Cellular respiration:
It is divided into few stages
Glycolysis
Pyruvic acid oxidation
Krebs’s cycle /citric acid cycle
Respiratory chain
- 1. Glycolysis:
Glycol means “Glucose” & lysis means “breakdown”. So this is the process of glucose break down & formation of Pyruvic acid. It can take place in absence/ presence of oxygen, both resulting in same product. A series of steps involved in Glycolysis require specific enzymes.
Glycolysis can be divided into two phases:
i. Preparatory Phase
ii. Oxidative Phase
Preparatory phase
The first step in Glycolysis in the transfer of phosphate group from ATP to the 6th carbon atom of glucose; as a result a molecule of glucose 6 phosphate is formed. An enzyme catalyzes the conversion of glucose 6 phosphate to its isomer, fructose 6 phosphate. At this stage another ATP molecule transfer a second phosphate group at 1st carbon atom of the glucose. The product is fructose-1, 6-biphosphate. The next step in Glycolysis is the enzymatic splitting of fructose-1, 6-diphosphate into 3-phospoglyceraldehyeand dihydroxy acetone phosphate. These two molecules are isomers and are readily interconnected by enzymes.
Oxidative Phase
In this phase two electrons or two hydrogen atoms are removed from the molecule of 3-phosphoglyceraldehde (PGAL) and transferred to a molecule of NAD. During this reaction of second phosphate group is donated to the molecule, which resulted in the formation of 1, 3-diphosphoglyceric acid (DPGA). The oxidation of PGAL is an energy yielding process. At the very next in Glycolysis ATP is formed. The end product of this reaction is 3-phosphoglyceric acid. In the next step 3-phosphoglyceric acid is converted to 2phosphoglyceric acid. From 2-phosphoglyceric acid a molecule of water is removed and the product is phosphoenol Pyruvic acid (PEP). PEP then gives up its high-energy phosphate to convert a second molecule of ADP to ATP. The product is Pyruvic acid (C₃H₄O₃).
- 2. Pyruvic acid Oxidation:
Pyruvic acid, the end product of Glycolysis does not enter the Krebs cycle directly. The Pyruvic acid is first changed into 2-carbon acetic acid molecule. One carbon is released as Co₂. Acetic acid on entering the mitochondrion unites with coenzymes A (CoA) to form acetyl CoA. In addition more hydrogen is transferred to NAD.
- 3. Krebs’s Cycle:
Sir Hans Krebs discovered Krebs’s cycle. It starts after the formation of Acetyl CO-A. Krebs cycle takes place in mitochondrial membrane & comprises of following steps:
i. The union of acetyl CoA with oxaloacetate to form citrate. In this process molecule of CoA is regenerated and one molecule of water is used. Oxaloacetate is a 4-carbon acid with two carboxyl groups. Citrate thus has 6-carbon atoms and three carboxyl groups.
ii. In the next reactions NAD mediated oxidation takes place and citrate is changed into Ketoglutarate.
iii. Ketoglutarate is then oxidized & decarboxylated simultaneously. Thus a new product Succinate is formed. One ATP molecule is also synthesized.
iv. The next step in the Krebs cycle is the oxidation of Succinate to fumarate. Once again, two hydrogen atoms are moved, but this time the oxidizing agent is a coenzyme called flavin adenine dinucloetide (FAD), which is reduced to FADH₂.
v. With the addition of another molecule of water, fumigate is converted to malate.
vi. Anther NAD mediated oxidation of malate produces oxaloacetate, the original 4-carbon molecule.
- 4. Respiratory Chain
NADH formed in Krebs’s cycle transfers its hydrogen atom to the electron carriers of respiratory chains. This transfer brings about transport of electron down to all carriers resulting in a series of reduction oxidation process & ultimately releasing O₂ & water is formed. Electron carriers are
i. CoenzymeQ
ii. Series of cytochromes enzymes
iii. Molecular oxygen
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