Final answer:
The last step of the investment phase of glycolysis, Step 6, involves the oxidation of glyceraldehyde-3-phosphate to produce NADH followed by the addition of a second phosphate group to form 1,3-bisphosphoglycerate without using another ATP molecule. This step marks the transition from ATP consumption to generation in glycolysis.
Step-by-step explanation:
The last step of the investment phase of glycolysis is Step 6, which features two crucial processes. First, the sugar molecule glyceraldehyde-3-phosphate is oxidized, leading to the extraction of high-energy electrons that are then picked up by the electron carrier NAD* to produce NADH. Subsequently, a second phosphate group is added to the oxidized sugar, which converts it into 1,3-bisphosphoglycerate. This addition of the second phosphate requires no external energy from an ATP molecule, as it leverages the high-energy electrons that were just removed.
This step is a turning point in glycolysis, transitioning from the investment phase, where ATP is consumed, to the payoff phase, where ATP is generated. It is also noteworthy because this phosphorylation, unlike earlier steps, does not necessitate further ATP expenditure. This culminates the initial investment of energy from ATP molecules into the glucose molecule, now ready for the subsequent energy-harvesting steps of glycolysis to take place.