Animals breathe in air containing oxygen and breathe out air with less oxygen and more carbon dioxide. The carbon dioxide comes from

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Quizplus · Accepted Answer

The carbon dioxide produced during respiration is a byproduct of the citric acid cycle, also known as the Krebs cycle. This cycle takes place in the mitochondria of cells and is part of the process of producing ATP, the energy currency of cells. Glycolysis and waste products also contribute to the production of carbon dioxide, but the majority comes from the citric acid cycle. Hydrocarbons do not play a direct role in respiration.

Animals Breathe in Air Containing Oxygen and Breathe Out Air