preparation for the final glycolysis reaction. The tenth enzyme transfers the remaining phosphate group to ADP, producing ATP and the three carbon sugar pyruvate. Glacoluses finally produces a net gain in ATP. The glycolysis pathway breaks down glucose into two molecules of pyruvate, gaining two ATP and two NADH in the process.