Question

At which stage of respiration does the CO2 release take place?

Answer 1

In aerobic respiration CO2 is released when pyruvate is converted to acetyl-CoA. It is then released again in the Krebs cycle(TCA-Citric acid cycle).
In anaerobic, CO2 is released when pyruvate is converted to ethanol.

Answer 2

Per glucose molecule 2 CO2 is produced during pyruvate decarboxylation (the reaction between glycolysis and the Kreb's Cycle) and 4 CO2 are produced during the Kreb's Cycle