substrates for respiration Carbohydrates Fats Fatty acids Glycerol Amino acids Proteins Glucose Fructose- , -Bisphosphate DHAP Glyceraldehyde - -Phosphate Pyruvic acid Acetyl CoA H O CO CO NH nation i m a e D Proteases Krebs cycle . . Electron Transport Chain (ETC) (Terminal oxidation) During glycolysis, link reaction and Krebs cycle the respiratory substrates are oxidised acid. Therefore, it is also known as Citric Acid Cycle (CAC) or Tri Carboxylic Acid (TCA) cycle .
It is followed by the action of different enzymes in cyclic manner. During the conversion of succinyl CoA to succinate by the enzyme succinyl CoA synthetase or succinate thiokinase, a molecule of ATP synthesis from substrate without entering the electron transport chain is called substrate level phosphorylation. In animals a molecule of GTP is synthesized from GDP Pi. In a coupled reaction GTP is converted to GDP with simultaneous synthesis of ATP from ADP Pi.
In three steps ( , , ) in this cycle NAD is reduced to NADH H and at step (Figure14. ) where FAD is reduced to FADH . The summary of link reaction and Krebs cycle in Mitochondria is Pyruvic acid 4NAD FAD 4H O ADP Pi 3CO 4NADH 4H FADH H O ATP. Two molecules of pyruvic acid formed at the end of glycolysis enter into the mitochondrial matrix.
Therefore, Krebs cycle is repeated twice for every glucose molecule where two molecules of pyruvic acid produces six molecules of CO , eight molecules of NADH H , two molecules of FADH and two molecules of ATP. . Significance of Krebs cycle: . TCA cycle is to provide energy in the form of ATP for metabolism in plants.
. It provides carbon skeleton or raw material for various anabolic processes. . Many intermediates of TCA cycle are further metabolised to produce amino acids, proteins and nucleic acids.
. Succinyl CoA is raw material for formation of chlorophylls, cytochrome, phytochrome and other pyrrole substances. . α-ketoglutarate and