Question

What goes on between glycolysis and the citric acid cycle? What are the reactants and products of this process if cellular respiration started with 1 glucose molecule. What are the reactants and products of the citric acid cycle if cellular respiration started with 1 glucose molecule?

Answer 1

1 molecule of glucose undergoes through glycolysis to produce 2 pyruvate molecules. In this process, 2 ATP and 2 NADH are produced. Now, pyruvate undergoes through a link reaction in order to convert it to Acetyl coA as pyruvate cannot directly enter the citric acid cycle, rather Acteyl coA enters the citric acid cycle. In the link reaction, Link reaction is also known as pyruvate decarboxylation or pyruvate oxidation and takes place by the help of enzyme complex, Pyruvate dehydrogenase. The chemical equation for link reaction is as follows:

1 Pyruvate + 1 NAD+ + CoA → 1 Acetyl-CoA + NADH + CO2 + H+

Thus, 2 pyruvates resulting from 1 molecule go through 2 cycles of link reaction to produce 2 Acetyl coA, 2 NADH and 2 CO2 molecules. One turn of citric acid cycle metabolizes 1 acetyl coA by assimilating it with oxaloacetate and then undergoing sequential oxidative decarboxylation to yields 3 NADH, 1 GTP ( ATP equivalent) and 1 FADH2 along with 2 CO2 molecules. Thus, 2 Acetyl coA will go through 2 turns of citric acid cycle yielding 6 NADH, 2 GTP (ATP equivalents), 2 FADH2 and 4 CO2 molecules.