Energy used:

As would be expected; the synthesis of glucose through gluconeogenesis required the input of energy. Two pyruvate molecules are needs to synthesize one molecule of glucose. Energy is needed at the following steps:

pyruvate carboxylase             1 ATP ( × 2)     2 ATP PEP

carboxykinase                         1 GTP ( × 2)     2 ATP

phosphoglycerate kinase         1 ATP ( × 2)     2 ATP

Total     6 ATP

This compares with only two ATPs as the net ATP yield from glycolysis. Therefore an extra four ATPs per glucose are needs to reverse glycolysis.

In reality, the glyceraldehyde 3-phosphate dehydrogenase reaction also consumes NADH, equal to two molecules of NADH for every molecule of glucose synthesized. Since each cytosolic NADH would commonly be used to generate around two ATP molecules through the glycerol 3-phosphate shuttle and oxidative phosphorylation, this is equal to the input of another four ATPs per glucose synthesized.