Step 4: ATP is made

• The bond between succinic acid and CoA is a high-energy bond—like a tightly wound spring.
• When this bond breaks:
  • Energy is released.
  • This energy is used to make one molecule of ATP (which is like a rechargeable battery that powers cell activities).
• CoA is released, and succinic acid continues in the cycle.

Step 5: Succinic acid becomes fumaric acid

• Succinic acid now loses two hydrogen atoms, which are picked up by a different molecule called FAD (flavin adenine dinucleotide).
• FAD becomes FADH₂.
• The resulting molecule is called fumaric acid (a 4-carbon molecule).

🧪 Why use FAD here instead of NAD⁺?
Because the energy from this step isn’t enough to reduce NAD⁺, but it’s enough to reduce FAD.

Step 6: Fumaric acid becomes malic acid

• A water molecule is added to fumaric acid, turning it into malic acid (still 4-carbons).

💧 This step is like adding water to soften the structure for the next reaction.

Step 7: Malic acid becomes oxaloacetic acid

• Malic acid is oxidized (loses hydrogen and electrons).
• NAD⁺ picks up the electrons and becomes NADH.
• The final product is oxaloacetic acid—the same 4-carbon molecule that started the cycle.

🔁 This completes the cycle, and the oxaloacetic acid is ready to combine with another acetyl group to begin the cycle again.

🔄 Summary of What’s Produced from 1 Turn of the Krebs Cycle:

From 1 molecule of acetyl-CoA, the cycle produces:

• 2 CO₂ molecules – released as waste
• 3 NADH molecules – store energy
• 1 FADH₂ molecule – stores energy
• 1 ATP molecule – usable energy

🧠 Final Thoughts (Like a Teacher Would Say)

Think of the Krebs cycle as a chemical machine in the mitochondria that takes fuel (acetyl-CoA) and squeezes out as much energy as possible by breaking down carbon bonds and capturing the energy in molecules like NADH, FADH₂, and ATP.

These energy-rich molecules will go to the next stage, called the Electron Transport Chain, where they will help make a large amount of ATP—the energy currency of the cell.