Krebs Cycle

We have learnt previously about the pyruvate molecule, and how two of those are produced for every glucose molecule. Now we will see in a greater detail what happens after.

Fermentation

This is anaerobic process, meaning that no oxygen is required for it to happen. Fermentation can lead to lactic acid or ethanol and carbon dioxide + 2 ATP molecules. Some organisms are able to only live from fermentation.

We humans cannot live only through fermentation. This process is just temporary for us, only when oxygen is not available. NADH is not recycled because, even though during the process of fermentation NAD is back, when oxygen is once again available, lactic acid is transformed in pyruvate, and NAD transforms into NADH.

As well as glycolysis, fermentation occurs in the cytoplasm.

The Bridge Reaction

After glycolysis is done, and if there is oxygen available, then pyruvate is transported into the mitochondrial matrix and will reacted with coenzyme A, which will transform it into acetyl-CoA. The result also include the formation of carbon dioxide and 2 NADH (one for each pyruvate.)

This reaction is called the bridge reaction because it links glycolysis to the Krebs cycle (citric acid cycle).

The Krebs Cycle

The Krebs cycle is a series of 8 connected reactions. The first reaction includes the reaction of acetyl-CoA with oxaloacetate, interesting enough the eight reaction will form this reactant, hence the name "cycle"

Just like glycolysis we will cite the some wonders of this cycle:

1. It consists of 8 connected reaction, which some of them are even coupled reactions.
2. It is a cycle because one of the reactants of the first reaction is formed in the last reaction.
3. Every reaction is exergonic.
4. Every reactions has its own enzyme.
5. It forms 2 ATP molecules (1 per pyruvate)
6. It forms 6 NADH and 2 FADH for both pyruvates. Both have a lot of energy in their bonds.

The formation of NADH and FADH leads to this idea that they must be recycled. Therefore, it is important the presence of oxygen, because it allow for the oxidative phosphorylation to happen, which we will discuss in the next posts.