Absence of __ would disrupt the citric acid cycle but not glycolysis.
ADP
FAD
ATP
CO2
NAD+
The correct answer and explanation is :
The correct answer is: NAD⁺
Explanation:
To understand why NAD⁺ is the correct answer, let’s first quickly review what glycolysis and the citric acid cycle (also called the Krebs cycle) are:
- Glycolysis is the process of breaking down one glucose molecule (6-carbon) into two molecules of pyruvate (3-carbon each). This happens in the cytoplasm and does not require oxygen. It produces a small amount of energy in the form of ATP and NADH (which is reduced NAD⁺).
- The Citric Acid Cycle occurs inside the mitochondria and requires oxygen indirectly. It fully oxidizes the pyruvate from glycolysis into carbon dioxide (CO₂) and captures high-energy electrons in the form of NADH and FADH₂, which are used later to produce a lot of ATP in the electron transport chain.
Now, the role of NAD⁺ is crucial for both glycolysis and the citric acid cycle, but glycolysis has a special workaround if oxygen (and thus NAD⁺ regeneration) is not available. In the absence of oxygen, cells can regenerate NAD⁺ through fermentation, allowing glycolysis to continue even without mitochondrial involvement.
However, the citric acid cycle absolutely requires NAD⁺ to function normally. NAD⁺ acts as an electron acceptor during several steps of the cycle. Without NAD⁺, critical reactions in the citric acid cycle, like the conversion of isocitrate to α-ketoglutarate (which produces NADH), could not occur. As a result, the entire cycle would halt.
Other choices explained:
- ADP is important for making ATP, but its absence would affect both glycolysis and the citric acid cycle equally.
- FAD is important for the citric acid cycle but not as broadly essential as NAD⁺ at multiple steps.
- ATP is the energy product, not a required input.
- CO₂ is a waste product of the citric acid cycle, not a reactant.
Thus, without NAD⁺, the citric acid cycle would stop, but glycolysis could still proceed via fermentation.