Question

How many times does oxidation occur in one Krebs cycle?

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

Hint:

In one Krebs cycle:
3 NADH + 1 FADH$_2$ are produced. Each corresponds to an oxidation reaction, so oxidation occurs four times.

Answer 1

The Correct Option is C

Concept: The Krebs cycle (also called the Citric Acid Cycle or TCA cycle) is a key metabolic pathway in cellular respiration that occurs in the mitochondrial matrix. It oxidizes acetyl-CoA to produce energy-rich molecules. During one complete turn of the Krebs cycle:
• Three molecules of NAD$^+$ are reduced to NADH.
• One molecule of FAD is reduced to FADH$_2$. Each reduction of NAD$^+$ or FAD corresponds to an oxidation step of a substrate molecule. Therefore, the number of oxidation reactions equals the number of reduced coenzymes produced.

Step 1: Identify oxidation reactions in the Krebs cycle. The following oxidation steps occur:
• Isocitrate $\rightarrow$ $\alpha$-ketoglutarate (NADH formed)
• $\alpha$-ketoglutarate $\rightarrow$ Succinyl-CoA (NADH formed)
• Succinate $\rightarrow$ Fumarate (FADH$_2$ formed)
• Malate $\rightarrow$ Oxaloacetate (NADH formed)

Step 2: Count the total oxidation events. \[ \text{Total oxidations} = 3 \text{ (NADH)} + 1 \text{ (FADH}_2\text{)} = 4 \] Thus, oxidation occurs four times in one complete Krebs cycle.