Q.
Refer the figure and answer the question.
Solution:
Glycolysis / EMP pathway:
(1) Discovery: It was given by Embden, Meyerhoff and Parnas in 1930. It is the first stage of the breakdown of glucose in the cell.
(2) Definition: Glycolysis (Gr. glykys= sweet, sugar; lysis= breaking) is a stepped process by which one molecule of glucose (6C) breaks into two molecules of pyruvic acid (3C).
(3) Site of occurrence: Glycolysis takes place in the cytosol and does not utilise oxygen. Thus, it is an anaerobic pathway. In fact, it occurs in both aerobic and anaerobic respiration.
Steps of glycolysis: Glycolysis consists of 10 steps divided into 2 phases, preparatory phase and pay off phase. Each step is catalysed by a specific enzyme. Most of the reactions are reversible.
Preparatory phase:
(i) First phosphorylation: The third phosphate group separates from adenosine triphosphate (ATP) molecule, converting the latter into adenosine diphosphate (ADP) and releasing energy. With this energy, the phosphate group combines with glucose to form glucose 6-phosphate, The reaction is catalyzed by the enzyme, hexokinase in the presence of Mg2+. Thus, a molecule of ATP is consumed in this step. This glucose 6-phosphate (phosphoglucose) is called active glucose.
(ii) Isomerization: Glucose 6-phosphate is changed into its isomer fructose 6-phosphate by rearrangement. The rearrangement is catalyzed by an enzyme, phosphoglucose-isomerase, or phosphohexose isomerase. In sperms, Fructose 6-phosphate may be formed directly from free fructose by its phosphorylation in the presence of an enzyme fructokinase, Mg2+ and ATP.
(iii) Second phosphorylation: Fructose 6-phosphate combines with another phosphate group from another ATP molecule, yielding fructose 1, 6-biphosphate and ADP, The combination is catalyzed by an enzyme phosphofructokinase in the presence of Mg2+ and appears to be irreversible. This phosphorylation, thus, consumes another molecule of ATP. Excess of ATP inhibits phosphofructokinase.
The second phosphorylation reaction activates the sugar and prevents its escape from the cell. They go uphill, increasing the energy content of the products.
(iv) Cleavage: Fructose 1,6-biphosphate now splits into two 3-carbon, phosphorylated sugars: DHAP and 3-phosphoglyceraldehyde (3-PGAL). The reaction is catalyzed by an enzyme aldolase. DHAP is converted into PGAL with the aid of an enzyme phosphotriose isomerase.
(v) Isomerization: DHAP is changed into its isomer 3-Phosphoglyceraldehyde by rearrangement. The rearrangement is catalyzed by an enzyme, phosphotriose isomerase.
Pay-off phase: In this phase, energy is released in the form of ATP and NADH2 is formed. The steps involved in ATP formation are as follows:
(vi) Formation of NADH2: During the conversion of 3-phosphoglyceraldehyde into 1, 3-biphosphoglycerate one molecule of NADH2 is formed. As each molecule of glucose yields two molecules of 1,3-biphosphoglycerate, hence each molecule of glucose form 2 molecules of NADH2. Each molecule of NADH2 during oxidative phosphorylation produces 3 molecules of ATP. Thus, a total of 6 ATP are produced in this step.
(vii) Formation of 3-PGA: High-energy phosphate group on carbon 1 of 1,3 diphosphoglycerate is transferred to a molecule of ADP, converting it into an ATP molecule. 1, 3-diphosphoglycerate changes to 3-phosphoglycerate due to the loss of a phosphate group. The reaction is catalyzed by an enzyme diphosphoglycerokinase.
(viii) Isomerization: 3-PGA is changed into its isomer 2-PGA by rearrangement. The rearrangement is catalyzed by an enzyme, phosphoglyceromutase.
(xi) Dehydration: 2-phosphoglycerate loses a water molecule in the presence of an enzyme, enolase and changes into phosphoenolpyruvate. The latter undergoes molecular rearrangement that transforms its phosphate group into a high-energy phosphate bond.
(x) Formation of Pyruvate: High-energy phosphate group of phosphoenolpyruvate is transferred to a molecule of ADP with the help of an enzyme pyruvate kinase.
(1) Discovery: It was given by Embden, Meyerhoff and Parnas in 1930. It is the first stage of the breakdown of glucose in the cell.
(2) Definition: Glycolysis (Gr. glykys= sweet, sugar; lysis= breaking) is a stepped process by which one molecule of glucose (6C) breaks into two molecules of pyruvic acid (3C).
(3) Site of occurrence: Glycolysis takes place in the cytosol and does not utilise oxygen. Thus, it is an anaerobic pathway. In fact, it occurs in both aerobic and anaerobic respiration.
Steps of glycolysis: Glycolysis consists of 10 steps divided into 2 phases, preparatory phase and pay off phase. Each step is catalysed by a specific enzyme. Most of the reactions are reversible.
Preparatory phase:
(i) First phosphorylation: The third phosphate group separates from adenosine triphosphate (ATP) molecule, converting the latter into adenosine diphosphate (ADP) and releasing energy. With this energy, the phosphate group combines with glucose to form glucose 6-phosphate, The reaction is catalyzed by the enzyme, hexokinase in the presence of Mg2+. Thus, a molecule of ATP is consumed in this step. This glucose 6-phosphate (phosphoglucose) is called active glucose.
(iii) Second phosphorylation: Fructose 6-phosphate combines with another phosphate group from another ATP molecule, yielding fructose 1, 6-biphosphate and ADP, The combination is catalyzed by an enzyme phosphofructokinase in the presence of Mg2+ and appears to be irreversible. This phosphorylation, thus, consumes another molecule of ATP. Excess of ATP inhibits phosphofructokinase.
The second phosphorylation reaction activates the sugar and prevents its escape from the cell. They go uphill, increasing the energy content of the products.
(iv) Cleavage: Fructose 1,6-biphosphate now splits into two 3-carbon, phosphorylated sugars: DHAP and 3-phosphoglyceraldehyde (3-PGAL). The reaction is catalyzed by an enzyme aldolase. DHAP is converted into PGAL with the aid of an enzyme phosphotriose isomerase.
(v) Isomerization: DHAP is changed into its isomer 3-Phosphoglyceraldehyde by rearrangement. The rearrangement is catalyzed by an enzyme, phosphotriose isomerase.
(vi) Formation of NADH2: During the conversion of 3-phosphoglyceraldehyde into 1, 3-biphosphoglycerate one molecule of NADH2 is formed. As each molecule of glucose yields two molecules of 1,3-biphosphoglycerate, hence each molecule of glucose form 2 molecules of NADH2. Each molecule of NADH2 during oxidative phosphorylation produces 3 molecules of ATP. Thus, a total of 6 ATP are produced in this step.
(vii) Formation of 3-PGA: High-energy phosphate group on carbon 1 of 1,3 diphosphoglycerate is transferred to a molecule of ADP, converting it into an ATP molecule. 1, 3-diphosphoglycerate changes to 3-phosphoglycerate due to the loss of a phosphate group. The reaction is catalyzed by an enzyme diphosphoglycerokinase.
(viii) Isomerization: 3-PGA is changed into its isomer 2-PGA by rearrangement. The rearrangement is catalyzed by an enzyme, phosphoglyceromutase.
(x) Formation of Pyruvate: High-energy phosphate group of phosphoenolpyruvate is transferred to a molecule of ADP with the help of an enzyme pyruvate kinase.