Question

Oxalosuccinic acid, a transient intermediary compound of Krebs cycle is
A.4-carbon compound
B.5-carbon compound
C.6-carbon compound
D.3-carbon compound

Answer 1

Hint: In plants, respiration is a complex process as they do not possess any specialized respiratory organ. In cellular respiration, the C-C bonds between the complex organic molecules break and release the energy.

Complete answer:
At the beginning of the process of respiration, glucose is broken down into pyruvate by a process known as glycolysis. The fate of the pyruvate then depends upon the availability of oxygen. In the presence of oxygen, aerobic respiration occurs.

In aerobic respiration, the pyruvate first gets converted into Acetyl CoA and then enters into the Krebs Cycle. The Krebs cycle is a process of cyclic oxidation which starts with the reaction of oxaloacetic acid with water and Acetyl CoA (2-carbon compound) to form Citric Acid with the help of the enzyme citrate synthase. Oxaloacetic acid is a 4-carbon compound and Citric acid is a 6-carbon compound. Citrate is then isomerized to isocitrate. Isocitrate is converted into Oxalosuccinic acid with the help of the enzyme isocitrate dehydrogenase. Oxalosuccinic acid is also a 6-carbon compound.

In the remaining steps, again oxalic acid is formed by two successive decarboxylations. As the combined result of glycolysis and Krebs cycle, 3 molecules of carbon dioxide, 8 molecules of NADH, $$\text{2FAD}{\text{H}}_{\text{2}}$$,2 ATP, and 8 hydrogen ions are formed.
But oxygen is not utilized in these steps. After the formation of a large amount of energy in the form of ATP, this energy is used in the step known as the Electron Transport Chain where oxygen acts as a final hydrogen acceptor.

Hence, the correct answer is option (C)

Note: Glucose is the molecule that is broken down mostly to yield energy in the process of respiration. Fats and proteins are also broken down.