The Citric Acid Cycle Explained: How ATP Powers Your Body

THE CITRIC ACID CYCLE

The citric acid cycle, or the tricarboxylic acid cycle (TCA) or the Kreb's cycle as it's also known. The final resting place of the intial glucose molecule from glycolysis. At this point glycolysis has broken down glucose into pyruvate (see below). The citric acid cycle is another series of reactions with the help of oxygen that creates an abundance of products which are then used in the production of ATP molecules. We've seen via The Cori Cycle that in the absence of oxygen, pyruvate is converted to lactate and through gluconeogenisis new glucose molecules are produced for the continuation of glycolysis. When oxygen is present and in abundance, the citric acid cycle is used. 

Before the citric acid cycle can start, the pyruvate molecule must undergo a prepratory step. Pyruvate is converted to acetyl CoA. This step also produces some CO2 and one NADH.

Upon entry into the citric acid cycle, it combines with a molecule called oxaloacetate to form citrate (citric acid - hence the name). This is the first of a series of reactions producing critical molecules such as NADH & FADH2, CO2 and ATP. NADH & FADH2 are what's called electron carriers and these are used in the electron transport chain. We'll address these a little bit further on and why they are important, but for now, let's stay in the citric acid cycle.

Although the above is a gross simplification of the process - you don't really need to know the in depth details. Using a range of different enzymes, citrate is oxidised several times. Each time it produces some CO2, some NADH, some FADH2 and some ATP as well. The citric acid cycle is a closed loop. The last part of the cycle reforms the first molecule used, thus starting the cycle again. Per pyruvate molecule, one turn of this cycle generates:

• 3 NADH
• 1 FADH
• 1 ATP

But, as there is two pyruvates produced from one glucose molecule during glycolysis, we get double:

• 6 NADH
• 2 FADH
• 2 ATP

If we remember back to the prepratory step converting pyruvate to acetyl CoA, during that step we also produced 1 NADH per pyruvate molecule so we ultimately end up with 2 NADH.

This runs on a loop provided we have enough input into the cycle. The most important part of this cycle is the production of NADH & FADH2 for use in the electron transport chain. This part of energy production yields the greatest amount of ATP.