what are the three phases of oxidative respiration and what generally happens in each?

Oxidative or Aerobic respiration:

It is a biochemical process that produces energy. In this process, cells of an organism acquire energy by the combination of oxygen and glucose and release carbon dioxide, water, and ATP (the energy currency of the cell).

Aerobic represents the “presence of oxygen”. Aerobic respiration is defined as the process in which respiration takes place in the presence of oxygen.

It takes place in three steps: A). Glycolysis, B). The citric acid cycle, C). An electron transport chain.

A). Glycolysis:

It is a metabolic pathway that involves the breakdown of one molecule of glucose into two molecules of pyruvic acid. The flux in the glycolysis is in response to the conditions present both inside and outside the cell.

Glycolysis steps:

1. In the first step, D-glucose is converted into glucose-6-phosphate. Hexokinase enzyme catalyzes this reaction.
2. In the second step, the rearrangement of glucose 6-phosphate occurs into the fructose 6-phosphate with the help of glucose phosphate isomerase enzyme.
3. In the third step, the conversion of fructose-6-phosphate into fructose- 1, 6-bisphosphate or FBP takes place.

      This reaction is similar to the reaction of step 1. A second ATP molecule provides the phosphate group which is added on to the F6P molecule. Phosphofructokinase or PFK catalyzes this reaction.

4. Aldolase enzyme splits the fructose 1, 6-bisphosphate into two sugars: dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP). These sugars are isomers of each other.
5. Triosephosphate isomerase enzyme quickly inter- converts DHAP and GAP molecules. Glyceraldehyde phosphate is used or removed in the next step of Glycolysis.
6. Glyceraldehyde-3-phosphate dehydrogenase or GAPDH dehydrogenates and produces a 1, 3-bisphosphoglycerate by adding an inorganic phosphate to the glyceraldehyde 3-phosphate,
7. Phosphoglycerate kinase forms an ATP and 3-phosphoglycerate by transferring a phosphate group from 1, 3-bisphosphoglycerate to ADP.
8. Phosphoglycero mutase enzyme forms the 2-phosphoglycerate by the relocation of phosphate from the 3rd carbon of 3- phosphoglycerate to its 2nd carbon.
9. Enolase enzyme forms a phosphoenolpyruvic acid or PEP by the removal of a water molecule from the 2-phosphoglycerate.
10. In the final step, phosphor-enol pyruvate is converted into pyruvate with the pyruvate kinase enzyme. In this reaction, phosphate group transfer occurs. The phosphate group linked to the 2nd carbon of PEP is then transferred to an ADP molecule and the formation of ATP takes place. Again, as there are two PEP molecules there will be the synthesis of 2 ATP molecules.

In glycolysis there will be production:

In the first and third steps, there will be = – 2ATP

In the seventh and tenth steps, thee will be = + 4 ATP

Net production of ATP = 2.

After the completion of glycolysis, immediately the cell continues to be in aerobic or anaerobic respiration. If it continues to be in aerobic condition then the cell undergoes a series of reactions (Krebs cycle) in the mitochondria.

B). The citric acid cycle:

The Krebs cycle is also known as the citric acid cycle. It is a chain of chemical reactions. It takes place in the mitochondrial matrix and is a part of aerobic respiration. This cycle is utilized by all the aerobic organisms to produce energy by the oxidative metabolism of acetyl units.

Citrate is oxidized through a chain of reactions. It releases two CO₂ molecules for each acetyl group and fed into the reaction cycle.

Krebs cycle starts when an acetyl group from the acetyl Coenzyme A (Co-A) gets a link to a four-carbon oxaloacetate molecule.

Citric acid cycle steps:

Step 1: In this cycle, the acetyl group from the acetyl Coenzyme A (Co-A) is linked to a four-carbon oxaloacetate molecule. This reaction produces a six-carbon citrate molecule. The enzyme involved in the catalysis of this step is citrate synthase.

Step 2: The citrate gets transformed into isocitrate. It is an isomer of citrate. The enzyme involved in the catalysis of this step is aconitase.

Step 3: Isocitrate becomes oxidized to form alpha-ketoglutarate (a 5-carbon molecule). It results in the production of carbon dioxide. There is a formation of one NADH molecule.

The enzyme involved in the catalysis of this step is isocitrate dehydrogenase. It is a rate-limiting step because this enzyme is allosterically regulated.

Step 4: Alpha-ketoglutarate becomes oxidized to form a 4-carbon molecule. It picks up a coenzyme-A and forms the succinyl CoA. This conversion produces an NADH molecule. The enzyme involved in the catalysis of this step is alpha-ketoglutarate dehydrogenase.

Step 5: Succinyl CoA gets changed into succinate (4 carbon molecule). One GTP molecule is formed. The enzyme involved in the catalysis of this step is succinate thiokinase.

Step 6: Succinate gets changed into a fumarate (4 carbon molecule). The molecule of FADH₂ is formed. The enzyme involved in the catalysis of this step is succinate dehydrogenase.

Step 7: Fumarate gets changed into malate (4 carbon molecule). The enzyme involved in the catalysis of this step is fumarase.

Step 8: Malate gets changed into oxaloacetate. NADH is formed here. The enzyme involved in the catalysis of this step is malate dehydrogenase.