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PinkMonkey Online Study Guide-Biology

(6) 4-C Succinyl Co-A is hydrolyzed to succinic acid (4-C) in the next step. One molecule of H2O is used and Co-A is regenerated. The reaction is exergonic. Energy released is used for the formation of GTP (guanosine triphosphate) from GDP and inorganic phosphate.

Subsequently, ATP is formed when GTP reacts with ADP

        GTP + ADP ® ATP + GDP

(Thus, there is the direct formation of one ATP when 5-C acid is converted to 4-C acid.)

(7) In the next step, 4-C Succinic acid is oxidized (by removal of hydrogen) to 4-C fumaric acid in the presence of co-enzyme FAD (flavin adenine dinucleotide). A reduced FADH2 is formed.

(8) Fumaric acid (4-C) is converted to another 4-C acid, malic acid, by the addition of H2O.


(9) In the final step of the TCA cycle, 4-C Malic acid is oxidized (by removal of hydrogen) to 4-C Oxaloacetic acid. NADH2 is formed in the process.

Thus, OAA is regenerated in the last step. It can combine withanother 2-c Acetyl Co-A to form Citric acid and participate in the TCA cycle again.

During various steps of the Krebís cycle, oxidation of substrate takes place by the removal of an electron hydrogen ion from substrate. It is accepted at each step by a suitable co-enzyme such as NAD or FAD to form a reduced co-enzyme molecule, NADH2 or FADH2, respectively.

The chart of the Krebís cycle indicates the result of the participation of only one molecule of pyruvic acid. It should be noted that 2 pyruvic acid molecules are formed at the end of glycolysis. Therefore, the final analysis of the Krebís cycle involving 2 pyruvic acids may be summarized as follows.

(i) Total number of reduced co-enzyme molecules formed = 8#NADH2 and 2#FADH2

(ii) Total number of ATP formed directly (when 5 - C acid is converted to 4 - C acid) = 2#ATP

(iii) Total number of CO2 molecules released = 6#CO2

(iv) Total number of H2O molecules utilized = 6#H2O

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Table of Contents

5.0 - Introduction
5.1 - Ultrastructure and Functions of Mitochondrion
5.2 - An Overview of Cellular Respiration
5.3 - Glycolysis
5.4 - The T.C.A. Cycle(Tricarboxylic Acid Cycle)
5.5 - Electron Transport Chain
5.6 - Fermentation
5.7 - Significance of Respiration

Chapter 6





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