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PinkMonkey Online Study Guide-Biology

The initial steps in glycolysis involve the phosphorylation of glucose into an unstable fructose 1, 6-diphosphate for easy breakdown. This is called the preparatory phase of glycolysis during which 2 ATP are used. The reactions are as follows.

(1) In the first step, 6-C glucose is converted into glucose-6-phosphate. One ATP is used in the reaction.

(2) The next step involves isomerization of glucose 6-phosphate into Fructose 6-phosphate.

(3) Fructose-6-phosphate is then phosphorylated to fructose-1, 6-diphosphate. This reaction utilizes one ATP.

Thus, two ATP are used up in the preparatory phase.


(B) Cleavage of fructose-1, 6 - diphosphate

(4) The molecule of 6-C fructose-1, 6-diphosphate undergoes cleavage to produce two molecules of 3-C triose phosphates. One is 3-phosphoglyceraldehyde (3-PGAL) and the other is dihydroxy acetone phosphate (DHAP).

The two triose phosphates can isomerize into each other. However, further reactions in glycolysis utilize only PGAL. Therefore, DHAP is first converted into PGAL.

Thus, in all, two molecules of 3-PGAL are formed from the cleavage of one fructose-1, 6- diphosphate.

(C) Formation of 3-C pyruvic acid

Further reactions in glycolysis result in the conversion of 3-C PGAL into 3-C pyruvic acid through the following intermediate steps:

(5) 3-PGAL is converted into 1, 3 diphosphoglyceric acid (1, 3 diPGA). This is an oxidation and phosphorylation reaction. It takes place in the presence of inorganic phosphate (P.) and co-enzyme NAD (nicotinamide adenine dinucleotide). The reduced co-enzyme NADH2 is formed in the process.

(6) In the next step, 1, 3-diPGA undergoes dephosphorylation to form 3-phosphoglyceric acid (3-PGA). Phosphate removed from each diPGA is transferred to by ADP to form ATP. Two ATP molecules are formed at this step.

(7) 3-PGA is transformed into 2-PGA.

(8) In the next step, 2-PGA is converted into 2-phosphoenol pyruvic acid (2-PEP) with the loss of water (dehydration).

(9) In the final step of glycolysis, 2-PEP is dephosphorylated to form 3-C pyruvic acid, which is the end product of glycolysis. The reaction produces 2 ATP.

Thus, when a molecule of 6-C glucose undergoes the reactions in glycolysis (EMP pathway), the overall process may be represented as follows.

However, 2#ATP are used up in the preparatory phase of glycolysis. Therefore the net gains of glycolysis are:

Phase-II of Aerobic Respiration

Phase-II of aerobic respiration comprises a series of enzymic reactions in which 3-c pyruvic acid (the end product of glycolysis) is broken down step-wise to form the end products of aerobic respiration, (i.e. CO2 , H2 O and ATP energy), inside the mitochondria.

Phase-II involves the following two main sets of reactions:

(1) The T.C.A. cycle

(2) Terminal oxidation and oxidative phosphorylation (E.T.S. and the redox reactions).

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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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