Question

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

Answer 1

Respiratory substrate is the substrate required for cellular respiration. The energy is derived from the respiratory substrate through oxidation. Carbohydrates, fats, and proteins are examples of respiratory substrates.

Breakdown of carbohydrates into pyruvic acid occurs in 10 steps via glycolysis.

Step I: The first step in glycolysis is the conversion of D-glucose into glucose-6-phosphate. Hexokinase enzyme catalyzes this reaction.

Step II: In this, the rearrangement of glucose 6-phosphate (G6P) into fructose 6-phosphate (F6P) takes place by glucose phosphate isomerase enzyme.

Step III: Phosphofructokinase enzyme in presence of Mg++ ion converts fructose 6-phosphate into fructose 1,6-bisphosphate.

Step IV: Enzyme Aldolase splits fructose 1, 6-bisphosphate into two sugars, namely dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP).

Step V: Enzyme triophosphate isomerase interconverts the dihydroxyacetone phosphate (DHAP) into glyceraldehyde 3-phosphate (GAP).

Step VI: Enzyme glyceraldehyde-3-phosphate dehydrogenase converts glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate. In this step, NADH++ is formed.

Step VII: Enzyme phosphoglycerate kinase converts 1,3-bisphosphoglycerate to 3-phosphoglycerate. In this step, ATP is formed.

Step VIII: Enzyme phosphoglyceromutase converts 3- phosphoglycerate to 2-phosphoglycerate.

Step IX: Enzyme enolase removes a molecule of water from 2-phosphoglycerate to form phosphoenolpyruvic acid (PEP).

Step X: Enzyme pyruvate kinase 2 molecules of pyruvic acids are formed from phosphoenolpyruvate (PEP). In this step, ATP is formed.