Question

In the course of glycolysis A. NADH is reduced to NAD+ . B. NAD+ is oxidized to NADH. C. glucose is degraded into two molecules of pyruvate. D. both (A) and (B).

Short answer

C. glucose is degraded into two molecules of pyruvate.

Step-by-step solution

Understanding Glycolysis

Glycolysis is a metabolic pathway that converts glucose (a 6-carbon molecule) into two molecules of pyruvate (a 3-carbon molecule) through a series of enzymatic reactions.

NAD+ and NADH in Glycolysis

In glycolysis, NAD+ is reduced to NADH, meaning NAD+ gains electrons and a hydrogen ion to form NADH. Therefore, option B (NAD+ is oxidized to NADH) is incorrect.

Pyruvate Formation

During glycolysis, glucose is broken down into two molecules of pyruvate. This confirms option C.

Checking Options A and D

Option A states NADH is reduced to NAD+, which is incorrect, as the correct process is NAD+ being reduced to NADH. Since both A and B are incorrect, option D is also incorrect.

Conclusion

The correct answer is option C, as it accurately describes the breakdown of glucose into two molecules of pyruvate during glycolysis.

Key concepts

Glucose Metabolism

Glycolysis is a critical process in glucose metabolism, transforming glucose, a 6-carbon sugar, into two molecules of pyruvate, each containing 3 carbons. This conversion occurs in a series of ten enzymatic steps within the cytoplasm of the cell. Glycolysis not only serves as a key source of energy but also provides intermediates for other metabolic pathways.
The steps of glycolysis are divided into two main phases: the investment phase and the payoff phase. In the investment phase, the cell uses up 2 ATP molecules to convert glucose into two 3-carbon molecules. Then, in the payoff phase, these molecules are further processed to produce 4 ATP molecules and 2 NADH molecules. Thus, the net gain from glycolysis is 2 ATP per glucose molecule.
This process does not require oxygen, making it an anaerobic pathway, essential for cells that lack mitochondria or oxygen.

NAD+ Reduction

During glycolysis, one vital reaction involves the reduction of NAD+ to NADH. This is crucial because it helps in maintaining the balance of redox reactions within the cell.
NAD+ (Nicotinamide Adenine Dinucleotide) acts as an electron carrier. In the glycolytic pathway, specifically during the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, NAD+ is reduced to NADH. This occurs when NAD+ gains two electrons and one proton (H+), transforming into NADH.
The equation representing this reduction is:
\[ \text{NAD}^+ + 2e^- + H^+ \rightarrow \text{NADH} \text{+ H}^+ \]
It’s important to note that NADH will later donate these electrons in the electron transport chain (ETC) during aerobic respiration, producing more ATP. In the absence of oxygen, NADH can be reoxidized to NAD+ in processes such as fermentation, allowing glycolysis to continue.

Pyruvate Formation

The end product of glycolysis is pyruvate. Each glucose molecule ultimately yields two pyruvate molecules, along with a net gain of two ATP molecules and two NADH molecules.
Pyruvate formation marks the entrance into either aerobic or anaerobic metabolism.

• In the presence of oxygen, pyruvate is transported into the mitochondria, where it undergoes oxidative decarboxylation to form acetyl-CoA. This acetyl-CoA enters the citric acid cycle for further energy production.
• In the absence of oxygen, pyruvate can be diverted into lactate production via lactate dehydrogenase, leading to fermentation in muscle cells or ethanol production in yeast.

This dual pathway ensures that cells can efficiently produce energy regardless of oxygen availability. Therefore, understanding pyruvate formation helps us grasp how cells adapt to varying conditions to meet their energy requirements.