Amphibolic pathway - class-XI

Sir Hans Adolf Krebs was a German born British physician and biochemist. He was the pioneer scientist in study of cellular respiration, a biochemical pathway in cells for production of energy.

Glycolysis produces 8 ATP (2 net ATP + 2 NADH yielding 6 ATP via shuttle), link reaction produces 6 ATP (2 NADH), and the Krebs cycle produces 24 ATP (6 NADH, 2 FADH2, 2 GTP/ATP). The total is 38 ATP.

Respiration is called amphibolic because it involves both catabolic processes (breakdown of substrates to release energy) and anabolic processes (use of respiratory intermediates for biosynthesis).

Statement ii describes the catabolic nature (breakdown of C-C bonds), while statement iii describes the anabolic nature (withdrawal of intermediates like Acetyl CoA for fatty acid synthesis), together defining the amphibolic pathway.

During the process of glycolysis 8 ATP molecules are produced in aerobic respiration. In the case of Krebs cycle 30 ATP molecules can be produced.

The process by which the energy of glucose is released under aerobic respiration, was discovered by Hans Krebs. The Krebs cycle is a part of cellular respiration. It is a series of chemical reactions used by all aerobic organisms to generate energy.

Aerobic respiratory pathway is appropriately called amphibolic pathway. It contains both catabolic and anabolic reactions. The pathway is catabolic because in this pathway, glucose is broken down into carbon dioxide and water. It is also anabolic pathway as energy rich ATP molecule is produced during this pathway. These ATP molecules are later used to carry out various reaction. 

The Krebs cycle is a major metabolic pathway that releases energy in the form of NADH, FADH2, and ATP/GTP. While it is a central hub, it is primarily responsible for the oxidation of acetyl-CoA.

The enzymes for the Krebs cycle are located within the mitochondrial matrix in eukaryotic cells.

In the Krebs cycle, each acetyl CoA oxidation yields 3 NADH, 1 FADH2, and 1 GTP (equivalent to 1 ATP), which collectively generate 12 ATP molecules through oxidative phosphorylation in older traditional calculations often referenced in textbooks. Thus, the formation of 12 ATP from each acetyl CoA is the standard representation for this pathway.

Glycolysis has 1 oxidation step (G3P to 1,3-BPG), the link reaction has 1 (pyruvate to acetyl-CoA), and the Krebs cycle has 4 (isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, succinate to fumarate, and malate to oxaloacetate).

The same respiratory process which acts as a catabolic pathway for respiratory substrates also acts as an anabolic pathway for the synthesis of various intermediary metabolic products and secondary metabolites. The respiratory pathway, therefore, acts both as catabolic as well as an anabolic pathway. Thus, it is better to call it the amphibolic pathway rather than a catabolic pathway alone.

Fatty acid synthesis takes place in the cytoplasm of the cell. It is the creation of fatty acids from acetyl-CoA and NADPH through the action of enzymes called fatty acid synthases. Most of the acetyl-CoA which is converted into fatty acids is derived from carbohydrates via the glycolytic pathway. 

The Kreb’s Cycle gives 3 moles of NADH, 1 mole of FADH and 1 mole of ATP per cycle of substrate i.e per 1 mole of Acetyl CoA. However net ATP i.e after ETS which would make the answer to 12 ATP(9+2+1) respectively.

TCA or Krebs' cycle is amphibolic (both catabolic and anabolic) because it provides a number of intermediates for anabolic pathways.

Mechanism of aerobic respiration was given by Kreb. Thus it was named as Krebs cycle. Aerobic respiration is the respiration, which involves the breakdown of the substrate in the presence of oxygen (oxidation process).

The conversion of pyruvate to acetyl-CoA is a linker step between glycolysis and Krebs cycle. After the entry of pyruvate in the mitochondria the acetylation takes place in the perimitochondrial space which is the space between the inner and outer mitochondrial membrane. Each molecule of pyruvic acid is oxidatively decarboxylated to acetyl coA by the action of an enzyme complex pyruvate dehydrogenase. The acetyl coA then enters the Krebs cycle.  Thus the correct answer is option C.

The pyruvic acid enters Krebs cycle in form of acetyl CoA where it is oxidatively decarboxylated into CO$ _2$. The complete Kreb cycle leads to the net formation of 2 molecules of ATP by substrate-level phosphorylation, 8 molecules of NADH plus 2 molecules of FADH$ _2$ (via succinate dehydrogenase). The NADH and FADH$ _2$ then enter the ETS to drive ATP production by substrate level phosphorylation. 

Kreb's cycle consists of 8 cyclic steps which take place in the mitochondrial matrix. Each step is catalyzed by a specific enzyme. In Kreb's cycle, the entrant molecule is 2- carbon acetyl coenzyme A and the receptor molecule is 4- carbon oxaloacetate. Since the process involves both catabolic and anabolic reactions. it is an amphibolic pathway. It degrades acetyl coenzyme A and yields carbon dioxide and energy which is used to produce NADH, H+, and FADH+

• The citric acid cycle is also known as the TCA cycle (tricarboxylic acid cycle) or the Krebs cycle. 
• It is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins, into adenosine triphosphate (ATP) and carbon dioxide.
•  In eukaryotic cells, the citric acid cycle occurs in the matrix of the mitochondrion.
•  In prokaryotic cells, such as bacteria, which lack mitochondria, the citric acid cycle reaction sequence is performed in the cytosol. 
• Hence, Mitochondria is the site for the Krebs cycle.
• So, the correct answer is 'Krebs cycle'.

In the Krebs cycle, two decarboxylation steps occur per acetyl-CoA molecule, releasing two molecules of CO2.