During glucose metabolism, carbon dioxide is formed in the mitochondria during the process of cellular respiration. Specifically, it is produced as a byproduct of the citric acid cycle, also known as the Krebs cycle.

The citric acid cycle is a series of chemical reactions that occur in the mitochondria of cells and play a central role in the breakdown of glucose for energy production. As glucose is broken down and oxidized, carbon atoms are removed and combined with oxygen to form carbon dioxide. These carbon atoms are released as carbon dioxide gas, which is then transported by the bloodstream to the lungs and exhaled.

The overall chemical equation for the citric acid cycle is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

This equation represents the complete oxidation of one molecule of glucose, resulting in the production of six molecules of carbon dioxide and six molecules of water.