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PinkMonkey Online Study Guide-Biology

(i) Formation of 2-C Acetyl Co-A

This is the initial step in Phase II of aerobic respiration. The reaction involves oxidation and decarboxylation of 3-C pyruvic acid to form 2-C acetyl Co-A. It in the cell cytoplasm and requires the presence of co-enzyme-A (Co-A) and NAD. It is catalyzed by complex enzymes. In this process, carbon dioxide is released and the reduced co-enzyme NADH2 is formed.

The 2-C acetyl Co-A formed in the cytoplasm then enters the mitochondria and takes part in the Krebís cycle reactions (the TCA cycle).

Thus, acetyl Co-A is the connecting link between glycolysis (in the cytoplasm) and Krebís cycle (in mitochondria).

(ii) T.C.A. cycle (Krebís cycle)

The Krebís cycle reactions occur in the matrix of mitochondria. All enzymes required for these reactions are present in the matrix. Each step is controlled by a specific enzyme and is reversible.

The various steps in the T.C.A. Cycle are as follows:

(1) Each molecule of 2-C acetyl Co-A that enters the T.C.A. cycle first combines with 4-C oxalo acetic acid (OAA), and a 6-C Citric acid is formed. One molecule of H2O is used in the reaction.

Thus, the first product in Krebís cycle is citric acid, hence it is also called the citric acid cycle. As citric acid is a tricarboxylic acid (with 3-COOH groups), it is also called tricarboxylic acid (TCA) cycle.

(2) In the next step, 6-C citric acid is first converted into 6-C -aconitic acid (deletion of H2O) and then into 6-C isocitric acid (addition of H2O).

(3) The next reaction involves the oxidation of isocitric acid (by removal of hydrogen) to form 6-C oxalo succinic acid. NADH2 is formed in the process.

(4) Decarboxylation of 6-c Oxalo succinic acid results in the formation of 5-c a -Ketoglutaric acid with the liberation of carbon dioxide.

(5) a-Ketoglutaric acid (5-C) then undergoes oxidation (by removal of hydrogen) and decarboxylation to form 4-C succinyl Co-A. The reaction is highly complicated and takes place in the presence of Co-A and NAD. NADH2 is formed and carbon dioxide is released.

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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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