Why are two ATP molecules required during the first phase of glycolysis?

• A. They are used to directly generate pyruvate from glucose.
• B. They are consumed to reduce NAD+ to NADH.
• C. They are needed to transport glucose into the cell.
• D. They provide the energy needed to phosphorylate glucose and fructose-6-phosphate, making them more reactive.

Answer: (D)

Verified step by step guidance

1. Understand the process of glycolysis: Glycolysis is the metabolic pathway that breaks down glucose into pyruvate, producing ATP and NADH in the process. It occurs in two phases: the energy investment phase and the energy payoff phase.
2. Focus on the energy investment phase: During this phase, two ATP molecules are consumed to provide energy for the phosphorylation of glucose and fructose-6-phosphate. This step is crucial for making these molecules more reactive and preparing them for subsequent breakdown.
3. Explain phosphorylation: Phosphorylation is the addition of a phosphate group to a molecule. In glycolysis, ATP donates phosphate groups to glucose and fructose-6-phosphate, forming glucose-6-phosphate and fructose-1,6-bisphosphate, respectively. This process increases the molecules' energy and reactivity.
4. Clarify why ATP is needed: ATP acts as an energy source in this phase. The hydrolysis of ATP releases energy, which is used to attach phosphate groups to glucose and fructose-6-phosphate. This step is essential for driving the reactions forward.
5. Summarize the importance of this step: The phosphorylation of glucose and fructose-6-phosphate ensures that these molecules are primed for further breakdown in glycolysis, ultimately leading to the production of pyruvate, ATP, and NADH in later steps.