Here's why:

* Cellular respiration is the process by which cells break down organic molecules (like glucose) to release energy in the form of ATP.

* Glucose is a simple sugar that is readily available in the body, either from the breakdown of carbohydrates we eat or from stored glycogen.

* Other organic molecules, like fatty acids and proteins, can also be used as fuel sources for respiration, but glucose is the primary one.

In summary, the carbon in the glucose molecule is oxidized during respiration, releasing energy and producing carbon dioxide as a byproduct.