13. Respiration and Energy Transfer - part 04 - Krebs Cycle

 

13. Respiration and Energy Transfer - part 04 - Krebs Cycle


Krebs Cycle ( TCA cycle/ Citric Acid Cycle):

  • Pyruvic Acid produced by glycolysis undergoes aerobic oxidation in the mitochondrial matrix through the TCA cycle.
  • This cycle serves a common oxidative pathway for carbohydrates fats and proteins. 
  • Moreover, some intermediates of the TCA cycle are used in synthesizing important biomolecules such as glutamate and aspartate.
  • Before participating in the TCA cycle pyruvic acid enters the mitochondrion. 
  • Here it is decarboxylated and the remaining 2-carbon fragment is combined with a molecule of coenzyme A to form acetyl-CoA.
  • This reaction is an oxidative decarboxylation process and produces H+ ions and electrons along with carbon dioxide. 
  • During the process NAD+ is reduced to NADH+H+. 
  • β-oxidation of fatty acids also produces acetyl- CoA as the end product. 
  • Acetyl-CoA from both sources is condensed with oxaloacetic acid to form citric acid. 
  • Citric acid is oxidized step-wise by mitochondrial enzymes, evolving carbon dioxide. This finally regenerates oxaloacetic acid to complete the cycle. 
  • There are four steps of oxidation in this cycle, catalyzed by dehydrogenases (oxidoreductases) using NAD+ or FAD+ as the coenzyme. 
  • The coenzymes are consequently reduced to NADH+H+ and FADH2 respectively. 
  • These transfer their electrons to the mitochondrial respiratory chain to get reoxidised. 
  • One molecule of GTP (ATP) is also produced for every molecule of citric acid oxidized.

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