Question

How many times does oxidation occur in the Krebs cycle?

• A. Two times
• B. Three times
• C. Four times
• D. Six times

Hint:

For every one molecule of acetyl-CoA entering the Krebs cycle, the cycle produces 3 NADH, 1 FADH\(_2\), and 1 ATP (or GTP) along with the release of 2 molecules of CO\(_2\).

Answer 1

The Correct Option is C

Concept: The Krebs cycle, also known as the Citric Acid Cycle (TCA cycle), is a crucial stage of cellular respiration. It occurs in the mitochondrial matrix of eukaryotic cells and plays a central role in the oxidation of metabolic fuels such as carbohydrates, fats, and proteins. The primary function of this cycle is to completely oxidize the acetyl group of acetyl-CoA and produce high-energy electron carriers. During the Krebs cycle, oxidation reactions remove electrons from intermediates. These electrons are transferred to the coenzymes NAD\(^+\) and FAD, forming NADH and FADH\(_2\). These reduced coenzymes then donate electrons to the electron transport chain to generate ATP. In one turn of the Krebs cycle, oxidation occurs four times. These oxidation steps produce:
• Three molecules of NADH
• One molecule of FADH\(_2\) These oxidation reactions are responsible for capturing high-energy electrons needed for ATP production.

Step 1: Entry of Acetyl-CoA into the Krebs Cycle.
Acetyl-CoA combines with oxaloacetate to form citrate, initiating the Krebs cycle.

Step 2: Identification of oxidation reactions.
During the cycle, several intermediates undergo oxidation reactions where hydrogen atoms and electrons are removed. The oxidation steps occur during the following conversions:
• Isocitrate $\rightarrow$ $\alpha$-Ketoglutarate (produces NADH)
• $\alpha$-Ketoglutarate $\rightarrow$ Succinyl-CoA (produces NADH)
• Succinate $\rightarrow$ Fumarate (produces FADH\(_2\))
• Malate $\rightarrow$ Oxaloacetate (produces NADH)

Step 3: Counting the oxidation events.
There are four oxidation reactions in one complete cycle. Thus, the correct answer is Four times.