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The Krebs cycle is the second stage in cellular respiration, following glycolysis, and it produces two ATP molecules for the cell. ATP stands for adenosine triphosphate and is an energy storage molecule used by cells.
You may sometimes hear the Krebs cycle referred to as the citric acid cycle, but they're both the same thing. The Krebs cycle occurs within the mitochondria of the cell and is an aerobic process, meaning it requires oxygen to occur. Pyruvate from glycolysis will move into the mitochondria and begin the preparatory steps of the Krebs cycle.
Remember that pyruvate is a three-carbon molecule that came from glycolysis, the stage of cellular respiration before the Krebs cycle. Carbon dioxide will be released, leaving you with a two-carbon fragment of pyruvate, which will then combine with something called coenzyme A. When that two-carbon fragment of pyruvate combines with coenzyme A, it produces something called acetyl CoA.
Once you have acetyl CoA, you’re ready to begin the actual Krebs cycle. Reference the diagram below as you learn about each step.
Step 1: We started with pyruvate that was turned into acetyl COA, which is a two-carbon molecule. Then we have a four-carbon molecule that's going to combine with that two-carbon molecule to produce a six-carbon molecule.
Step 2: Then carbon dioxide will be released. A hydrogen will be removed from NAD+ and transferred to NADH. NADH is an electron carrier that will carry electrons into the electron transport chain to make more ATP later.
Step 3: We had a six-carbon molecule, which had a carbon removed to leave us with a five-carbon molecule. The steps then repeat and CO2 is released again. NAD+ will donate a hydrogen, leaving us with NADH, an electron carrier molecule.
Step 4: When the five-carbon molecule releases a carbon, we're left with a four-carbon molecule. ADP is now going to join with a phosphate group, transforming it into ATP, our energy storage molecule. We haven't changed any of the carbons, so we're still at a four-carbon molecule.
Step 5: NAD+ will become NADH and FAD will become FADH2. A hydrogen will be removed and transferred, giving us FADH2—another type of an electron carrier molecule. We still have the four-carbon molecule that we started with.
Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND AND KELSEY Perreault