Question

What is the respiratory substrate? Enumerate the steps involved in the breakdown of carbohydrates up to pyruvic acid.

Answer 1

Carbohydrates, fats, and proteins are considered to be respiratory substrates. The pathway by which carbohydrates (glucose) is converted to pyruvic acid is called glycolysis and is an example of energy production via oxidation.

Complete answer:
Respiration is the process of utilizing oxygen to produce energy within the body. This is done with the aid of several different biochemicals that are oxidized by the air we breathe in to form viable energy within our body. The various biomolecules used for respiration is called respiratory substrates. Carbohydrates are used as respiratory substrates in the glycolysis and Krebs cycle, fats are used as respiratory substrates in the beta-oxidation cycle, and proteins are used as a respiratory substrate in the gluconeogenesis cycle.
The breakdown of carbohydrates into pyruvate is the main step involved in the respiration using carbohydrates, complex carbohydrates are broken down into glucose molecules which enter the cytoplasm of the cell. In the cytoplasm, the glucose undergoes glycolysis which is a 10-step process.
Step I: D-glucose is converted into glucose-6-phosphate by hexokinase.
Step II: Glucose-6-phosphate (G6P) is converted into fructose-6-phosphate (F6P) by glucose phosphate isomerase.
Step III: Fructose-6-phosphate is converted into fructose 1,6-bisphosphate by phosphofructokinase.
Step IV: Fructose 1, 6-bisphosphate is converted into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP) by aldolase.
Step V: Dihydroxyacetone phosphate (DHAP) is converted into glyceraldehyde 3-phosphate (GAP) by triphosphate isomerase.
Step VI: Glyceraldehyde-3-phosphate is converted into 1,3-bisphosphoglycerate by glyceraldehyde-3-phosphate dehydrogenase.
Step VII: 1,3-bisphosphoglycerate is converted into 3-phosphoglycerate by phosphoglycerate kinase.
Step VIII: 3- phosphoglycerate is converted into 2-phosphoglycerate by phosphoglyceromutase.
Step IX: 2-phosphoglycerate is converted into phosphoenolpyruvic acid (PEP) by enolase.
Step X: Phosphoenolpyruvate (PEP) is converted into 2 molecules of pyruvate by pyruvate kinase.

Note: The conversion of glucose to pyruvate is also an energy-producing pathway where indirect high energy compounds such as NADPH are formed and even 2 ATP molecules are formed. The pathway followed when using fats as a respiratory substrate is known as beta-oxidation of fatty acids. This pathway produces less energy when compared to carbohydrates and bypasses the glycolysis pathway and directly begins the Krebs cycle with acetyl CoA obtained from oxidation of fatty acids.