This is also known as the Citric Acid Cycle, but I will be referring to it as the Krebs Cycle for the rest of the post. This occurs in the inner compartment, the matrix, of the mitochondria.
Preparation for the Krebs Cycle
Before the Krebs Cycle begins, glycolysis occurred, and glucose turned into two pyruvate molecules. This also produced 2 net ATPs (used 2, generated 4), and 2 NADHs.
In order to be ready for the Krebs Cycle, the pyruvate has to be oxidized.
So, from that, an NAD+ is reduced to NADH, carbon dioxide is released, and the pyruvate molecule becomes acetyl CoA. Now, we are ready for the Krebs Cycle.
Note that Acetyl CoA is a two carbon compound (backbone has two carbons, Acetyl CoA has 23 carbons). Acetyl CoA merges with Oxaloacetic Acid, which is a four carbon compound, to form Citric Acid, which is a six carbon molecule.
First, the Citric Acid rearranges itself twice, before reducing a NAD+ to an NADH. Note: Khanacademy's diagram is order is out of order. Right after that, a CO2 is released, changing the molecule to a five-carbon compound. The molecule then reduces another NAD+ and releases a CO2. After that, a GDP is turned into a GTP, (Guanosine Triphosphate- very similar to ATP), but for now, we can just say it makes an ATP. Finally, it reduces an FAD to an FADH2, and reduces an NAD+ to an NADH.
Recall that the aforementioned process happens to each of the two pyruvates.
1 in the preparation phase, and 3 in the Krebs cycle.
1 + 3 = 4 NADH x 2 (4 per pyruvate) = 8 NADH + 2 NADH (from glycolysis) = 10 NADH total
1 ATP in each = 2 ATP + 2 ATP (from glycolysis) = 4 ATP total
1 FADH2 in each = 2 FADH2 (0 in glycolysis) = 2 FADH2 total
The NADH and FADH2 will be very useful in the Electron Transport Chain.