Mechanism of Respiration:

Aerobic respiration is a continuous process, but for convenience, we can divide it into three main stages;

a- glycolysis,          b-Krebs cycle, and         c-electron transport chain.

Glycolysis occurs in the cytoplasm and oxygen is not involved at this stage. That is why it occurs in both types of respiration i.e. anaerobic and aerobic. The other two stages occur within mitochondria where the presence of oxygen is essential.

Glycolysis:

In glycolysis, the glucose molecule is broken into two molecules of pyruvic acid. Following is the summary of glycolysis.

1. Two ATPs add phosphates to glucose molecules to make them reactive.
2. The reactive compound breaks into two molecules of the 3-carbon compound
3. Each 3-carbon compound is oxidized and its hydrogen atoms are transferred to NAD⁺. During this step, one more phosphate is added to each compound.
4. Each 3-carbon compound gives up phosphates to form 2 ATPs and then it is converted to pyruvic acid.

Krebs Cycle:

In the Krebs cycle, the pyruvic acid molecules are completely oxidized into CO₂ and H₂O. Before entering in Krebs cycle, pyruvic acid is changed into a 2-carbon compound called acetyl-CoA. One molecule of CO2 and one NADH are also produced during this reaction.

After the formation of acetyl, Co-A, the reactions of the Krebs cycle start:

1. The Co-A reacts with a 4-carbon compound and produces a 6-carbon compound.
2. The 6-carbon compound releases CO₂ in two steps and is changed into a 4-carbon compound. Two NADH and one ATP are also produced in these steps.

3. The 4-carbon compound goes through several reactions and is finally converted to the original 4. carbon compound of the first step. One FADH2 and one NADH are produced during these reactions.

Note:

A British biochemist, Sir Hans Krebs discovered this series of reactions that is why it is called the Krebs cycle.