Mechanism of Respiration - Oxidative decarboxylation Krebs cycle
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Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle 

Mechanism of Respiration – Oxidative decarboxylation , Krebs cycle

Oxidative decarboxylation of pyruvic acid

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The two molecules of pyruvic acid formed from a glucose molecule
move into mitochondria and are oxidized, decarboxylated to two molecules
of acetyl coenzyme A (acetyl Co~A).

These 2 carbon compounds are formed by

2 Pyruvic acid + 2NAD+ →  2 Acetyl Co~A + 2NADH2+ 2CO2

decarboxylation and dehydrogenation.

This reaction is catalyzed by pyruvic dehydrogenase and two molecules of NAD+ are reduced to NADH2.

Duringthis reaction two molecules of CO2 are released.

Oxidative decarboxylation of pyruvic acid occurs only under aerobic condition.

Under anaerobic conditions, the pyruvic acid is reduced either to lactic acid or ethyl alcohol depending on the nature of the organism.

Krebs cycle

In 1937, Sir Hans Adolf Krebs described the catalytic role of pyruvic
acid for the production of energy in the cell.

The series of cyclic reactions involved in converting pyruvic acid to carbondioxide and water in mitochondria is called Krebs cycle.

It is also known as citric acid cycle or tricarboxylic acid cycle – TCA cycle.

1. In the first reaction of citric acid cycle, one molecule of acetyl
Co~A combines with oxaloacetic acid to form citric acid. This reaction
is catalyzed by citric acid synthetase. Citric acid contains three carboxylic
acid groups.

2. Citric acid is dehydrated to form cis-aconitic acid in the presence
of aconitase

3. The same enzyme aconitase catalyzes the formation of isocitric
acid from cis-aconitic acid by the addition of a molecule of water. Citric
acid, cis-aconitic acid and isocitric acid contain three carboxylic acid
groups.

4. The isocitric acid is oxidatively decarboxylated to a – ketoglutaric
acid. This reaction is catalyzed by isocitric dehydrogenase. During this
reaction, one NADH2 is formed.

5. The a – ketoglutaric acid is oxidatively decarboxylated to form
succinyl Co~A. This reaction is catalyzed by a – ketoglutaric
dehydrogenase. The energy released during this reaction is conserved in
NADH2.

6. The succinyl Co~A is hydrolysed to succinic acid in the presence
of succinyl Co-A synthetase. In this reaction, ADP is phosphorylated to
ATP. This is called substrate level phosphorylation.

7. The succinic acid is oxidized to form fumaric acid by succinic
dehydrogenase. Here, FAD+ is reduced to FADH2.

8. The fumaric acid is converted to malic acid by the addition of a
molecule of water. This reaction is catalyzed by fumarase.

krebs cycle

9. The malic acid is oxidized to oxaloacetic acid by the enzyme malic
dehydrogenase. Here, NAD+ is reduced to NADH2.

Significance of Krebs cycle


2 molecules of acetyl CoA enter into Krebs cycle which on subsequent
oxidation generate 6NADH2, 2FADH2.

When 6NADH2, 2FADH2 enter into the electron transport system generate 22ATP molecules.

In one step, there is substrate level phosphorylation whch directly yield 2ATP molecules.

So, during Krebs cycle, every 2 molecules of acetyl CoA enter into Krebs cycle 24 ATP molecules are generated.

So, primarily it is a energy producing system.

Since, Krebs cycle involves with both anabolic and catabolic processes, it is also described as amphibolic process.

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For more information about Krebs cycle click here

Other links 

Plant tissue culture – origin and techniques


Plant physiology – photosynthesis and its significance


Site of photosynthesis and Mechanism of photosynthesis


Electron transport system and photophosphorylation types


Dark reaction


C3 and C4 pathways


Photorespiration or C2 cycle


Factors affecting photosynthesis


Test tube and funnel experiment, Ganong’s light screen experiment


Mode of nutrition – Autotrophic, Heterotrophic


Chemosynthesis


Mechanism of Respiration – Glycolysis


Mechanism of Respiration – Electron Transport Chain, Energy Yield


Ganong’s respiroscope, Pentose phosphate pathway


Anaerobic respiration, Respiratory quotient, Compensation point, Kuhne’s fermentation tube experiment


Plant growth and Measurement of plant growth


Phytohormones Auxins


Phytohormones Gibberellins


Phytohormones Cytokinin, Ethylene, Abscisic Acid, Growth Inhibitors – Physiological Effects


Photoperiodism and vernalization, Phytochromes and flowering

 

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